Modern Tug Design

Modern Tug Design with Particular Emphasis on Propeller Design, Maneuverability, and Endurance BY DOROS A. ARGYRIADIS, 1 ASSOCIATE M E M B E R

T u g b o a t design, although of utmost importance, has been disregarded by the naval architect to a great extent and only a limited amount of information is available to the designer in the form of technical papers. In this study, an attempt is made to correct this lack by presenting some of the important features peculiar to tugboat design. Hull form and lines are treated briefly. Some design formulas are presented and a c o m p a r i s o n is made between British and American designs. T h e stability of tugs is presented at some length and the lines o f some m o d e r n boats as well as the particulars of several others are given. Arrangements and accommodations usually are based upon the wishes of the tug o w n e r and are treated very briefly. T h e importance of a g o o d preliminary

weight and trim calculation is emphasized. Several different types of main p r o p u l s i o n machinery p o w e r plants are discussed and the merits of each one are presented. Propeller design is discussed at some length. Preliminary desig n formulas are given for both the bollard pull and the t o w i n g thrust. Comparisons between the different types of propellers are made and a m e t h o d for calculating the performance of the propeller at any speed of the boat is presented. Maneuverability is also discussed and covers both r u d d e r design and engine controls. Formulas are given for the rudder area and the relative merits of some of the several types of tugboat rudders are analyzed. Finally, endurance and engine performance at reduced speeds are discussed.

of an old steam tug helping a sailing vessel dock or und0ck was fairly common in the major ports of Tugboat design is a subject that has been disthe world around the middle of the past century. regarded by most naval architects, with the nota- Without these handy vessels in and around our ble exceptions of Roach (24) 2 and Caldwell (8), harbors, a major portion of the world's shipping despite the obvious usefulness of these boats. In could not operate successfully and efficiently, and searching the libraries for appropriate literature, the docking, undocking, salvage and the carriage the author has been amazed b y the lack of written of cargoes in barges would have beeh impossible. material on the subject. This inadequacy is Tugs can be subdivided into three main catedifficult to understand, especially if one remem- gories or classes; namely, (a) small harbor and bers that the design of tugboats dates back to the utility tugs, (b) large harbor and coastwise tugs, earliest days of steam-driven boats and the sight and (c) ocean-going and salvage tugs. The small harbor tug represents the workhorse of the harI N a v a l Architect, J o h n J. M c M u l l e n Associates, Hoboken, N. J. 2 N u m b e r s in parentheses refer to the B i b l i o g r a p h y a t t h e end of bor, and its services would include the performt h e paper. Presented a t t h e A n n u a l M e e t i n g , New York, N. Y., Noance of a number of rather small towing jobs and v e m b e r 14-15, 1957, of T I ~ SOCIETY OF NAVAL ARCHITECTS AND the docking of small vessels. The utility tug may MARINB ENGINEERS, 362 INTRODUCTION

MODERN

TUG DESIGN

range from 40 to 65 ft in length, while the large harbor and coastwise tug usually has a length of from 70 to 120 ft. The services of this second class would include the docking of large vessels and the towing of barge and lighter fleets within the harbor or along the coast. Finally, the salvage tug is mainly concerned with long ocean towing services and, as its name implies, salvage jobs. The length of this tug is usually over 125 ft ancl its freeboard is normally more t h a n the freeboard found in its smaller counterparts, in order to allow a safe and dry ocean crossing. The three types mentioned can actually be dealt with simultaneously, since, a p a r t from physical dimensions, there are few fundamental differences between them. In general, the tug designer is limited in his choice of principal dimensions b y the specifications of the owner relative to power, m a x i m u m allowable draft, and free running speed, while he also has to take into account such practical aspects as stability, limited length in connection with maneuverability, engine room size, propeller dimensions, hull form, and so on. Off hand, it might seem peculiar to the uninitiated t h a t length and speed are treated so nonchalantly and assigned specific values for different classes of tugboats without a thorough investigation of the effects of length on speed and power. This subject has been treated b y L. A. Baler in references (3) and (4) in which he shows t h a t in m a n y cases it is advantageous to increase the length of the boat to obtain the best resistance characteristics. However, the design of a tugboat does not allow the selection of the most efficient length for the power available because of other, more important, considerations. A tugboat is essentially a floating powerhouse and its p r i m a r y mission is to help other vessels to maneuver in restricted quarters or to tow them to their destinations. AcCordingly, most of its power is absorbed on the towline and only a small percentage is used for the propulsion of the boat itself. With the exception of ocean-going salvage tugs and some coastwise tugs, one can safely say t h a t the resistance of the tug itself, while towing, is only a small percentage of the over-all towrope pull exerted, with the result t h a t hull-form characteristics can have little influence on the towing speed. However, since the speed/length ratio of these boats will.be high in the free running condition owing to the large available power, and since normally the owner will specify some particular speed to be attained while running free, the designer should give careful consideration to the selected prismatic COefficient, longitudinal center of buoyancy and fineness of b o d y fore and aft in order to obtain the best hull form possible. Free-

363

board forward, which frequently limits the free running speed, m u s t also be considered. The trend of t o d a y seems to be to increase the available power over older tugs without a n y change in the over-all length of the boat. One reason for this is the better and more powerful engines, such as the supercharged Diesel, available on the m a r k e t today. Again, the length of the ship-handling tug cannot be increased considerably o v e r 100 ft, since any size above t h a t length would tend .to m a k e the b o a t awkward in maneuvering in and out of tight spots. I t follows, then, that the main problem of the designer, after the preliminary form characteristics have been established, would be to fit a propeller which would give m a x i m u m possible towrope pull at some o p t i m u m towing speed and which, at the same time, would allow the b o a t to attain the desired free running speed. Finally, careful consideration should be given to the main propulsion machinery control, since the maneuverability and hence quite a bit of the success of the tugboat, be it large or small, will depend largely on the response of the propulsion plant to the orders given to it from the bridge. In addition to the pilothouse control station, an additional control station on the deck aft of the pilothouse is recommended for harbor work. HULL-FORM CHARACTERISTICS I t has been mentioned before t h a t the effective horsepower of a tug at normal towing speeds will be very small as compared to the total towrope pull exerted. However, the designer should give careful consideration to the hull form, so as to obtain the m a x i m u m possible thrust available for towing and at the same time meet the owner's requirements regarding free route speed. F r o m purely theoretical considerations, the prismatic coefficient should be somewhere between 0.57 and 0.67, since most tugs will have a free route speed/ length ratio of about 1.10 to 1.40 and a towing speed/length ratio of from 0.60 to 0.70. I t would appear, off hand, t h a t a greater prismatic could be used if the hull were to be designed for towing speeds, b u t an investigation of the resistance of a h e a v y displacement hull (say displacement/length ratio equal to 400) with a prismatic of 0.70 at a speed/length ratio of 1.15 shows a twofold increase in total resistance per ton of displacement over the same hull having a prismatic of 0.60, while the reduction in resistance of the lower prismatic hull over the:~0.70 prismatic hull at a speed/length ratio around 0.60 amounts to less than 10 per cent of the total resistance at t h a t speed. On the other hand, if the speed/length ratio of the tug when running free is over 1.25, as is often the case in

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TUG DESIGN

a modern tugboat, the resistance of the low prismatic coefficient hull at these high speed/length ratios becomes prohibitive. C o n t r a r y to the belief and practice of m a n y tugboat designers, it would appear t h a t the most suitable prismatic for a vessel of this type would be the one corresponding to a speed/length ratio of a b o u t 1.10, or a prismatic of between 0.57 and 0.60. This would tend to reduce the abnormally high resistance t h a t most boats of this type show when running free, and at the same t i m e give a reasonably low resistance over the whole range of operation. Residual resistance contours of tugboat forms are presented in Appendix 3. These contours are derived from the same d a t a as the ones appearing in C.D. Roach's paper on " M o d e r n Tug Design" (24), with the difference t h a t resistance-coefficient curves have been plotted for different prismatic coefficients against the more commonly used speed/length ratio. Several curves for displacem e n t / l e n g t h ratios of from 200 to 450 in increments of 50 are shown and it is hoped t h a t this type of presentation m a y facilitate interpolation between different speed/length ratios, displacem e n t / l e n g t h ratios and prismatic coefficients. T h e block coefficient of a tugboat is usually much lower than the prismatic coefficient, and is sometimes as low as 0.45 or 0.46. This is mainly due to the fact t h a t the bilges have to be as slack as practicable to allow an easy fairing of the lines into the fine fore-and-aft body. Average values of block coefficients range from 0.45 to 0.55 and corresponding values for the midship-section coefficient v a r y from 0.75 to 0.85, with the most commonly used value being very close to 0.80. The foreb0dy lines should be as fine as possible and the half-angle of entrance of the load waterline ranges from 15 to 30 deg with the median around 20 deg. The waterlines sometimes have a slight reverse to allow the fairing of the curve into the half-beam at or near amidships. The load waterline aft should be as full as possible to allow for m a x i m u m coverage and protection for the propeller. T h e afterbody lines below the load waterline should be fair and fine in order to give the propeller the m a x i m u m possible amount of solid water, and reverse curvature of these lines is practically a necessity. The fineness of the afterbody lines below the waterline cannot be overemphasized, since in m a n y tugboats the propeller does not seem to receive the required amount of solid water, tending to pull down air from above, and in this way m a y cause serious and objectionable vibrations. Reference (5) gives a good analysis of the reasons of stern vibrations on single-screw vessels. Although t h a t reference deals mainly with G r e a t Lakes ore carriers, the findings can be applied to

365

tugboats as well. In particular, the authors state t h a t it seems to be apparent t h a t wide variations in wake distribution have far greater effect on hull vibrations than do close clearances between the propeller and the hull. An interesting sidelight of the vibration problem of tugboats is t h a t boats fitted with K o r t nozzles show much less hull vibrations than boats with open p r o p e l l e r s . I t might be added here t h a t in order to avoid the sucking of air b y the propeller, a case t h a t m a y happen if the wheel does not receive sufficient solid water from ahead, some of the late river towboats have the b o t t o m shell plating extending somewhat past the side-shell plating in the vicinity of the propeller, thus using in effect the same technique t h a t L. A. Baler and J. Ormondroyd used to reduce the fantail vibrations of G r e a t Lakes ore carriers. T h e longitudinal center of buoyancy location is also quite important. T h e fine form of the after body will tend to force the center of buoyancy amidships or even forward of amidships. Some designers seem to be satisfied with this condition and even recommend such a location. However, the author believes t h a t the best location of the longitudinal center of buoyancy for the proposed design speed/length ratio is from 2 to 2.5 per cent aft of amidships, a value t h a t cannot always be obtained. A compromise is here necessary, and a longitudinal center of buoyancy of approximately 0.01 L (or 1.0 per cent) aft of amidships seems to be the best one can hope for. I t m a y be found advisable, at times, to lengthen the vessel b y a few feet to obtain a reasonable location for the longitudinal center of buoyancy, since in most designs it seems to fall forward of amidships if the design is based strictly on arrangements and accommodations. Several authors give preliminary design formulas a n d / o r proportion figures which m a y prove helpful to the tugboat designer at the preliminary stages of the work. Most of these are derived for British tugboats which differ from the usual American design in t h a t they have shorter deckhouses and are somewhat underpowered according to modern American practice, with the result t h a t the figures presented should be used with care. Table 1 gives some of the most i m p o r t a n t and widely used figures in Britain in comparison with some representative values for similar United States built tugs. Some more comparisons between contemporary British and American designs are shown in Figs. 1 and 2. Additional information and proportions of several types of British tugs m a y be found in W. Pollock's "Small Vessels," (22). A. R. T a y l o r (28) gives the following formula for the preliminary estimate of the block coefficient

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M O D E R N TUG D E S I G N PROPORTIONS OF TUGBOATS--BRITISH VERSUS AMERICAN PRACTICE

TABLE i Class Item Vk/%/L (free)

~ a 1.04 4.75 0.85 0.56 0.85 0.66 0.7.0 0.09 9.5 5.75 c 1.5 -2.5 250 1000 & up

L/B KG/D ~ Cb C~I Cp Cwp m'~

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1-ocean b -5.75 0.85 0.475 --0.70 0.089 9.0 -

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. a 1.04 4.25 0.90 0.55 0.85 0.65 0.75 0.09 8.5 5.75 1.35 -2.5 380 600 to 1000

2-coastwise - - - - - . b c --1.25 4.50 3.70 0.82 0.91 0.53 0.48 0.80 -0.66 0.75 0.75 0.091 0.091 8.0 7.2 3.9 -1.2 1.75 3.30 2.5 2.2 300 400 -1200 to 1800 -

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~ a 1.20 3.75 0.80 0.52 0.85 0.61 0.71 0.09 8.0 5.75 1.25 -2.5 310 300 to 600

3-harbor - - - ~ b c -1.30 4.50 3.30 0.78 0.88 0. 464 0.50 0.78 -0.64 0.702 0.74 0.091 0.092 8.0 7.8 3.6 -1.1 1.75 2.0 2.5 2.9 380 400 -300 to 900 -

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" BM coefficient for use with Simpson's formula for beam. b No representative American design listed, since most of the river craft in the United States are towboats. our own small harbor tugs are similar to the river tugs listed. Based on the addition of forecastle. NOTI~S: Figures under "a" represent values recommended by A. Caldwell, reference (8). Figures under "b" represent values recommended by A. R. Taylor, reference (28). Figures under "c" represent modern American practice. This f o r m u l a has b e e n t e s t e d in accordance with m o d e r n A m e r i c a n practice a n d gave c o n s i s t e n t l y good results

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T h i s f o r m u l a seems to give low results for A m e r ican practice, a n d the a u t h o r wishes to propose the following modification of Caldwell's f o r m u l a to b r i n g i t in a g r e e m e n t with m o s t m o d e r n designs

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2.0 2.5 310

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Block coefficient from 0.48 to 0.55M i d s h i p section coefficient a b o u t 0.75 D r a g of keel from 0.04L to 0.05L M i n i m u m freeboard a b o u t 10 per c e n t of m a x i m u m "beam M e t a c e n t r i c h e i g h t : a m i n i m u m o f ' a b o u t 2.5 ft in loaded condition. I n a d d i t i o n to these figures, Mr. S i m p s o n states t h a t the deck line should be full, especially aft, in order to protect a n d provide coverage for the propeller, a n d should show c u r v a t u r e all along its l e n g t h to facilitate control alongside other ships. T h e i n f o r i n a t i o n a p p e a r i n g in the foregoing references a n d in n u m e r o u s other articles to be f o u n d in m a r i n e magazines from t i m e to t i m e has been consolidated into one plot. I t is h o p e d t h a t these sets of curves, as a p p e a r i n g here in Fig. 3, m a y help t h e t u g designer in t h e p r e l i m i n a r y stages of the design. F i n a l l y , Fig. 4 m a y aid t h e designer in establishing a p r e l i m i n a r y sectionalarea c u r v e for a c o n v e n t i o n a l boat. So far t h e d i s c u s s i o n h a s b e e n l i m i t e d to c o n v e n tional t y p e s of t u g b o a t s : However, r e c e n t l y some n o v e l types of b o a t s h a v e b e e n a p p e a r i n g in t h e E u r o p e a n h a r b o r s a n d their designers h a v e b e e n claiming i n v a r i a b l y t h a t t h e y are b e t t e r t h a n a n y other t u g b o a t afloat. F o r this reason, a brief discussion of these b o a t s m i g h t p e r h a p s be necessary. E. C. B. C o r l e t t (9) discusses two of these new t y p e s of t u g b o a t s ; t h e "Voith W a t e r T r a c t o r " a n d a t u g with a " h y d r o c o n i c " t y p e hull. Figs. 5 a n d 6 show t h e profile a n d m i d s h i p section of the latter. T h e s e b o a t s h a v e n o t as y e t a p p e a r e d in U n i t e d S t a t e s harbors, a n d the i n t e r e s t e d reader is re-

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times the depth of the b e a m and deep knees should be fitted on every third frame forward and aft of the machinery space. T h e shell plating of a tug of moderate size from

377

the after machinery bulkhead to the stem should be at least 1~ in. thick. T h e sheer strake should be at least ~/~ in. thick. The remainder of the plating can be of rule size. The gunwale angle should be a t least a 31/~ in. )4 31~ in. X ~ in. angle. Preferably it should be fitted below the deck to allow the stepping-in of the bulwarks. Deck plating. The stringer plate should be at least 0.35 in. thick all the w a y . The remainder of the plating c a n b e of rule size. Bulwarks should be approximately 1/~ in. thick and should be fitted about 6 in. inboard of the shell whenever possible. Their height should be a b o u t 36 in. for larger tugs, but as low as safety permits for small harbor tugs. T h e sheer of the bulwarks forward should be a b o u t 6 to 12 in. more than t h e corresponding deck sheer, while it can be a b o u t 3 in. less than the corresponding deck sheer aft. T h e floors in the engine room should be at least 1~ in. thick. The guards should consist of at least a 4 in. half:round section for the main guard and proportionately less for the secondary guards. This requirement depends mainly upon the type of servi c e the tug will be required to perform and can v a r y widely. " The installation of bilge keels is recommended for a t least the midship one-third length of the boat. More and more tugs today are equipped with formidable fire-fighting equipment:. T h e usual location of the fire monitors is on the top of the pilothouse and either directly forward or abaft the pilothouse. One last word about accommodations and arrangements. I t is obviously i m p o r t a n t to keep the tug trimmed under all loading conditions in such a way as to have the propeller submerged at all times. T h i s means t h a t the designer m u s t give careful consideration to the location of fuel tanks and ballast tanks and m u s t make good preliminary trim calculations to ensure t h a t the propeller does not come out of the water under any loading or.trimconditions. A trim calculation cannot be made, of course, without a good weight estimate. In the preliminary stages of the design, the figures given below m a y prove of use to the designer. D. S. Simpson (27) gives the following average weights for preliminary c_alculations for moderate sized tugs: (a) (b) (c) (d)

Steel hull weight . . . . . . . . . . . . L B D X 0.003 Deckhouse weight . . . . . . . . . . . lbd X 0.001 Gear and equipment weight . . . . . . L )< 0.35 Joiner and carpenter work . . . . L B D X 0.001

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TUG DESIGN

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T h e foregoing figures are in long tons a n d the s y m b o l s h a v e the' following m e a n i n g : B = b e a m inside guards, m a x i m u m , ft L = l e n g t h on t h e waterline, ft D = d e p t h a m i d s h i p s (or a t m a x i m u m b e a m section), ft l l e n g t h of deckhouse, ft b = a v e r a g e b r e a d t h of deckhouse, ft d = average d e p t h of deckhouse, f t /

Mr. S i m p s o n also states t h a t the r e m a i n d e r of the weights d e p e n d s to a great e x t e n t u p o n t h e o w n e r ' s r e q u i r e m e n t s a n d c a n n o t be e s t i m a t e d w i t h o u t some knowledge of w h a t t h e m a n n i n g a n d a c c o m m o d a t i o n s are to be, how m u c h a n d w h a t t y p e of power is to be installed, a n d w h a t t h e r a n g e of the b o a t is expected to be. I n c o n j u n c t i o n with weight, space a n d range req u i r e m e n t s , the d a t a given i n T a b l e 3, e x t r a c t e d from the U n i t e d S t a t e s N a v y r e q u i r e m e n t s for t u g b o a t s , m i g h t prove useful in the p r e l i m i n a r y stages of the design. r~ O

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TABLE 3

S T O R E S FOR T U G B O A T

item

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Dry provisions. . . . . . . . . . . . . . Refrigerated provisions: Freeze . . . . . . . . . . . . . . . . . . . Chill.. . . . . . . . . . . . . . . . . . . . Dairy . . . . . . . . . . . . . . . . . . . Total refrigerated . . . . . . . . . . Clothing and small stores . . . . . Ship's store and ship's service store . . . . . . . . . . . . . . :... Special clothing . . . . . . . . . . . . . Potable water . . . . . . . . . . . . . . .

3.25 1.16 2.37 0.26

3.79 0. 146

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77 107 92 120 98.5 267 169 3.25 b

a Pounds per man. b Cubic feet per-man. e G a l l o n s p e r m a n p e r d a y for t o t a l t i m e for r a n g e .

F i n a l l y , A. Caldwell (8) gives some relative weight d a t a for hull outfitting, etc., for t u g b o a t s . T h e s e d a t a are shown i n T a b l e 4. A n y o n e u s i n g this t a b l e should r e m e m b e r t h a t t h e y refer to British designs which differ from c o n t e m p o r a r y A m e r i c a n designs. Therefore, the c o m m e n t s reg a r d i n g T a b l e 1 a p p l y to this t a b l e as well. Fig. 11 shows the general a r r a n g e m e n t a n d o u t b o a r d profile of a m o d e r n tug. W h i l e this t y p e of a r r a n g e m e n t is c o m m o n , it is b y no m e a n s t y p i c a l a n d m a n y v a r i a t i o n s of the same t h e m e can be obtained. TABLE 4

Item ~

APPROXIMATEHULL AND OUTFITWEIGHTS IN PER CENT Castings Equipment and and Steel Wood forgings outfit

Classes 1-Ocean . . . . . 2-Coastwise.. 3-Harbor . . . . 4-River . . . . .

70 70 75 80

10 10 8 7

. 10 10 10 8

10 10 7 5

580

MODERN

TUG DESIGN

MAIN PROPULSION MACHINERY

In years gone by, m a n y successful tugs have had steam reciprocating machinery as their main power of propulsion. The main advantages of such an installation are, of course, rather obvious. With the slow turning steam engine a propeller with a large diameter can be used, the pitch-todiameter ratio can be close to unity and the fuel burned is generally much cheaper than Diesel oil. One also can control the propeller revolutions from practically zero to full power revolutions and in this way obtain a great degree of maneuverability. However, m a n y disadvantages have forced the steam engine out of the picture and most boats of today are equipped with Diesel engines. Several authors have commented on the relative advantages of steam and Diesel drives, such as E. F. Moran, Jr. (18), C. D. Roach (24), and P. G. Tomalin (30). All of them come out in favor of the Diesel engine and the trend of t o d a y justifies them completely. Some of the disadvantages of the steam engine as compared to the modern Diesel drive are: The space taken up b y the boilers; the large crew required to operate the steam power plant; the cost of stand-by operation; the time necessary to bring up the steam to the prescribed temperature and pressure; and last, b u t not least, the high specific fuel consumption of the steam power plant. Diesel engines, with some method of connection between the engine and the propeller, are almost universally employed as main propulsion units in American tugboats. Four different ways of connecting the engine to the propeller .~e listed here as representative of modern trends : (a) Directly connected, reversible Diesel. (b) Nonreversible Diesel with reverse reduction gear drive or torque converter. (c) Diesel-electric drive (nonreversible Diesel). (d) Nonreversible Diesel with conventional reduction gear and controllable-pitch propeller. The direct-connected, reversible Diesel is clearly an a t t e m p t to incorporate all the advantages of the steam reciprocating engine into a power plant t h a t would not have the inherent disadvantages of steam. These are slow, h e a v y - d u t y engines and starting is normally done b y air. In a harbor tug power plant, where maneuvering is a prime requirement, a large capacity of air must be present at all times, resulting in large and cumbersome air tanks and compressors. An accidental loss of air will mean loss of maneuverability and might prove disastrous. Accordingly, the United States Coast Guard regulations state that sufficient air m u s t be available for twelve starts, with the compressor capable of recharging the air tanks in 60

rain. This is hardly sufficient today, when most controls of a tugboat are on the bridge and the master, under adverse circumstances, m a y use up all, or very nearly all of the air before realizing that the engines have reached sufficient revolutions for starting. To overcome this difficulty, m a n y tug owners specify larger air tanks than this with a capacity of as m a n y as 40 starts. Another serious disadvantage of the direct-connected Diesel engine is its weight, since this slowturning engine weighs much more than the conventional moderate to high-speed Diesel. Finally, some of the directly connected Diesels have a high enough starting r p m so as to start the boat with a jerk, an action which often results in broken lines and hawsers. To avoid all the disadvantages of the slow-running Diesel, clutch-operated, nonreversible Diesel drives have been developed. These systems inelude, in addition to the regular Diesel engine, some kind of mechanical clutch and reversing mechanism plus a conventional reduction gear, or a clutch without a reversing mechanism and a reduction gear incorporating a reversing feature. Several types of clutches with or without reversing features have been developed, the most i m p o r t a n t of which are the Falk Airflex type, the Maybaeh, the American Blower, the Westinghouse, and the Elliot clutches. T h e last two are electric, the American Blower is hydraulic, the M a y b a c h is mechanical, and the Falk is operated b y air. Some companies also have developed reverse-reduction gears which, as the name implies, incorporate a reversing feature. De-Laval Steam Turbine C o m p a n y has developed the so-called H i n d m a r c h - D e L a v a l reverse reduction gear and Western Gear Works has developed a similar unit. Several other companies have prototype reverse reduction gears in the making, b u t only the DeLaval and Western Gear units have been actually installed on boats. One of the most serious disadvantages of this type of propulsion with any of the systems mentioned is t h a t the propeller is not able to absorb all the power developed b y the engine at all times and t h a t a corresponding reduction in engine revolutions would be necessary as the ship's speed is changed from the design speed to any other lower speed. Again, if the design speed is some towing speed, the revolutions will have to be kept constant from there to free route speed to avoid overspeeding the engine, with a resultant serious loss in total thrust available. To overcome this serious disadvantage, National Supply C o m p a n y has developed a torque converter, similar to its industrial type A 342-100 converter, but with the addition of a second stator, pump, and turbine unit to incorporate a re-

MODERN

TUG DESIGN

381

versing feature. While this unit is still in the ex- ciency of the generator and motor would be apperimental and testing stages, it promises to in- proximately 92.5 per cent each so t h a t the total corporate all the advantages of the Diesel-electric transmission efficiency would be a b o u t 83.7 per drive without the h e a v y transmissioli-losses asso- cent allowing 2.2 per cent for Voltage drops, inciated with it. cidental electrical losses, and so forth. Thus the T h e Diesel-electric drive uses the Diesel engine shaft horsepower developed would be 1256, a loss as a generator to produce electric power and a pro- of 244 horsepower from B H P to SHP. On the pulsion m o t o r to convert the electric power thus other hand, if a torque converter were used in generated back into mechanical power. I t is cus- conjunction with a conventional reduction gear, t o m a r y to use a conventional reduction gear in the slippage losses in the torque converter would conjunction with this t y p e of drive, so as to avoid amount to 3 or 4 per cent and the total transmisan unusually large and bulky electric motor. T h e sion efficiency would be approximately 95 per cent, propulsion m o t o r can be either of the single or giving an S H P of 1425 for the same 1500-bhp double-armature type and b y weakening or engine, a net gain of 169 hp, or 11.3 per cent of the strengthening the field of the motor, full power ab- total power developed. sorption at all speeds of the vessel can be realized. A system employing a controllable-pitch proThis advantage, plus the fact t h a t maneuver- peller consists of a nonreversible high or mediumability and control of the system is excellent under speed Diesel engine, a conventional reduction gear all conditions, have made the Diesel-electric drive and, of course, a controllable-pitch propeller. one of the most popular ones in modern tugboats~ i Several such propellers are in the m a r k e t today, Reversing is obtained b y reversing the field of the such as the K A - M E - W A , the Liaaen-Wegner and motor, and reversing times of from 2 to 3 sec have the Baldwin-Lima-Hamilton, to mention just a been quoted. few of a still expanding field. T h e advantages of Several other advantages of the Diesel-electric the controllable-pitch propeller are fairly obvious drive over the conventional direct-connected Die- and would include the significant reduction in sel are listed, see also references (10, 18, 24 and weight over the Diesel-electric or reverse-reduc3 0 ) ; the abilil~y of the power p l a n t t o obtain tion-gear drives and the ability to m a t c h the about 80 per cent of the Free or towing speeds with wheel, to some extent at least, to the main engines only one of two prime movers; the free selection of at all speeds. However, this, like every other syspropeller and engine speeds, thus avoiding as tem, has some disadvantages t h a t m u s t be conmuch as possible "compromise" designs; the sidered and evaluated before a decision as to the constant rotation of the engine in one direction; method of propulsion for a particular tugboat can the ability ,of the power plant to furnish large be reached. Some of the disadvantages are strictly quantities of electricity to other boats and shore hydrodynamic in nature and will be discussed installations; the use of electric auxiliaries to later on, b u t one serious disadvantage should be start the engine, to operate the steering engine mentioned here; namely, complication of control and 'the possibility of having an automatic towing and increased maintenance expenses. I t is true machine. T h e advantages of the latter are dis~ t h a t controllable-pitch propellers have not been cussed at some length in reference (18) b y E. F. on the m a r k e t long enough to say one way or Moran, Jr. T h e author states t h a t in long-dis- another if their complicated control system will retance towing the automatic electric towing ma- quire more extensive and expensive maintenance chine has been found to p l a y a most important work than, say, the Diesel-electric system, b u t all part through its inherent ability to select and indications so far-point towards t h a t conclusion. maintain line tensions and keep the towline length Be this as it may, the advantages of the controlat the desired scope without constant supervision. lable-pitch propeller are such t h a t it merits very These primary advantages, the writer continues, serious consideration in every design. One also have come to attention m a n y times, particularly should keep in mind t h a t several controllableunder adverse weather conditions. Through the pitch propellers are available t o d a y and t h a t ability of the machine to p a y out in time of exces- one particular wheel m a y be far superior from sive pull and to reeve in when too little pull pro- both the mechanical and h y d r o d y n a m i c points of duces an excessive bite, the wear and tear of the view to another propeller, similar in all appeartowing cable has been reduced substantially and ances. In short, it is the beliefof the author t h a t the life of the cable has been prolonged. the whole field of controllable-pitch propellers T h e main disadvantages of the Diesel-electric should be investigated carefully before such a drive is the transmission loss of the system. For method of propulsion is discarded in favor of the example, in a conventional installation with the more conventional fixed-pitch wheel. main engine developing, say, 1500 hp, the effiLately, several novel systems of propulsion

382

MODERN

TUG DESIGN

have been discussed in the trade magazines. M o s t of these systems are not directly applicable to the field of tugboats, b u t one merits perhaps somewhat more attention; namely, the application of a free-piston gas generator to a tugboat. The free-piston gas generator has been discussed at some length in several recent technical papers and

with one half of its total power is better than a tug with no power at all. Add to this the savings in fuel costs t h a t a two engine installation will m a k e possible when the b o a t has to operate under reduced power and the pendulum seems to swing towards the two-engine installation. The author believes t h a t a twin-engine installation should be

~~

Tow resistance

/

i

~

Tug

HP a v a i - l a b l e

I v k (towing) F~G. 12

RELATION BI~TWEEN TOW RESISTANC~ AND TUG H P AVA'ILABLE FOR TOWING

is beyond the scope of this study, b u t the possibility of the installation of such a unit in a tugboat might be worth discussing, especially since such a system was actually sketched out b y N. L. H a w k s (11). The particular advantages of this system would be the low weight-to-horsepower ratio in comparison with any Diesel or steam system (Mr. H a w k s states t h a t this ratio for the free-piston gas generator is only 60 per cent of the corresponding figure for a directly connected Diesel engine) and the possibility of having a smaller engine room. Reference (11) shows a sizable reduction in machinery-space requirements for this system as comPared to the direct-connected Diesel power plant. Aside from the inherent disadvantages this system would have, its main, and perhaps prohibitive, disadvantage is t h a t it exists only on paper and t h a t it has never been tried, even in a remotely similar application. Sooner or later, every tug designer faces the problem of a one-engine installation versus a twoengine installation. The answer to this problem m a y not be as obvious as it appears, when one takes into consideration endurance and speed and fuel consumption at reduced power. A one-engine installation has t h e . a d v a n t a g e of simplicity, be it of the reversible or nonreversible type, and in addition a saving in weight and space m a y be realized. On the other hand, a twin-engine installation provides reliability in the form of a stand-by unit in case of trouble, since even a tug

given first consideration for coastwise and ocean tugs at least, unless specific reasons and design considerations force a single engine power plant adoption. PROPELLERS

AND

PROPELLER D E S I G N

The main reason for the existence of a t u g b o a t is the pulling and pushing of large vessels. I t follows, then, t h a t one of the most i m p o r t a n t parts, if indeed not the most i m p o r t a n t part, of a tugboat, is its propeller. C. D. Roach (24) says: " T h e cost of present-day harbor tugs is in the order of $15 to $17 per pound of bollard pull. The difference between a well-designed wheel and a wheel-designed for. other than o p t i m u m towing conditions, m a y well result in differences of 20 to 30 per cent in bollard pull . . . . " F r o m these figures alone one can see clearly how i m p o r t a n t the propeller design can be in a tugboat" and it is doubtful t h a t there would be a n y b o d y with even an outsider's knowledge of tugboats t h a t will argue the point t h a t a proper wheel is the most import a n t factor in the success of a tugboat. Although this fact seems to be common knowledge, very few designers p a y particular attention to the propeller design, and most propellers are designed for maxim u m efficiency at free running speed, with practically complete disregard of the performance of such a wheel at towing speeds. T h e reason for this attitude is not very clear, unless, perhaps, it

MODERN

...........

TUG DESIGN

is because it is much easier t o base the design of the propeller on the free route speed and forget a b o u t its performance at any other condition. Some designers figure out a propeller based on the free running speed of the vessel and then cut the. speed-to-diameter ratio down from 5 to 15 per bent to obtain a wheel suitable for towing. This is ~/-ratfidr a r b i t r a r y vcay of designing a propeller, to say the least, and as such does not merit .any serious consideration. I t is the firm belief of the author that, in general, tugboat propellers m u s t be designed for reasonable towing speeds of from 4 to 8 knots. This statement, perhaps, requires some clarification. To begin with, it is not m e a n t to apply in any condition without any further investigation. M a n y times, for example, a tug m a y be designed with a particular tow or run in mind and the owner m a y even specify the particular speed of the tow t h a t he wants. In t h a t particular case, the propeller-design problem is clear and the tugboat horsepower available for towing m u s t be matched to the resistance of the tow. Fig. 12 gives a typical graphical representation of the problem and its solution. Again, in some other cases, and especially in the case of a tug t h a t is designed specifically for the handling of large vessels during docking and undocking maneuvers, it m a y be wise to design the propeller for m a x i m u m pull at practically zero speed of advance of the boat. However, if a tug is to be occupied fully in a large harbor, quick dispatch m a y be just as important as delivering m a x i m u m possible thrust at bollard pull. I t can be shown t h a t this condition would become critical if the tugboat is equipped with a reversereduction-gear-drive or a directly connected Diesel engine. In a Diesel-electric drive on the other hand, a propeller designed for some towing speed other than zero will lose only a very small percentage in bollard-pull thrust in comparison with a wheel specifically designed for m a x i m u m possible thrust at the bollard. Particular calculations for such a eomparlson have been carried out for the case of a 1500-shp tugboat. One propeller was designed for towing at 7 knots and showed a bollard pull of 46,300 lb and another was designed for m a x i m u m efficiency at bollard pull. Both wheels were of the same diameter and the latter developed a bollard pull of 47,500 lb, or an increase of about 2.5 per cent. Consequently, it appears t h a t the loss in bollard pull in the Dieselelectric drive for a propeller designed for some towing speed other than zero can be disregarded " in most cases. The problems mentioned are only two of the cases where the propeller of the tugboat might be

383

designed with a particular goal in mind. However, most of the time a tug is designed for general work in and around the harbor and rigid requirements cannot be quoted, except t h a t the b o a t should have a certain free running speed and t h a t it should show a specific minimum bollard pull. For preliminary design purposes, a well-designed wheel should-develop a b o u t 30 lb of thrust a t bollard pull per shaft horsepower installed, although this figure m a y v a r y slightly from design to design. L . C . Norgaaxd (21-) estimates the expected boUard pull at 33.6 lb (0.015 tons) per delivered horsepower, a difficult figure to reach at times. E . F . Moran, Jr. (18) states t h a t the expected bollard pull should be from 28.5 to 30 lb per SHP, irrespective of type and size of machinery installation. A . R . Taylor (28) gives the towing effort in tons as equal to the estimated I H P times 0.01125 irrespective of towing s p e e d - - a rather novel idea. D . S . Simpson (27) points out t h a t when the speed of the tow is specifically stated, the resistance and hence the required power is difficult to predict since it involves investigation of the resistance of the tow as well as of the tugboat, tide and current conditions, and, if total time is an object, the examination of the terminal handling. He further states t h a t when the towrope pull and speed are known, the required delivered horsepower can be approximated b y the following formula

where

D H P = VP/IO0 V = speed, knots P =: towrope pull, Ib

i

To this result, the power required for the tug must be added. Finally, the brake horsepower can be estimated by B H P = k X (total from above) k = 1.10 for direct drive or geared Diesel k = 1.25 for Diesel-electric drive Mr. Simpson also states t h a t the bollard pull will be equal or close to 22.4 lb (0.01 tons) per S H P irrespective of size or type of machinery. Some propeller-sizing charts for three and fourbladed propellers for both the towing and free running conditions are given in reference (39). Unfortunately, the charts do not extend above a Bp value of 60, thus rendering them of slight practical use for modern tugboat propeller design. For preliminary design, purposes, the same reference gives the bollard pull in pounds as equal to Bollard pull in pounds = where

5250 X B H P X T~ RPM X D

384

MODERN

~Uu~1, ibs = Bollard • R Bollard PM 60 ~ BHP RPM D Tn Tr

12

TUG DESIGN

0.i0

5250 x BHP x To RPMx D 2700 x BHP ' RPM x, DSx T r

!

= Brake horsepower . Revs/min at Design Conditions = Propeller Diameter in Feet = Thrust/Torque Constant (from c u r v e , = Torque Constant (from curvi' ~

0.09

11

0.08

o

'

v

l& Blede~

'que Con 5tent E~

0.07 t / ' r o r q u,

stent ~

w

3 Bled ~s

= o o

0 o

o

o

=9

0.06

~3 Bled(

% o

, L BIa~

o

Io

~8

0.05

7

b

5

/

/

J

O.O&

0.03

if i

0.6

0.7

0.8 F x o . 13

l

0.9 1.0 i.i Pitch/Diameter RB tic

TUGBOAT

PROPELLER PERFORMANCE

AND

1.2 BOLLARD PULL

1.3

O. 02 l.&

MODERN TUG DESIGN

385

2h 22

in Ibs is:

20

• ~ effic Vk x0.3065 available horsepower : speed in

16 @

o 12 e~ @

io

h 2 0 0

IO

20 30 ~o 50 60 Percent of Maximum Wheel Efficiency

FIG. 14

APPROXIMATE TOWING THRUST IN POUNDS

~6o

~ U p p e r Limit IQ

~ 3o

Lo~ e r ~ t m i l ) ~

~

~

~

~ 2o o

o

8 I0 12 lh 16 18 SHP/Disk Area of Wheel

FIG. 15

20

22

2h

APPROXIMATE BOLLARD THRUST IN POUNDS

B H P = brake horsepower for which propeller was designed R P M = propeller revolutions per minute at design conditions D = propeller diameter, ft Tc = t h r u s t / t o r q u e constant, from Fig. 13 T h e revolutions per minute at bollard pull can be found from the following equation ( 2700 × B H P R P M at bollard = 60 \ R P M - X ~D~ X

~'J'

Tr]

where T, -- torque constant, from Fig. 13, and remaining symbols have the same meaning as before. NOTE : T h e foregoing figures and values are for

a disk-area ratio of 0.50 and for airfoil-type blade sections at the roots only, sweeping out to the conventional circular back sections from 0.50 radius to the tips. Finally, A. Caldwell (8) gives some empirical formulas and curves for both the towing thrust and the bollard pull of tugboats. These plots have been reproduced in Figs. 14 and 15, modified somewhat to agree with modern American practice. In the case of the bollard-puU curve, an envelope rather than a single curve is given to allow for differences in propeller types and design speeds. I n the general case of a tugboat with no particular route or tow in mind, the propeller should be designed for a reasonable towing speed of from 4 to 8 knots. T h e exact speed is a m a t t e r of judg-

386

MODERN

TUG DESIGN

m e n t and up to the individual designer in conjunction with the owner, who must take into consideration the particular service of the boat. There are three methods of propeller design commonly in use t o d a y a n d all three are discussed at some length in reference (6). In brief, these methods are: (a) The statistical method developed from the 'early m o m e n t u m theory to the present vortex theory. (b) The self-propelled model test plus the open' water test of the propeller. (c) The propeller-series chart method. In tugboat propeller design, the third method is the one most commonly used and will be discussed at some length. There are, in general, two types of chart coefficients in use t o d a y (see also reference 6). The first one uses the following symbols J Kt K,, Kq eo

= Va/nD = T/P,2D 4 = T / P V~,2D 2 (eliminating revolutions) = Q/P,~2D5 = KtJ/2Kq

These are nondimensional coefficients and especially useful when the slip is high and J approaches zero. The second type uses D H P , Vk and E H P instead of Q and T. The usual coefficients are B Uo =

Uol/'N/ Va 2.~

BP S KU KP

P ' / ' N / V a 2.5 ND/I~5 D V a l . 5 / U o V' D V a l . 5 / P '/'

= = = =

The last two coefficients, which have been developed b y Professor Baier, do not contain N and normally can be used to determine directly the propeller characteristics with m a x i m u m efficiency. Either one of the two systems mentioned can be used to determine and select the most efficient wheel at a particular design speed other than bollard pull. However, when one tries to determine the propeller conditions at 100 per cent slip (bollard pull) both systems fail, since they include either the speed of advance of the boat (which at bollard is clearly zero), or else an unknown quant i t y in the face of the revolutions, which normally will change as the speed of the vessel changes. F. W. Benson (7) was the first one, to the author's knowledge, to develop a system of coefficients eliminating both the revolutions and the speed of advance, thus enabling the designer to proceed in the design of propellers of very high slip and at low speeds of advance. Unfortunately, his work is based completely on the metric system and is not directly applicable to common

American practice. C o m m a n d e r Richards T. Miller (16) has modified the Benson system and made it applicable to our pound-foot-second system. The method shown in Appendix 1 is based on both the original Benson system and Commander Miller's modifications. Actually nothing new has been added, except perhaps t h a t some of the coefficients proposed b y Benson and Miller have been manipulated to arrive at the desired results. Aside from the mathematics of the design, a few other considerations m u s t be taken into account. T o begin with, the type of propeller most suitable for the boat in question m u s t be determined. This is a rather i m p o r t a n t decision, since different types of propellers are available, each having its own characteristics and the designer m a y find it difficult to choose between them. The two most common types are the Troost and the T a y l o r wheels. Under riormal operating conditions, the Troost propeller (airfoil-type sections) is slightly more efficient than the Taylor wheel, b u t if the boat in question has to do some serious work while backing, a Taylor propeller with symmetrical (ogival type) sections m a y be a wiser choice. A. J. C. Robertson (25) estimates t h a t a symmetrical propeller m a y be as much as 50 per cent more efficient in the astern condition than a propeller with rounded back blades. While an airfoil wheel is not exactly a propeller with rounded back blades and although this estimate is unusually high for airfoil-type wheels, empirical results seem to indicate t h a t a Taylor propeller m a y be as much as 10 per cent more efficient while backing than a similar Troost wheel, while in normal ahead conditions the Taylor propeller m a y b~. expected to be only 2 per cent less efficient than the equivalent Troost wheel. I t should be mentioned here t h a t exact data on propellers operating in the astern condition are not exactly a b u n d a n t and very few tests comparing the efficiencies of different types of wheels in both the ahead and astern conditions have been carried out. The choice of the type of wheel is then again left up to the designer, but it is believed that, as most tugboats will require good backing qualities, a T a y l o r wheel m a y be the final choice, although each ease should be treated individually and without prejudice. Finally, another question t h a t has to be answered is how the turning m o m e n t is applied, t h a t is to say one has to know if the turning moment is constant or if constant power is being developed. T h e answer to this question is fairly simple if the type of machinery has been decided upon definitely. However, a word of caution m a y be necessary here. While a Diesel-electric installation m a y provide constant power for all

MODERN h

TUG DESIGN

/

/

/

Q

387 p

/ ./

/ "

C

@

1

0 O

.

@ r..~N

3

"/ / ID

10o Fio. 16

105 Shafthorsepower

- 110 and RI~

115

VARIATION IN SHAFTHORSEPOWER AND REVOLUTIONS PER MINUTE WITH PROPELLER-TIP CLEARANCE FROM NOZZLE

operating ranges and give great flexibility to the design, the transmission losses m a y be such as to render some other type of power plant a more suitable one, especially if the towing speed at which the propeller is being designed is rather high, say above 60 per cent of the free route speed. T h e discussion on propellers has been limited so far to conventional, or fixed-pitch propellers. Various ~ types of controllable-pitch propellers have appeared in the m a r k e t over the past decade a n d some manufacturers promise reliability and "effieiencies equal to those of the fixed-pitch wheels. In connection with the relative efficiencies of controllable and fixed-pitch propellers, reference (1) indicates t h a t well-designed controllable-pitch propellers come v e r y close to the efficieneies of Troost propellers. I t is true t h a t this reference has compared only one of the m a n y controllablepitch propellers to the Troost wheels; n'amely, the Lips-Schelde propeller, b u t the results of these tests can be taken as a general trend in controllable-pitch-propeller design, which has come a long way since the first wheels of this type appeared on the m a r k e t a few years ago. While the relative efficiencies of the controllable versus the fixed-p!tch propellers m a y be argued one way or another, t h e simple fact t h a t more complicated controls are added would tend to indicate t h a t the probability of a breakdown is higher in the case of the controllable-pitch wheel. With the exception of one or two models, all the control mechanisms of the controllable-pitch propeUer are within the hub, so t h a t access to them while the tugboat is on the high seas is impossible. However, servomechanisms and controls have i m -

proved so much in the past few years, t h a t an annual inspection of the hub, propeller and controls should suffice to ensure proper function of the system. Aside, then, from a slight loss in efficiency due to the large hub involved and perhaps a rather high original investment, the most serious disadvantage of the controllable-pitch propeller as compared to the fixed-pitch wheel is, strange as it m a y sound, its poor ability to deliver astern thrust. This will v a r y with the t y p e of wheel in question. T h e author has had access to very limited test results; the figures seem to indicate that a controllable-pitch propeller can deliver only about 80 to 85 per cent of the astern thrust as compared to a well-designed T a y l o r wheel. Reference (2) shows t h a t the results of a series of tests conducted with comparable Troost and Lips-Schelde propellers indicate t h a t the Troost wheels were able to deliver about 20 per cent more thrust in the astern condition than the controllable-pitch wheels, provided the Troost wheel was designed for the bollard and not the free running condition. F r o m other tests (references 20 and 25) it appears as if a Taylor wheel' should be able to deliver even more thrust in the astern condition than a comparable Troost propeller and even if the Taylor wheel is designed for some other rhasonable towing speed and not strictly for bollard pull, it is expected t h a t it will deliver about 20 per cent more thrust in the astern bollard condition than a comparable controllable-pitch propeller. T h e inability, so t o speak, to deliver astern thrust, might, of course, be a very serious objection to the installation of a controllable-pitch propeller in a tugboat, especially if the vessel is a hat-

388

MODERN

TUG DESIGN

22

/

//

2O ¢D

4.$

/

12 o

o

l0

/ J 0

Relation Betweer Propeller Diameter and Tip Clearance in ~c Nozzles

J 0.I

Recomnended

0.2 Clearance

0.3 C.h O. in inches (Blade Tip to Nozzle}

FIG. 17

bor tug and is expected to dock and undock large ships. As far as design goes, the controllablepitch propeller is usually designed b y the manufacturer and not the naval architect. This being the case, very little can be said about design considerat!ons, except t h a t from past performance data and purely theoretical considerations it is known that the uniform pitch should be set for some low p i t c h - t o - d i a m e t e r r a t i o and an allowance should be made for the blades to turn to their higher-than-normal pitch ratio at high speeds or free running conditions. This undoubtedly will raise the efficiency of the wheel at towing speeds, while it will not seriously hamper the efficiency at free running speed. An interesting exception to the normal procedure of having the manufacturer supply the design of the ccmtrollable-pitch propeller is a paper recently presented b y Prof. Laurence Troost (32) in which a simplified method of designing a controllable-pitch propeller for a tugboat is presented. In this paper,' Professor Troost discusses at some length his proposed method and shows various ways in which revolutions, thrust and efficiency can be obtained for two different types of propellers, type A and type B. T y p e A propellers are designed for m a x i m u m possible diameter and efficiency at an o p t i m u m number of revolutions b y the proper selection of gear ratio between propeller and engine. T y p e B wheels are designed for the m a x i m u m possible efficiency at o p t i m u m diameter and given revolutions and are mainly used for directly connected Diesels. M o s t tug-

boat propellers, Professor Troost says, will be of the A type and he carries out some calculations for a B4-55 controllable-pitch wheel for m a x i m u m thrust and efficiency at the free route and bollard pull conditions. Unfortunately, the formulas developed are geared to the Troost-type propellers only, since they are based on ~-g charts t h a t are to be found in reference (33), but not in reference (29) or (37). I t is true t h a t ~ and ~ are related to Kt and Kq, 4 but if only Kt and Kq charts are available, the calculation for the determination for ~ and ~ and the subsequent application of the Troost method becomes somewhat laborious, if not difficult to follow. Since the tugboat propeller is usually rather" heavily loaded, the application of a K o r t nozzle might merit serious consideration. The improvem e n t of propulsive efficiency obtained with the nozzle is mainly due to the contraction of the race behind the propeller and is most pronounced at high slips. If the draft of the tugboat is restricted for some reason and an o p t i m u m diameter wheel Cannot be fitted, several authors claim (12, 23, 26) t h a t a K o r t nozzle m a y improve the efficiency of a conventional, small diameter wheel b y as much as 30 or 40 per cent. If the tugboat is expected to work with long tows up and down along the coastline, and if the foregoing figures can be trusted as representing true conditions outside the test basin, a K o r t nozzle can save a lot of m o n e y for its owner. 4 T h e r e l a t i o n b e t w e e n ~, ¢, Kc a n d Kq is

#

=

1/(Kq)l/l Ki

"

=

;)~ Kq

MODERN

TUG DESIGN

But if the tug is expected to do aU kinds of work in and around the harbor, then a K o r t nozzle m a y not be worth considering, since the already bad astern characteristics of the propeller will be made even worse with the addition of the' nozzle.: Of course, one m a y argue t h a t the shape of the nozzle m a y be modified to allow for reasonable astern thrust, in which case, though, the ahead improvem e n t in efficiency m u s t be sacrificed. Theoretically at least, there are m a n y arguments in favor of a Kort-nozzle application. Dr. J. D. van M a n e n (14) e,cen mentions t h a t in certain cases the K o r t nozzle m a y prove advantageous at relatively low propeller loadings. In particular, he states t h a t systematic experiments conducted at the Wageningen naval t a n k have proven t h a t if the length of the nozzle is kept to no more than 30 per cent of the propeller diameter and the blade-tip clearance has a certain value as recommended b y the writer, an increase in efficiency can be realized b y the addition of the nozzle even at very low propeller loadings of the order of Bp = 13. T h e writer gives what he considers to be the best tip clearances and the figures given in reference (14) are reproduced here as Figs. 16 and 17. Fig. 16 gives the variation in shaft horsepower and revolutions per minute with propellertip clearance from the nozzle and Fig. 17 gives the tip clearances recommended b y Dr. van Manen. In both cases the values given have been converted to the equivalent English system of measurements to make them directly applicable to propeller and nozzle design in the United States. F r o m these two figures it can be seen clearly t h a t the advantage gained b y the addition of the nozzle m a y be nullified b y increasing the tip clearance. I t also can be seen t h a t very small clearances are recommended, so small as to make them impractical in actual applications. From Fig. 17 one can see t h a t a tip clearance of approximately 0.3 in. is recommended for a 13-ft-diam wheel, and from Fig. 16 it is seen t h a t about 10 per cent more horsepower is-needed in the bollard condition to deliver the same thrust if the tip clearance is increased to a more attainable, but still rather small, value of 1.00 in. Nevertheless, it appears that, from a hydrodynamic point of view, the use of the K o r t nozzle need not be restricted to heavily loaded screws, provided the structural and operational difficulties of a nozzle with very small tip clearances and practically no connecting supports between the nozzle and the hull can be overcome. The-main disadvantage of a Kort-nozzle application lies in just t h a t fact: In theory, the nozzle is more efficient than the open propeller; in practice, the small tip clearances recommended and the need for an absolute minimum of structural

589

members between the hull and the nozzle are practically impossible t o achieve, not to mention the difficulties in the construction of the normal nozzle in itself. I t also might be mentioned here t h a t several towboat skippers operating on the inland waterways, and notably on the Mississippi river run, have been complaining t h a t a Kortnozzle application has been resulting in bad steering qualities of their boats at towing speeds. I t is entirely conceivable, though, t h a t in the near future most of the problems associated with the K o r t nozzle of t o d a y will be solved b y the most advanced engineering of tomorrow and thus, what appears to be impractical in most applications at present, m a y become entirely sound practice in the future. The K o r t system of propulsion has both its followers and its detractors. Both consist of large groups of naval architects or h y d r o d y n a m i cists and each group claims to be correct. T h e author has listened to both sides of the argument and it appears t h a t both groups are right, at least partially. T h e pro group has a sound theoretical basis and can easily prove b y tests or m a t h e m a t i cal arguments t h a t the K o r t nozzle is definitely, at least in most cases, an efficient shrouding for almost any propeller. The con group can argue, with the same effectiveness, t h a t although the theory is right, the practice proves the nozzle impractical in all b u t a very few, extreme cases. When all is said and done, however, it might still be to the advantage of the tugboat designer, even for plain educational purposes, to look into the Kort-nozzle system of propulsion and the possibility of its application to any one design. MANEUVERABILITY The rudder design and consequent influence on maneuverability of a tugboat is perhaps nearly as i m p o r t a n t as the selection of the proper engines and the propeller design. The ability to turn quickly, though, is .not entirely a function of the size or design of the rudder. Even the best possible rudder will not be sufficient, unless the shape of the hull is proper and such factors as keel drag have been taken into consideration. Keel drag is not very seriously looked upon in this country as aiding in the steering ability of the boat. T h e author agrees here with C. D. Roach (24) who, commenting upon tugboat keel drag in this country, says t h a t more drag to the keel m a y very well improve tugboats as far as response to the helm is concerned. T h e reason for this is quite clear when one remembers t h a t adding drag to the keel also moves the center of lateral resistance aft, thus improving the response of the boat to the helm. Fig. 18 is what the author believes

390

MODERN

TUG DESIGN

\\ Re, : o m m e ,ded

,ine

3

Rot c h ' s iMean l.if e

,

o M

i 0

20

FIG. 18

40

60 80 100 LBP, Feet

140

160

]80

TUGBOAT KEEL DRAG IN PER CENT OF L B P

to be a good drag to L B P ratio, plotted against length between perpendiculars. Thirty-four different points have been plotted, representing thirty-four modern tugboats, and one can see easily from the figure t h a t hardly a n y of the points falls above the recommended curve. C. D. Roach's mean line, as appearing in reference (24), has been added to this figure to show the difference between the actual and the recommended amount of keel drag found in most modern designs. As Roach observecl, the tugs above the mean line, and hence closest to the recommended line, have the reputation of being very maneuverable and as having exceptionally quick response to the helm. A tugboat, unlike other vessels, is called upon to perform freak operations, such as turning a tow m a n y times its length and displacement efficiently and in the minimum possible radius under adverse wind and weather conditions. As a result of these peculiar requirements, m a n y different and peculiar rudder types for tugboats have been developed and in an effort to increase maneuverability as much as possible, the ratio of rudder area to the lateral plane area for tugs is unusually high. This ratio is, of course, quite important. Numerous formulas for its determination have been given at one time or another b y m a n y authors. These normally help the designer in determining the rudder area required, but in most cases the area thus determined is on the low side for tugboats. The following formulas are quoted here as giving good results and as being the most appropriate ones in tugboat rudder design: (a) A. Caldwell (8) states t h a t the minimum rudder area should be equal to "At m i n i m u m = 0.0275A~

120

where Ar = rudder area A ~ = midship section area (b) L . C . Norgaard (21) states t h a t a rudder area of 6.3 per cent of the lateral plane area in a harbor tug of moderate size has given good maneuvering characteristics. (c) D . S . Simpson (27) gives the minimum percentage of the rudder area to the lateral plane area as approximately equal to 5 per cent (a somewhat low figure) and states t h a t this figure would decrease with increasing size. (d) A . R . Taylor (28) gives the recommended rudder area as equal to 1/45 of the area of the immersed waterplane. No hard and fast rule can be applied in this case, since one vessel that is very maneuverable as far as one skipper and d u t y is concerned, is slow in response for another and vice versa. In very general terms it can be stated, though, t h a t 6 to 6.5 per cent ratio of rudder area to lateral plane area will give a maneuverable harbor tug. For ocean-going and salvage tugs, a somewhat smaller ratio can safely be used. Once the rudder area has been determined, the shape and type of the rudder m u s t be decided upon. I t is a very common misconception and belief t h a t an airfoil-type section will give maxim u m lift. The actual fact is t h a t a symmetrical, aerodynamic section will give m a x i m u m lift to drag ratio, or, in other words, have minimum resistance, b u t t h a t does not indicate b y any means t h a t a section of t h a t type has m a x i m u m lift characteristics. Lift, of course, is directly pro-

MODERN TUG DESIGN ~Rudder

391

Rudder axis

axis

- sFishtaill

J L.E. (a) Flat

A - A

Plate

Rudder

I..E.

T.

(b) Symmetrical

B- B Rudder

Rudder axis

FlapC~ T'E°L_L.__ -

c

.E.

C -C

( c ) S y , n , n e t r i c ' , l Rud,ler w i t h A d j a a t a b l e FIG. 19

fflap

TYPES OF TUG RUDDERS

pdrtional to turning force, and maximum turning force is what the tugboat skipper is after. In general, we can distinguish three different types of tug rudders, namely: (a) The conventional fiat plate rudder with or without "fishtails" at the trailing edge. (b) The symmetrical, aerodynamic section rudder, with or without "fishtails" at the trailing edge and (c) A symmetrical, aerodynamic section type rudder with an adjustable flap. Fig. 19 gives a general idea of what the three different types of rudders mentioned might look like. All types are shown as balanced, a fairly common procedure in American design, and, for simplicity reasons, the assumption of balance is retained in the discussion that follows as well as in Appendix 2, where an analysis of the relative merits of the three types is presented. The most common tugboat rudders of today are of the conventional type (type a in Fig. 19) with wedge flaps or fishtails. This type of rudder gives high lift when the vessel is going ahead and consequently is excellent for maneuvering in that particular condition. The drag, or resistance, of this type of rudder is rather high, but can be accepted as the penalty one has to pay for high ahead maneuverability. On the other hand, backing with it is unsatisfactory because of the perpendicular rectangle presented to the water in this condition, and a tug equipped with this type of rudder does not steer as well in the astern con-

dition as one equipped with a streamlined (airfoil-type) rudder. The symmetrical, airfoil-type rudder (type b) without fishtails, gives maximum lift to drag ratio but less lift than type a. In this way it provides less steering ability when going ahead, but is superior to the flat-plate rudder when going astern. If this rudder is fitted with fishtails at the trailing edge, it becomes in effect a type a rudder and the discussion on that type of rudder applies in general terms here as well. The symmetrical rudder with adjustable flap (type c) possesses all the advantages of both types a and b rudders. The flapped rudder will provide for high lift for ahead maneuverability and, since the rudder is of symmetrical airfoil sections, it will have minimum resistance or drag when on t h e centerline. Control of the adjustable flap can be by means of a mechanical linkage that would adjust the flap automatically to the proper angle as the main rudder is operated. Some will argue correctly that the flapped rudder will be more vulnerable than either the flatplate or the airfoil rudder and t h a t for the sake of reliability this type of rudder should not be used. The point of maintenance and repair bills for such a rudder also has been raised, and there is no doubt that the repair bills on a symmetrical rudder with adjustable flap will be much higher than the practically nonexisting repair bills of the more conventional flat-plate or airfoil-type rudders. Both arguments are well taken. Consequently,

392

MODERN TUG DESIGN

the discussion presented in Appendix 2 is by no means intended to convince all tugboat designers and owners that they should use a flapped rudder, but is shown in order to indicate clearly the relative advantages this type of rudder m a y have, if ever perfected, over the more conventional ones. The linkage system of this rudder is by no means foolproof and a slight shock to the flap m a y result in jamming it in a most unfortunate position, thus rendering the tugboat virtually helpless. I t is hoped, though, that some ingenious designer will come up with a safety device which will cause the flai~ to return to the centerline when damaged, so that even a damaged rudder of this type will never be worse than a corresponding symmetrical one. As for the repair bills to the linkage mechanism, o n e may argue that servomechanisms are becoming more reliable every day and that in the forseeable future they may become as reliable as the relatively simple steering engines of today. However, until such a day when both the jamming and the expected high repair bills of this rudder are overcome, the tugboat designer will do well to stick to the reliable flat-plate or symmetrical airfoil rudders, despite the hydrodynamic advantages a symmetrical rudder with an adjustable flap m a y possess over them. Before closing the rudder section, several other more novel types of rudders should be discussed briefly. The most common of these is perhaps the flanking rudder which is used mainly on towboats, but could very well be adapted for use of tugboats as well. These rudders, usually installed in pairs forward of the conventional rudder location and on the side of the hull so as not to interfere with the stream of water delivered to the propeller have been fairly successful, although they did not come up to expectations in all cases. It is believed that such rudders could be helpful in steering a tugboat, especially when the boat is going astern, but lack of experimentation in this field makes it impossible to reach a definite conclusion one way or another. Centerboard-type rudders that lift into the hull when not in use also have been tried on barges and towboats, but the results were disheartening. The rudders worked fine the first few times, but an interference in the form of some floating object, such as a log, would jam them into place, thus raising havoc with the tow. Finally, C. D. Roach suggested some time back that a reversed symmetrical airfoil-type section rudder m a y prove to possess all the advantages of both the flat-plate and the conventional airfoil types. This rudder would have the leading and trailing edges of the airfoil reversed, thus eliminating to a considerable extent the need for fishtails

at the trailing edge and at the same time would retain all the advantages of the airfoil rudder when backing. To the author's knowledge, this type of rudder has not been tried as yet. I t is hoped that some day comparative tests between Roach's rudder and the more conventional ones will be carried out and it will be interesting to see the results of these experiments. The second factor that must be taken into consideration as far as maneuverability is concerned is engine controls. A good rudder and good and reliabl e engine controls work together to make the tugboat maneuverable and successful. Complete pilothouse control of the engines is practically a necessity and an engineer answering bells in the engine room is as out of date as stiff collars and silk hats. The advantages of having the instant control of the movement of the vessel under one person are too numerous to list and most of them are readily apparent. No one familiar with tugboat operations will argue that in the delicate operation of approaching a large vessel at sea in rough weather for passing a towline, the maneuvering can be accomplished much more quickly and safely if the boat is under a one-man control, thus reducing the danger of contact with the other vessel and serious damage. Again, when the . tug has to maneuver in close quarters with or without a tow, a one-man control of the boat increases the confidence of the pilot and enables him to perform the work safer and quicker. E . F . Moran, Jr. (18) states that in passing through the New York State Barge Canal locks, pilothouse control has reduced the time for locking through to about 50 per cent of that formerly required. Pilothouse control also can be used to advantage in most cases to reduce the rope breakage to a minim u m by its ability to take up the strain slowly, and in several cases has reduced the engine-room personnel by eliminating an engineer answering bells. In most modern tugboats, two interconnected engine control levers operating from the same console are provided in the pilothouse, one port, one starboard, on either side of the steering wheel. It is also customary to provide a control station on top of the deckhouse, abaft and to the starboard side of the stack, complete with steering wheel and engine controls. Several tugboat pilots have indicated that this additional control is extremely important when the boat is maneuvering in very close quarters and has been a life saver several times. Anyone who has ever tried to turn a boat around in slightly more than its own length can appreciate the absolute necessity for such an additional control station. Consequently, all harbor and coastwise tugs at least should have this additional control station.

MODERN

TUG DESIGN

393

I00

90

X'"

80 7o 4a

//

60

_e

5o

3o _

_

_

/

/< !

0

I0

20

30

&O

50

Peroont

60 RPM

70

80

9O

i00

FIG. 20 RECOMMENDED OPERATING LIMITS OF DIESEL ENGINES

A one-lever control of the~ engines is highly recommended. Such a control usually can be supplied either as an electrical or compressed-air unit. Either system has certain advantages over the other, but an analysis of each system and relative comparisons with the other are beyond the scope of this paper. The choice, then, of one particular system over another is left up to the individuM designer, the owner and the prospective master of the tugboat. Finally, the location of the towing b i t t will affect the maneuverability of the tugboat while towing. I t has been mentioned before t h a t no exact d a t a for the best location of the towing point are available in the form of test results, but the opinions of several writers on the subject have been given. I t is repeated here that, in the opinion of the author, the best location of the towing point, taking into consideration the necessity for decent arrangements and accommodations in the deckhouse, is about 60 per cent of the length of the boat aft of the bow. ENDURANCE

Endurance is generally disregarded in tugboat design, since most tugs, under normal operating conditions, are close enough to their base where plenty of fuel is available. This is true up to a point only. Another factor t h a t would come under endurance is the prolonged operation under some reduced speed a n d / o r power of the power plant and it is well known t h a t t h e continuous operation of a Diesel engine under other t h a n recommended brake mean effective pressure ( B M E P )

values m a y very well result in serious carbon deposits 'and unnecessary wear and tear of the engine. In a multiengine installation this can be avoided to some extent b y operating under a reduced number of engines, b u t in a single-engine install/ttion not much can be done except to operate the engine at some acceptable B M E P value. I t would then be helpful to the skipper and operator to know w h a t his b o a t can do under reduced power and how far it will go with the available fuel. N o t much can be said as far as endurance is concerned for the steam reciprocating engine, except t h a t the fuel consumption is approximately directly proportional to the power developed. If one then knows the power required to propel the ship at a certain speed, the calculation of the endurance a t t h a t particular speed becomes elementary. F o r this reason, and since most tugboats of t o d a y are equipped with Diesel engines, the diseussion t h a t follows will be mainly concerned with the Diesel engine. Very little has been published concerning the selection of engines and the methods of calculating endurance for Diesel power plants. Mr. J. J. T u r n e r (33) has treated this subject in a recent paper, and although his work is mainly concerned with naval vessels and their particular problems, most of the findings can be applied successfully to a n y Diesel machinery installation. As Mr. Turner points out, certain calculations and assumptions have to be made before one can select the proper power plant for the design in question and before the fuel requirements for a specific eruis!ng radius can be estimated. Some

394

MODERN

TUG DESIGN

of the items necessary for calculating endurance or selecting the proper engines are listed in order to give a general idea of. what is required before one can commence with the actual calculations: (a) Full power shaft horsepower. (b) Full power speed. (c) Full power propeller revolutions. (d) Cruising speed and endurance required. (e) Cruising propeller revolutions. (f) Total brake horsepower required. (g) Speed-power curve for the boat. The actual methods for calculating the foregoing items should be fairly straightforward if the necessary data for the calculation of each item can be collected. At' times, however, data are not readily available and in such a case Mr. T u r n e r ' s paper (33) gives some assumptions t h a t are valid and can be used in most cases. I t might be mentioned here t h a t several engine manufacturers have been very reluctant in the past to supply information to the designer as to the actual performance of their engines. In endurance and fuel-consumption calculations the following data are absolutely necessary before the designer can reach a conclusion as to what the engines actually can deliver and at what speed they . can be operated safely: (a) B H P and engine R P M versus BYIEP. (b) B H P and engine R P M versus fuel consumption. These curves can be supplied in one plot and should be made available b y all engine manufacturers to the designers upon request. The reluctance of some manufacturers to supply the naval architect with all necessary information for the evaluation of a specific power plant is hard to understand and every effort should be made b y all parties concerned to correct this unhealthy situation. To do justice to all, though, it m u s t be mentioned here t h a t several engine manufacturers have been co-operating with the designers in every respect and t h a t the author has found the United States N a v y ' s machinery section of the Bureau of Ships very co-operative in supplying engine d a t a t h a t otherwise were impossible to get. The Diesel engine, like any other internal-combustion engine, is essentially a constant-torque engine and as such has certain limitations as far as torque and B M E P are concerned. The rated values for both of the above items cannot be exceeded without harmful effects to the engine, unless, of course, overload provisions have been made. Mr. Turner in his paper on Diesel engines (33) gives certain recommendations on limitations imposed to protect the engine. Fig. 20 presented herein is very similar to Mr. Turner's Fig. 2. I t has been included here for the sole pur-

pose of emphasizing the i m p o r t a n t limitations and the limited operating range of Diesel engines, since the author has found t h a t several tugboat owners and skippers have no exact idea on the limitations of these engines, except the indefinite one t h a t the engine should not operate below 50 per cent rated R P M . As can be seen from Fig. 20, the recommended operating area for Diesel engines has been restricted to ranges above the 50 per cent B M E P values rather than above the 25 per cent B M E P value shown in reference (33), because the author believes t h a t even between 25 and 50 per cent B M E P , and at certain low R P M for the engine, some carbon deposits are formed, resulting in unnecessary wear and tear of the engine. At the same time, it is felt t h a t if the engineer is given some rather strict limitations on the operation of the power plant, he will be more reluctant in exceeding them. The truth of the m a t ter is, t h a t undoubtedly in most cases these limitations will be disregarded to some extent and it is felt t h a t the tendency will be to disregard them at the low side, or below the 50 per cent B M E P values. For these reasons, the area between the 25 and 50 per cent BY[EP lines is marked as emergency operation only and the explanation of the term emergency is left up to the discretion of the engineer. Tugboats have usually a relatively high speed/length ratio at free route speed. A twin-engine installation would probably prove best for a tugboat, whereby one engine could be used for cruising or endurance and the full horsepower of the power plant could be used for towing and full free running speed. Naturally, the cruising or endurance conditions should fall within the desired operating area of the engines. After a satisfactory engine combination has been selected, and if sufficient data are available, the fuel-consumption curves for the particular engines being considered should be plotted on the speed-power curves of the vessel t h a t have been established previously. I t is not considered necessary to present the exact procedure in this study, especially since Mr. Turner (33) has covered it very well in his recent work on Diesel engines. Consequently, the interested reader is referred to this reference for a complete analysis of the subject, including curves and examples, and it is believed t h a t with the aid of that reference, an easy computation of the endurance requirements for any Diesel-powered boat can be accomplished quickly and efficiently. When the fuel-consumption curves for the engines in question have been plotted on the speedpower curves of the boat, a complete picture of the performance characteristics of the main propulsion power plant will be available, as Mr. Turner points

MODERN

TUG DESIGN

395

I00 .

90 80 70

~

60

•o ®

J

50 J,0

~ 0 . 2 ~ / /

30 20

//~

lo p / / ~ _ ~ 0

~I00~ of engines /~/ operating ~ //'~50~ of enginesope-

0

10

20

I 30

~--~'

~/~ZU'h~U///

/

. . . . ~ _ I I ~0 50 6 0 7 0 P e r c e n t R~.q

rat ing or towing

~rates

I 80

in Ibs/BHP-hr

imaginary and does I mot correspond to 90 1 0 0 a n y a c t u a ~ p o w e r plant.

FIG~ 21 PERFORMANCE CHARACTERISTICSOF A DIESEL MAIN PROPULSIONPOWER PLANT out. An imaginary plot of t h a t sort, similar to Mr. Turner's Fig. 3, is shown here as Fig. 21. After the fuel requirements of the main power plant have been established, the requirements of the auxiliaries m u s t be taken into account. These auxiliaries would include, but are not necessarily limited to, the following: (a) Ship's service generators. (b) Auxiliary boilers, galley, laundry and hot water loads. (c) Steering engine. (d) Engine controls. (e) Any separate generators for electronic equipment normally in operation. (f) Towing engine. (g) Any p u m p t h a t m a y normally be working, such as fire pumps. T h e loads used for the foregoing auxiliaries in estimating the fuel consumption of each should be the average loads for 24 hr cruising conditions times the total number in days for endurance, as determined from the original endurance requirements. In addition to the fuel consumption thus calculated, a n y haxboi load on the auxiliaries normally present should be added, provided such a harbor load can be thought of as being a part of the normal workday of the boat, as the case will be for a harbor tug. The addition of the auxiliary and main engine fuel requirements will give the total amount of fuel required for consumption for endurance. An additional 10 per cent of this total Should be added for any emergency t h a t m a y arise. This new total will then give the total weight of fuel required. The t a n k capacity of the boat should then be figured on the basis that the tanks will be able to carry that total when 95 per dent full. Aside from fuel-oil requirements, potable water, ship's stores and ship's service stores and consumable goods and foodstuffs enter into the ca]=

culations for endurance. I t would be indeed futile to have sufficient fuel capacity for a specific cruising radius and lack in consumable goods or water for the same radius. Accordingly, these items should be figured out carefully, and it is hoped t h a t Table 3, presented previously in this study, m a y help the designer in arriving at reasonable values for the items mentioned. A final item t h a t m u s t be taken into account is the lube-oil consumption of the engines. This can be taken as equal to a b o u t 0.0025 or 0.0026 l b / h p h r for most modern Diesel engines. Hence, knowing the total horsepower of the engines and the total time required for endurance, one can easily calculate the a m o u n t of lube oil needed for it. T o this, a reasonable amount m u s t be added for a n y emergency t h a t m a y •arise. T h i s a m o u n t will mainly depend on • the total horsepower available and in no case should be less than 10 per cent of the consumable lube oil figured previously. Again, the lube-oil t a n k capacity should be figured on the basis t h a t the tanks will be able to carry the total mentioned when 9,5 per cent full. ACKNOWLEDGMENTS The author wishes to express sincere thanks to the following individuals a n d / o r organizations, without whose kind help and advice this paper would not have been possible. Prof. L. A. Baler, University of Michigan. Prof. H a r r y Benford, University' of Michigan. C o m m a n d e r Richards T. Miller, USN (Bureau of Ships). Mr. Ulysses A. Pournaras, naval architect. " Mr. Lester Rosenblatt of IV[. Rosenblatt & Son. Mr. James J. Turner of the Bureau of Ships. Codes 420, 430, 440 and 554 of the Bureau of Ships. T h e Small Boat Section of the David Taylor Model Basin.

396

M O D E R N TUG DESIGN

The Marine Terminal Division of the Transportation Research and Engineering Command of the United States Army, Ft. Eustis, Va. In addition to that, several towing companies have been very obliging in supplying data and information for several boats. Some of these companies have asked to remain anonymous and in respect to their wishes, no names shall be mentioned. However, the author wishes to extend a sincere "thanks" to all those who contributed to the data presented in this paper. BIBLIOGRAPHY 1 "Comparison between the Open water Efficiency and Thrust of the 'Lips-Schelde' Controllable Pitch Propeller and Those of the Troost Series Propellers," by J. A. van Aken and K. Tasseron, International Shipbuilding Progress, vol. 2, no. 5, January, 1955. 2 "Results of Propeller Tests in the Astern Condition for Comparing the Open Water Efficiency and the Thrust of the 'Lips-Schelde' Controllable Pitch Propeller and the Troost Series Propellers," by J. A. van Aken and K. Tasseron, International Shipbuilding Progress, vol. 3, no. 26, October, 1956. 3 "The Efficient Length for a Given Form and Speed," by L. A. Baler, Trans. SNAME, vol. 42, 1934. 4 . "Power-Length-Speed," by L. A. Baler, Motorship, May, 1948. 5 "Vibration at the Stern of Single Screw Vessels," by L. A. Baler and J. Ormondroyd, Trans. SNAME, voL 60, 1952, pp. 10-25, 35-39. 6 "Propellers and Propulsion," by L. A. Baler, SNAME Great Lakes Section Meeting, February 3, 1956. 7 "Propellers for Tugs and Trawlers," by F. W. Benson, Transactions of NEC Institution for Engineers and Shipbuilders, vol. 54, 19371938. 8 "Screw Tug Design," by A. Caldwell, Hutchinson's Scientific and Technical Press, London, England, 1946. 9 "Trends in Tug Design," by E. C. B. Corlett, The Shipping World, January 12, 1955. 10 "Diesel-Electric Drive for Tugs," by L. M. Goldsmith, SNAME Philadelphia Section Meeting, October 17, 1947. 11 "Free Piston Gas Generators for Marine Pr6pulsion," by N. L. Hawks, SNAME Bulletin, October, 1952. 12 "Beitrage znr Theorie ummantelter schiffsschrauben," by F. Horn, Jahrbuch der Sehifbautechnischen Gesellschaft, 1940. 13 "Some Problems Involved in the Design of Small Harbor and Coastal Vessels," by Eads

Johnson, SNAME New York Metropolitan Section Meeting, October 24, 1947. 14 "Recent Research of Propellers in Nozzles," by J. D. van Manen, SNAME New York Metropolitan Section Meeting, October 19, 1956. 15 "Small Craft Types," by J. A. Mavor, The

Association of Engineering Draughtsmen, 1937-1938.

and

Shipbuilding

16 Discussion of reference (27), by R. 2-. Miller, Trans. SNAME, vol. 59, 1951, pp. 605606. 17 "Theory of Flight," by Richard.von Mises, Brown University Press, Providence, R. I., 1942. 18 "Long Distance Towing and Tug Design," by E. F. Moran, Jr., SNANIE New York Metropolitan Section Meeting, September 21, 1950. 19 "Resistance and Trim of Heavy Displacement Standard Series Ships," by A. B. Murray and S. A. Barklie, Stevens Institute of Technology Experimental Towing Tank Report No. 279, January, 1945. 20 "Screw Propeller Characteristics," by H. F. Nordstr6m, Paper No. 9 of Swedish State Shipbuilding Experimental Tank. 21 "The Design of Tugs for the San Fx:ancisco Bay Area," by L. C. Norgaard, SNAME Northern California Section, April 12, 1956. 22 "Small Vessels," 'by W. Pollock, Tunbridge Wells, Kent, England, 1946. 23 "The Theory and Practice of the Kort System of Propulsion," by A. M. Riddel, Trans. INA, vol. 84, 1942. 24 "Tugboat Design," by C. D. Roach, Trans. SNAME, vol. 62, 1954, pp. 593-626, 641-642. 25 "Propeller Backing Power in Tugboats," by A. J. C. Robertson, Trans. SNAME, vol. 36, 1928. 26 "Wirtschaftliche und wissenschaftliche bedeutung ummantelter Schiffsschrauben," by E. K. Roscher, Jahrbuch tier Schiffbautechnischen Gesellschaft, 1939. 27 "Small Craft, Construction and Design," by D. S. Simpson, Trans. SNAME, vol. 59, 1951, pp. 554-582. 28 "A Note on Tug Design," by A. R. Taylor, Trans. INA, vol. 84, 1942. 29 "The Speed and Power of Ships," by D. W. Taylor, U. S. Government Printing Office, 1943. 30 "Marine Engineering as Applied to Small Vessels," by P. G. Tomalin, Trans. SNAME, vol. 61, 1953, pp. 590-634. 31 "Open Water Test Series with Modern Propeller Forms," by L. Troost, Transactions N.E.C. Inst. of Engineers and Shipbuilders, vol. 67, 1950. 32 "A Simplified Method for Preliminary Powering of Single Screw Merchant Ships," by

MODERN

TUG DESIGN

L. Troost, S N A M E New England Section Meeting, October, 1955. 33 "Selection of Diesel Propulsion Plants for N a v a l Vessels," b y J. J. Turner, Journal of the American Society of Naval Engineers, vol. 69, no. 3, August, 1956. 34 "Series-versus Parallel-Conneeted Generators for Multi-Engine D-C Diesel-Electric ShipPropulsion Systems," b y J. A. Wasmund, Trans. A I E E , 1954, Paper No. 54-144. 35 U.S. Coast Guard Stability Regulation and Rules .and Regulations for Cargo and Miscellaneous Vessels, 1955. 36 "Principles of N a v a l Architecture," edited by H. E. Rossell and L. B. Chapman, published b y THE SOCIETY OF NAVAL ARCHITECTS AND MARINE

ENGINEERS, New York, N. Y., vol. I, 1939. 37 "Principles of N a v a l Architecture," edited by H. E. Rossell and L. B. Chapman, published b y THE SOCIETY OF NAVAL ARCHITECTS AND MARINE ENGINEERS, New y o r k , N. Y., vol. II, 1939.

397

38 "Small Sea Going Craft and Vessels for Inland Navigation," b y A. Roorda, et al, (Dutch edition), N.V. de Technische Uitgeverij, H. StareHaarlem, Holland, 1955. 39 " T h e Pulling Power of Tugs," Yachts and Yachting, M a y 13, 1955. 40 " C o n t e m p o r a r y T u g Propulsion," b y A. C. Hardy, The Shipping World, June, 1954. 41 " H i g h Spots in Small Boat Design~" by Eads Johnson, Marine Engineering, November, 1938. 42 " T h e Pitch Distribution of Wake-Adapted M~rine Propellers," b y L. Troost, Trans. S N A M E , vol. 64, 1956, pp. 357-367. 43 "Model Resistance D a t a S h e e t s Nos. 18, 70, 71, 72, 73, 74,. 75, 76, 80, 83, 86, 93, 94," published b y The Society of N a v a l Architects and Marine Engineers. 44 "Hydroeonic Construction Progress," The Shipbuilder and Marine Engine Builder, vol. 64, no. 586, February, 1957.

Appendix 1 A procedure for the design of a tugboat propeller for any one of several design conditions is presented in this Appendix. T h e design follows the propeller-series chart method and is based upon the Benson-Miller method with some additions and manipulations of the coefficients to arrive at the desired results. To facilitate matters and avoid confusion, the following nomenclature is used throughout this Appendix :' B H P = brake horsepower ( = S H P / e m , e., = mechanical efficiency) S H P = shaft horsepower, aft of gears and thrust block D H P = delivered horsepower ( = S H P ) < es, where e. represents losses in the stern tube and bearings) T R H P = tow-rope horsepower, = hull resistance useful horsepower, = D H P )< e UHP = open water efficiency of propeller e = 1--t effective horsepower, = U H P × -1 --72) EHP = l--t

hull efficiency

I --w t

V = Va

=

thrust deduction wt = T a y l o r ' s wake fraction speed, fps speed of advance, fps, = v(1 - wt)

Vk = speed, knots Va = speed of advance, knots, =

Vk(1

--

wt)

D H N n T

= = = = =

propeller diameter, ft propeller pitch, ft propeller revolution per minute propeller revolutions per second thrust (total), pounds D H P × 550 × e Va

Tp = thrust to propulsion Ta = thrust available for towing D H P X 550 Q = torque, ft-lb, = 27rn T T Ct = thrusteoefficient, = n2H2D2 = n2a~D4 = thrust coefficient, = Cte~2/p C~ = torque coefficient Q Q 1 --sCt - n2H3D 2 n2a"D 5 2rr e Kq = torque coefficient, = C~a3/o p = density of water Kt

7)a

J = coefficient of advance = n-D =

a(1

-- s)

550e • T n D F = relation coefficient = - J - I)HP DHP 27rCqol 3 G -- power coefficient n3D ~ 550

398

MODERN TUG DESIGN 2O 18

16 12 10

~6 ~4 ~

-

2 0

tll 1~

14 FIG. 2 2

N ( B H P ) 1/, Va2. 5

Bu =

16

18

20 2 Wake p e r c e n t a g e

FREQUENCY DISTRIBUTION OF THE W A K E S OF

- Taylor's basic propeller power coefficient

N ( U H P ) 1,

- Taylor's basic propeller coefficient a = pitch ratio, = H I D = ND/I~

s = slip =1

101.33 X Va TIN

-1

101.33 ~a

To select a suitable wheel, the following procedure can be used : Assume a reasonable towing speed, and consequently propeller design speed, say between 4 and 8 knots. Determine Va by estimating the wake fraction wt. Values for wt can be found in the appropriate literature, such as references (24, 29, 37, 38), and so on. Normally the wake of a singlescrew tugboat will range from 15 to 30 per cent, with 23 or 24 per cent being good median values. Fig. 22 is a histogram of wake percentages of 47 tugboats. All wakes have been taken at the towing speed, which in turn was taken at one half the free route speed. No separation of the wake percentage b y length or any other parameter has been attempted, since the amount of wake depends on a combination of factors. However, this figure should give a general idea of the average w a k e variation in tugboats. After the value of V~ has been established, the designer must decide upon the type of wheel he wants, be it Taylor's, Troost's, Schaffran's, or of any other type. Now, since normally the largest diameter will give the maximum efficiency and thrust, one must look at the lines and establish the maximum propeller diameter that can be used; while doing this, one must keep in mind that tip clearances are quite important and cannot be below certain values if one wants to obtain a b o a t reasonably free of propeller vibrations. At first

47 TU6BOA'rS

this statement m a y appear to be at odds with the previously quoted belief of several authorities in the field of ship vibrations (5) who state that poor distribution of the wake along the disk of the propeller appears to have much more effect on hull vibrations than close clearances between the tips of the wheel and the hull. However, one must remember that no matter how fine the lines of a tugboat are made, the form of the boat is such as to practically insure poor wake distribution in the vicinity of the hull at the after end. Thus, by increasing the tip clearance between the propeller and the hull, one actually reduces the poor wake distribution along the disk of the propeller and hence reduces the danger 9f vibrations. The tip clearance at the top of the wheel should have a minimum value of 7 per cent of the diameter of the screw and preferably be in the order of 10 to 15 per cent of the diameter, while the bottom clearance need be only one half of the top clearance. Next one must establish the number of blades for the proposed propeller. I t might be mentioned here that in so far as vibrations are concerned the designer m a y be better off by adopting a fourbladed wheel. If a three-bladed wheel with a certain thrust loading is replaced by a four-bladed one with the same loading, the vector sums of the vibration exciting forces will remain for all practical purposes the same. The four blader will begin to excite hull vibrations at a somewhat lower speed thala-the three-bladed propeller, but at full power the results will be about the same. However, if the diameter is the same, the loading on the three-bladed Wheel will be higher than the corresponding one on the four blader and consequently the forces will be lower for the four-bladed screw. On the' other hand, if the loading is the same the diameter of the four blader can be reduced with the result that it will excite less amplitude in stern vibrations than the three-bladed

MODERN TUG DESIGN

399

120

A 115

A &

II0 O

A A

105 A

0

0

~4

X

A

A a

/.£Mi,anA L:

~A

I00

a~

a

Z

95

/ /,

90

0 m

A

a

A 85

a

a

&

80

I

75

0

I0 FIG. 2 3

20

J 50

30 40 Speed in % of

60 free

70 80 route

90

i00

VARIATION OF W A K E WITH SPEED OF" BOAT

screw, because the blade tips will not reach so far up into the poor wake distribution close to the hull of the boat. Normally, aside from vibration considerations, there will be very little choice between a three and a four-bladed wheel, b u t nevertheless, after the decision is made as to the most suitable number of blades and the design is completed, a similar screw but with a different number of blades should be tried to ensure that the propeller chosen is the most efficient one. The design actually should be based on maximum absorption of thrust at the design speed. However, since

Tv 2rrQn designing for maximum efficiency automatically will give maximum thrust absorption, since v is set and the product Qn is equal to the power available. T h a t being the case, the design procedure is as follows: Pick out at reasonable R P M and calculate the Bp and ~max values; 6max will be based upon the maximum propeller diameter that can be used as established previously from the lines drawing. From the applicable propeller charts, determine the values of e and a. Repeat this procedure for four or five more R P M values, until a definite trend has been established. P16t propeller RP1V[ versus a and e and determine the revolutions that will give maximiam efficiency. The a-value

corresponding to the best R P M can be read directly from the curve. The same procedure can be followed for a wheel with a different number of blades than the one first decided upon and a final decision can be reached as to the most efficient propeller. A loading check must now be made to ensure that the propeller does not have an abnormally high load (even 15 psi is acceptable for a tugboat propeller) and finally a cavitation check must be made. Several cavitation criteria can be found in the appropriate literature (29, 37). One particular formula is mentioned here; not because it is the best or most appropriate one for tugboat propeller design, but simply because the author has not been able to find it anywhere else. This formula is used by the United States N a v y to determine the R P M at which cavitation is most likely to start with a given propeller and is as follows: (Nor)2 = 37,500(11iD-- s)2s h MWRBT~ where N~r = R P M at which cavitation is most likely to start h = absolute hydrostatic head to hub, ft M W R = mean width ratio B T F = blade-thickness fraction and the remaining symbols are explained elsewhere. Another cavitation formula that might be men-

400

MODERN

TUG DESIGN

tioned here is the one proposed b y P. G. Tomalin (30). This is actually a modification of the original Bower's formula, as the author points out, and has given consistently good results in relatively small vessels where most of the other cavitation formulas seem to fail (N,)2 = 52,000 (0.86 -- s) h M W R liD BTF where h = total head of water in feet at the ¢_ of shaft s = true slip and the remaining symbols have the same meaning as before. After selecting the most appropriate propeller, the thrust, efficiency, slip, and revolutions at any other speed can 13e determined. Here a differentiation m u s t be made between' the two different ways in which the turning m o m e n t can be applied; namely, if there is constant power developed (as in the case of a Diesel-electric drive or a Diesel drive with a torque converter), or if the turning m o m e n t is constant, as in the case of a Diesel engine in conjunction with a reverse reduction gear drive, or a reciprocating steam engine, or a direct-connected Diesel. (a) In the case where constant power is being developed, different values of J can be assumed, from J = 0 to J = m a x i m u m at or near free route speed. F r o m the J-values thus assumed, the K , and Kq (or Ct and Cq) values of the propeller in question can be determined from propeller-characteristic curves, such as the ones in references (29), (31), (37). Now, we have t h a t F

_

Ct 550 Cq 2 r a

_

b y definition and t h a t T - --

e=~ the efficiency can be determined. Finally, knowing t h a t Va = 0.592 J n D , we can get Va, from which Vk can be determined easily. Here a word of caution m a y be necessary. For most tugboat hull forms, the wake is fairly constant over the whole range of speeds of the boat, b u t some hulls do show some unusual variations in the wake. Fig. 23 shows wake figures in per cent of towing wake (towing speed being in all cases equal to one half of the free route speed). Points for twenty different hulls have been plotted and the average wake curve shows very little variation from full to zero speeds of the boat. Finally, if the astern thrust of the propeller is required, the same procedure just outlined can be followed. Reference (20) will give Ct and Cq-values for propellers operating in reverse. This, together with reference (2) to some extent, are the only publications, to the knowledge of the author which have any model test information on propeller characteristics in reverse operation. However, since the wheels tested in both references are of one particular t y p e only, judgment m u s t be used in interpreting the results of the tests relative to the design in question. (b) In the case where constant turning m o m e n t is being developed, we know t h a t b y definition D H P o / n o will be c o n s t a n t

where DHP0 = power at design speed (maximum power) no = revolutions per second at design speed From elementary considerations, it also "can be shown easily t h a t the following relation will hold true

DHP -

-

f

nO

We also have, though, t h a t T Ct = n2H2D2

or t h a t T = C~n°'H2D 2

and b y subtracting the two equations for T the value of n can be determined. Thus we have n = \CtH2D3]

Then, b y substituting n into any of the foregoing formulas for T the thrust can be found and, since

n = ~

\ noD

-

Again we have two distinctive cases. T h e first one, assuming t h a t a governor is supplied with the engine to avoid overspeeding, is the case of speeds higher than the design speed. In this case N will be constant. The second case is the one of all speeds below the design speed. Each case is actually independent of the other and is treated as such in the analysis t h a t follows. Calling the design speed Va, we have 1 Vk larger or equa! to Vd. In this case, Va and ~ can be determined, since N is known and constant. F r o m the propeller characteristic charts Bp and e-values can be determined at the calculated 8 and a-values, from which all the desired characteristics of the p r o -

MODERN TUG DESIGN peller (T, s and BHP) can be determined easily. 2 V, smaller than Va. Here again a set of J-values from J = 0 to a value of J such as to make V, nearly equal to V~ can be selected and the slip can be calculated. From either the J or the slip values and the appropriate propeller characteristics chart, Ct and Cq or Kt and Kq-values can be found and G can be calculated. Knowing G, one can determine n from the foregoing formula and hence D H P , since DHP0 DHP = - n no

Knowing n one can calculate V~ ( = J n D )< 0.592) and Vk. Finally, to find the thrust we can set DHP

T=--F nD where

401 F = C~ 550. Cq 21ra

The efficiency can be easily found, knowing that

e=~ I t is suggested that after the propeller characteristics have been determined by the methods just discussed, a plot be prepared with Vk as the abscissa and the following ordinates: 1 Thrust (total). 2 Revolutions per minute. 3 Efficiency. 4 Slip. 5 Thrust to propulsion. 6 Thrust available for towing. This way, a complete picture of the characteristics of the boat at any speed can be had at a glance.

Appendix 2 A hydrodynamic analysis of the relative advantages of a cambered section over the more common airfoil-type section is presented in this Appendix. S o m e simplifying assumptions have b e e n made, which, in the opinion of the author, do not affect the interpretation of the results. Rudder groups a, b, and c refer to Fig. 19 of the text. In general terms it can be stated that essential differences exist only between the a and b groups on the one hand and the c group on the other. Differences among types a and b are only the effective results of different drag characteristics. This will be clear if one recalls that part of the rudder-stock torque is due to drag. The drag contribution to the turning moment in general will depend upon the value of

If we assume then that the difference in drag is the only difference in the turning moment between type a and type b rudders, we can return to our original assumption that essential differences exist only between groups a and b and group c. If we assume for reasons of simplicity that the drag of the groups under consideration is the same, then the value of the maximum turning moment will depend on the maximum lift coefficient of the rudder in question, CL m~x. The value of C~ mo~ for cambered sections is higher than the corresponding value of symmetrical airfoil-type sections. However, cambered sections are characterized by their "irreversibility;" i.e.

cp~ sin 7T

where CL is the lift coefficient, and a, -- angle of attack of the rudder. This characteristic, plus the fact that cambered sections show an objectionable profile drag at a --- 0, makes any fixed camber section unusable in boat design. However, the advantages of the higher CL ms, of the cambered section can be utilized with a trailing-edge flap added to a symmetrical section, resulting in a rudder similar to the c-group, of Fig. 19.... The~_:fotlowing analysis indicates the degree of advantage one m a y expect from such a section. The slope of the lift curve (CL versus a) for both the cambered and the symmetrical sections will be

where

cps = distance of center of pressure from rudder stock ¢_ 7T = rudder angle Now, if we set 7~ = 7, (where 7 , = angle of relative flow) b y neglecting small angles of relative flow, we can say that the rudder-stock torque and turning moment are equal and that they can be represented by (Drag) cps sin 7r for the drag component of the turning moment.

ICL at a, = a[ # ICL at at = --al

402

MODERN TUG DESIGN

Lift

~

T~---G -

-

FLOW Center~

•

Turning

~ /

Moment = L i f t x L + Drag x d

FIG. 24 LIFT-DRAGRELATIONSHIPTO THE TURNINGMOMENT

C [

~

[ ~ / [ /

^C~o~

-~

~%~ r / ~ L i f t _.~0~/~dCL

c u r v e f o r cambered i s f o r NACA 6 3 2 0 1 5

curve

section

for

symmetrical

is f o r NACA 0015

+~

0

@ ~L=0 ~ . i _

Lift section

~/dCL

~

FIG. 25 LIFT CURVESFOR SYMMETRICALAND CAMBEREDSECTIONS

dCL d~.=

5.7 for aspect ratio (AR) equal to infinity 3

or

dCL

5.7 d--~ = 1 + 2 / A R for any aspect ratio

5

~

---g

a~ = 36 ° + s0 = 43 ° ---- 0.80 radians so that

From the foregoing and with the help of Fig. 25 we obtain • (CL)

=

2.53 -- 1.98 - 0.28 or 28 per cent 1.98

5.7(~ + Is0[) 1 + 2/AR

where s0 is the angle of attack at CL = 0. Or, we can generalize by saying that

CL=

5.7a~ 1 + 2/AR

where a, is the angle of attack referred to s0. If we select an N A C A 0015 (t/w = 0.15) symmetrical section for the rudder, we will have 5.7a~ (eL)

.....

Irlcal =

for aspect ratio (AR) 0.628 radians

1 + =

3.16 X 0.80 = 2.53

Then the per cent increase due to the flap will be

5.7a . . . . . trical = 1 + 2 / A R

(CL)oa~borod

(CL) flapped.=

2/2.5

2.5;

and

=

3.16~,

at ~

=

36 ° =

(CL).ym~o,r,°,~ = 3.16 X 0.628 = 1.98 Now, assuming that the flap has a flap angle of 35 deg and that the flap area is equal to 1/5 of the rudder area, we have

I t should be noted here that, in order to simplify matters, no corrections have been made for submergence and Reynolds number. However, the order of magnitude of the advantage of the flapped versus the unflapped rudder will be reasonably close to the one shown in the foregoing after the corrections mentioned have been made. This can be proven as follows: The corrections will result in a change of slope of the lift-coefficient angle of attack curve for both the symmetrical and the flapped sections. Since the change of the slope curve will be practically the same for both sections, the corrections for Reynolds number and submergence will not seriously affect the relative increase of the lift determined previously. The d y n a m i c turning response for both the symmetrical and the cambered sections also will be the same. This follows from the fact that the lift-curve slope is the same in both cases. In mathematical form, it would mean that

MODERN

TUG DESIGN

403

lift-curve slope of a fixed camber a n d / o r symmetrical rudder CL

/

dCL 5.7 -- = = 3.16 for A R =2.5 da 1 + 2/AR

I

The lift curve for the flapped rudder will be

S

(-)

• FIG.

26

dCL = 3.16 a0 + 3.16 am.x ---- 3.16 + 3.16 a0 /

0

.

(+)

I

¢-'...>'....-" /._/ /_

a'~

_

/

~

_

"*~

~

/~-f

6-~'J~,~ ~

/

~ , e,o~

oe'~" ~ I

-

./

,~/ Lengfh of Normal S.S. J Hvdroconic Tuns ,I.$ \ ./.~;"

Ioo_

g 80 __

IOO

iI

o~ , 2 0 _ IO II0

"5

oo

I

_

.

.o

@

/.

_

,.

_.00 _

Pracf~ce

1400

IgO0

lBO0

4oo k5

_

2000

200

2_200

Shai:f Horsepower FIG. 35

C O M P A R I S O N OF H Y D R O C O N I C T U G W I T H A M E R I C A N AND B R I T I S H D E S I G N P R A C T I C E

dom of a distinct design philosophy which may, if desired, be applied to tugs and is based upon the use of simple.developable surfaces. This approach uses the trade name "Hydroconic," although if the author had not mentioned it, I would feel averse t o using a trade name with which I am connected a t a-professional conference~ M a n y tugs have been..built to this system and, mistakenly, are ,often thought of as simple straight-line forms such as, for. instance, the British Tid tugs .built during •the last war. This is entirely incorrect and in fact m a n y of .the principles of design used m a y be ap.plied to chine tugs or to tugs of conventional bilge •shape. Initially we intended to design tugboats of inherently lower first cost, but of average performance. Development of the design approach Soon allowed the production of tugs of outstanding performance, seaworthiness and strength, all much above average and cost saving is now merely a bonus. At the m o m e n t there is a group -of round-bilge tugs building embodying most of the Hydroeonic design features. I would like, therefore, in this discussion, to supplement the in:formation given b y the author, as the design ap.proach in these tugs and the resulting proportions and performance are so different from British-or American practice t h a t they certainly cannot be considered by the plots in the figures given in the paper. For example, Fig. 35 herewith shows, the

author's Fig. 1 replotted with the length and displacement of normal single and twin-screw Hydroconic tugs on a basis of shaft horsepower. I t will be seen that the length of these on a basis of shaft horsepower is intermediate between t h a t of British and American tugs, but the curve is similar in character to t h a t of American practice. On the other hand, the displacements, while conforming closely to the values obtained in American practice up to 1000 shp, follow the general characteristics of the curve of displacement for British tugs. The remaining dimensions, such as depth and draft, also v a r y from both these practices, Fig. 36. The draft is generally in excess of normal British practice and, for instance, is between British and American practice up to 100 ft and in excess of either above that. The depth generally conforms closely to the curve shown for British vessels, while the percentage drag of keel is much less variable with length than those shown. The design approach used in these tugs is quite different from t h a t outlined in the paper, for example, no variation of block coefficient with speed is permitted, neither is any such variation made with respect to LCB position. This will be explained. The design is centered on the propeller, and the towing performance of a screw behind a normal tug hull is necessarily lower than in open-water conditions whether it is to be on the bollard, or at towing speeds, or running free. We provide

MODERN

TUG

DESIGN

419

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IGO

Perpendlculors-Feef

COMPARISON OF LENGTH B P VERSUS DEPTH AND DRAFT OF HYDROCONIC T U G WITH AMERICAN AND BRITISH DESIGN PRACTICE

operating conditions as good as are possible and boundary layer separate from that of the skeg therefore keep the center of buoyancy well for- This allows the clearance from the hull to be reward, giving a long easy run to the hull. The duced to the absolute minimum. For example, sections are made extremely wide in way of the with a 10-ft 8-in. screw transmitting 1350 shp, we propeller and all buttock lines are kept straight • have used clearances from the hull of as little as 8 for at least 20 per cent of the length of the vessel in. with entirely satisfactory results and no sign of from t h e transom, or alternatively, are hooked blade-excited vibration. By careful design, this type_of form can be made over the propeller. In this manner, with the LCB For example, a at o r slightly forward of amidships, the tendency of inherently low resistance. of the hull to squat at speed-length ratios in the given trig'will always plot favorably against a tug region of 1-1.4 is entirely avoided as has been from the S N A M E test sheets where the two have shown by measurement during model testing and similar coefficients. We do not strive for fine by observations on trials. The resulting sections waterline entrances and, in fact, the type of stern give complete coverage to the propeller and do not being by definition one of flow along buttock lines, allow air drawing even under severe provocation. it is advantageous to avoid imparting a markedly Owing to the shape of the foregoing sections in waterline characteristic to the flow. in the foreway Of and forward of the propeller, we can make body. From this will be seen that the Hydroconic form this rather large. As the pull of the tug of given power is, to the first order, linearly proportional to is a distinct one and the basic design premises dif• the propeller diameter, the revolutions are ad- fer from those a d o p t e d in normal British or justed by choice of gearbox ratio to suit the charac- American tugs. The block coefficient has been. interistics of the hull and the propeller. While this vestigated in extensive tank and full-scale testing large propeller assists in obtaining a good pull the and it has been found that to give optimum requestion of clearances arises. By designing the sults, it should be kept between 0.45 and 0.48. hull as a discrete body and adding thereto an ap- By suitable adjustment of the dimensions, it is pendage in the form of a skeg fairing into the possible to design so that any tug function m a y be hull well forward of the propeller, a relatively uni- performed keeping within this range of block coform wake distribution is' obtained with the hull efficients a n d one must emphasize that this point

420

MODERN TUG DESIGN

Fore Peak

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FIG. 37

HYDROCONICTUG, SYDNEY COVE

is i m p o r t a n t , as vahies in excess of 0.50 have been f o u n d to raise the t h r u s t d e d u c t i o n a p p r e c i a b l y e v e n i n bollard conditions. T h e a u t h o r m a k e s reference to a n u m b e r of articles b y the writer o n these tugs, l e a v i n g the q u e s t i o n of p e r f o r m a n c e r a t h e r in the air. Firstly, I should like to s t a t e it is a fact observed is cons t r u c t i o n t h a t , in tugs a b o v e 50 ft length, m a n h o u r savings of over 30 per cent in the steel labor of t h e hull are o b t a i n e d c o n s i s t e n t l y . T h i s is p a r t l y due to the s i m p l i c i t y of c o n s t r u c t i o n a n d p a r t l y due to a novel form of p r e f a b r i c a t i o n t h a t is pos.sible w i t h this t y p e of hull a n d which has been f o u n d so successful in practice t h a t the aforem e n t i o n e d savings can, in fact, be realized con: s i s t e n t l y in specialist yards. N a t u r a l ! y , this saving m a y be utilized either to produce a cheaper vessel or to increase t h e b u i l d i n g profit a n d m a y n o t be reflected fully in a bid. N o r m a l l y , we design tugs to give full power ab-

sorption a t speeds r a n g i n g b e t w e e n 0 a n d 6 knots, F o r example, the m o t o r t u g Sydney Cove, Fig. 37. has the following p a r t i c u l a r s : Length overall, ft-in . . . . . . . . . . . . . . . . . . . . . . . . Length between perpendiculars, ft-in . . . . . . . . . Breadth molded, ft-in . . . . . . . . . . . . . . . . . . . . . . Depth at side, ft . . . . . . . . . . . . . . . . . . . . . . . . . . Draft aft, ft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Draft forward, ft . . . . . . . . . . . . . . . . . . . . . . . . . . Drag of keel, ft . . . . . . . . . . . . . . . . . . . . . . . . . . . . Freeboard, ft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Block coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . V~ V L free running . . . . . . . . . . . . . . . . . . . . . . . . Shaft horsepower: Normal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maximum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RPM, Maximum . . . . . . . . . . . . . . . . . . . . . . . . . . Propeller diameter, ft in . . . . . . . . . . . . . . . . . . . . Type of rudder . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bollard pull : Long tons at 1200 shp . . . . . . . . . . . . . . . . . . . . Long tons at 1350 shp . . . . . . . . . . . . . . . . . . . .

115-0 105-0 29-2 13 13 8 5 3.4 0. 475 1.25 1200 1350 160 10-8 Bulb 20.0 21.8

T h i s vessel was designed w i t h a screw i n t e n d e d for the a b s o r p t i o n of 1350 shp a t zero forward

MODERN 4__

3_

~

18.0Tons 1.77 Ton/lO0SlipI 21.6 Tons 1.60 TonllO0SHP P_2.aTon,s 1.56 Ton/lO0SHP

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OBSERVED AND CALCULATED P U L L - - M T

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BOLLARD T R I A L S

speed. I t was found t h a t a Troost-type section was therefore, is considerable. Subject of course to an suitable and the propeller was four bladed with a owner allowing choice of propeller revolutions and 40 per cent disk-area ratio. The contract pull on o t h e r parameters, it is then possible for the dethe bollard was 18 tons and this was obtained with signer to guarantee a pull figure of 1.6 to 1.7 tons 1015 shp measured on a Siemens torsion meter. against penalty. The obtainable speeds are less The contract horsepower of 1350, in fact, pro- nondimensional, of course, as the installed horsepower/ton displacement tends to drop in the duced 21.8 tons. Fig. 38 shows a s u m m a r y of the bollard trial larger ships. I t should be remarked t h a t Fig. 5 in the paper results of this vessel and it will be seen t h a t the observed pulls are well in excess of the calculated shows a 92-ft tug in very outline arrangement, o p e n - w a t e r thrusts of the screw. The shape of while Fig. 6 shows the midship section of a small these tugs is so arranged that, while no measur- 42-ft launch/tug. On the subject of stability and freeboard, the able thrust deduction takes place on the bollard, With the type of rudder fitted which incorporates tug Sydney Cove has an operating metacentric a h e a v y streamlin e bulb rudder post with a flat- height with full free surface correction in the deepp l a t e trailing edge, actual thrust augments are load condition of about.5 ft. I t has been found in obtainable. This rudder operates as a contra-sys- practice t h a t this does not affect in a n y way the tern capable of accepting, high incidence of flow seakindliness of the vessel which was found on deproducing an augment varying between 5 and 15 livery to Australia to be .outstanding, b u t has per cent of the total open-water thrust of the most desirable results in ability to handle tows in screw, depending upon the level of the disk difficult conditions and at close quarters. Where loading. I t m a y be felt t h a t speeds in the region of possible rolling chocks should be fitted, of course. V/x/L = 1 . 2 5 - 1.35 in association with Generally speaking, we arrange t h a t Hydroconic bollard pulls of between 1.6 and 1.9 tons/100 shp, tugs have high metacentric heights ranging from are so high as to be questionable, but nearly 40 3 to 5 ft after correction for free surface, dependent tugs of this t y p e have been constructed in the past upon type. I n association with this, we feel t h a t 5 years ranging from 160-1400 shp, m a n y of which adequate freeboard is most i m p o r t a n t and we would generally rather increase the freeboard for achieve these figures. The practical background,

422

MODERN TUG DESIGN

some of tile vessels given in Table 2, although so~e have freeboards which are entirely adequate; for example, the William and Y. AIoran. At this point, it is as well to mention that an attractive alternative to the diesel-electric installation is to fit a multispeed gearbox, as in s~me European vessels, or to allow an overspeed at light load when running free. As an example of Continental practice, the very interesting French salvage tug Jean Bart of 2500 shp is fitted with. a three-speed gearbox allowing the screw to absorb full power either running free, or at 100 per cent slip, or running astern. M a n y diesel-engine manufacturers will permit, with absolute confidence, an increase of approximately 10 per cent in the operating revolutions of their machinery running free, providing the torque does not exceed a figure of, say, 75 per cent of the continuous rated maximum. If this is done, it will be found that many of the characteristics of the diesel-electric installation can then be Gbtained with a single-ratio geared diesel tug, the propeller being designed for full-power absorption at normal revolutions at perhaps 2 knots forward speed. This was not the case with the Sydney Cove. Where this has been possible, a 56-ft Thames tug has produced a bollard pull of 5.5 tons at 324 shp while registering a speed on the measured mile of 10.78 knots, a rather remarkable figure. Here, approximately 10 per cent overspeed of the machinery allowed absorption of about 85 per cent of the rated power. The cost per pound of bollard pull on the average high-efficiency Hydroconic tug designed without limitations on revolutions or draft and built in Britain, is of the order of $7 per lb pull. Tile cost given in the paper, compares favorably with this, bearing in mind the difference in shipbuilding costs between the two countries. The pull per S H P of these tugs is taken, for normal purposes, as ranging from 35-40 lb per delivered horsepower; good figures approaching 40. There are other points upon which one would like to touch. No detailed mention is made in the paper of twin-screw tugs. Recently a series of four Hydroeonic twin-screw tugs was built for the Manchester Ship Canal Company; 88 ft in length, these 1200-shp vessels were fitted with three different sets of screws in order to assess their suitability for canal working from an operational point of view. The first set consisted of two 7 ft 3 in. diameter screws of ogival or "Taylor" sections. These cast-iron screws gave an ahead pull of just over 17 tons and an astern pull of just under 10 tons. The second set was of aerofoil section to about the 0.5 radius and lentieular from thereon. These screws gave a bollard pull ahead of 15.5

tons and a bollard pull astern of 13.5 tons. The third set of screws is of Tr~ost aerofoil type and wbile performance figures are not yet to hand, it is anticipated that the bollarcl pull ahead will be approximately 16 tons and the bollard pull astern approximately 12 tons. The writer cannot agree that the ogival screw will give a relatively high astern pull, for the reason that the mean effective pitch astern is lower in relation to the pitch ahead than ill the case of the aerofoil screw. We have observed this in practice on carefully measured trials and, at any rate in our context, it is a fact. I t is interesting to note that where propellers are designed for full power absorption at low speeds (0-2 knots), each power has an associated optimum diameter and hence an associated pitch and revolutions, to give the maximum pull per 100 h.p. We know, before entering standard series charts, what pitchdiameter ratio will give these optimum results in a tug, that is to say, what the diameter should be for a given power and what tile gear ratio should be. Calculations largely serve to check chosen characteristics. Tug-propeller design is a fascinating subject and doing a considerable amount of tug design, we find it profitable to do our own detailed work and, in fact, to introduce original variations in design. On occasion, it has been found possible to produce as much as 4 per cent increase in thrust over, for instance, the standard Troost-type screws without any increase in power by 'varying the blade outline. All single-screw tugs of this type are fitted with a patent rudder which, as mentioned earlier, gives an increase in thrust, due to a contrarotation influence on the screw race. This rudder has subsidiary advantages as it is cheap, enormously strong, and gives a high lift coefficient at small angles of incidence. I t consists of a streamline bulb rudder post of high thickness ratio, the stock being as near tile tail of the bulb as possible and the fiat trailing rudder being used mainly to stimulate circulation, the lift when going ahead coming almost entirely from the bulb. This rudder has the great advantage t h a t its rotational energy recovery capabilities are high at high values at slip and therefore large angles of intake incidence. We have found that the normal Hydroconic tug fitted with a bulb rudder is capable of equalling or bettering the towing abilities of similar vessels fitted with a Kort-nozzle rudder in tile context of given free-running characteristics, but naturally the maneuverability astern is not nearly as good. Where a fixed nozzle is fitted, we consider that a "drooped leading edge" to the bottom section of the nozzle may be necessary in

MODERN

TUG DESIGN

a hull with strong buttock flow characteristics, as the inflow incidence of the lowermost section is apt to be unduly high, possibly leading to some incipient breakdown of flow. Regarding the area of rudders, I agree with the author t h a t 6-61/~ per cent profile ratio gives a maneuverable harbor tug. We prefer to be cautious in reducing this figure, even for seagoing tugs, but the fixed post m a y be included, where a bulb rudder is fitted. In the case of twin-screw vessels, we always use twin rudders and we place these inboard of the screws. We fit large tip plates to the rudders which are usually of a symmetrical NACA type section. By careful attention to the afterbody, and to all appendages, we.have found-itpossible to produce twin-screw tugs of quite outstanding maneuverability, which, for instance, can steer against the thrust of one screw with 5-10 deg of helm only. This type of tug has a definite place in the picture considering the diffficult maneuvers normally m e t with in handling big ships in canals and n a r r o w rivers. In conclusion, I should like to thank the author f o r the amount of work he has put into this excellent paper and for his interest in what I. feel to be a fascinating, rewarding, and neglected field of naval architecture. M y differences of opinion outlined in the foregoing do not constitute a disagreement with any part of the paper, they merely detail a design approach to the tugboat problem rather, different from either European or American practice. DR. J. F. LEATHARD, Foreign Affiliate Associate Member: The paper has been read with a great deal of interest, and the following comments are concerned primarily with the approach to propeller design for tugs. I t has been found t h a t the use of the ~ -= a charts represents the simplest and most direct approach to the problem of the design of bollard or towing propellers while the KT -- KQ charts areuseful for estimating the freerunning speed of the vessel. By mathematical analysis of systematic series data for a particular propeller type, for example, the Dutch T a n k B.4.40, it is possible to find the pitch ratio which will give the m a x i m u m ahead pull for any given values of diameter and engine torque. There is, therefore, a corresponding o p t i m u m value for revolutions. By this means, it should be possible to eliminate some of the arithmetical work proposed in Appendix 1 of the paper. I t is interesting to note t h a t some considerable deviation from the o p t i m u m pitch ratio does not produce an appreciable difference i n t h e bollard pull.

423

In a similar manner it is also possible to approximate to the o p t i m u m pitch ratio for a given screw series to ensure t h a t the free-running speed of the tug will be the highest in association with a given minimum bollard pull. I t cannot be emphasized sufficiently that a propeller m a y be designed for only one forward speed at which m a x i m u m power will be absorbed at the designed revolutions. Once this forward speed is departed f r o m , either the propeller will be required to absorb higher torque at the given maxim u m revolutions, or for normal m a x i m u m torque, the revolutions will have t o be increased. The majority of modern tugs in the United Kingdom are diesel propelled and the problem of obtaining both satisfactory bollard pull and ahead speed m a y be approached in one of three ways: (a) The use of a multispeed gearbox between engine and propeller. (b) The fitting of a controllable-pitch propeller. (c) The use of acceptable ratings for the engine different from the continuous rating. Condition (c) is undoubtedly the simplest approach. Consider a propeller designed for a given forward speed, say, 4 knots. If revolutions are maintained constant on the bollard, the torque will have to rise and it will be found t h a t it will! correspond very closely to the value for the 1 2 - h r (10 per cent overload) ratingof,the engine. " On the other hand, when the tug is free running, unless revolutions are allowed, to increase, the power absorbed will be relatively small with the torque considerably below t h a t corresponding to the continuous rating of the engine. • Several engine builders, however, p e r m i t - a certain percentage overspeed when free running provided the absorbed power is within certain limits. By these means, it is possible to design a propeller which will give both. high bollard pull and reasonable free-running speed. The question of section shape in relation to the astern pull is of importance and the writer c a n n o t agree with the suggestion t h a t segmental sections are better than aerofoil sections in this respect. I t is usually found t h a t the no-lift angle of a segmental section having given thickness ratio is: higher than the corresponding no-lift angle for a normal aerofoil section. I t m a y be argued t h a t the associated difference in eenterline camber ratios should be reflected when going astern in a reduction of the effective astern pitch for a segmental section as compared with a corresponding aerofoil section. I t would therefore be anticipated t h a t f o r g i v e n revolutions, the astern pull for the: segmental screw would be less than for the aerofoil screw. The use of symmetrical sections is, of course, the best proposition and it is s u b m i t t e d

424

MODERN

TUG DESIGN

that the performance would decrease in accordance with the following order--symmetrical, aerofoil, segmental. The question of blade thickness and propeller material is also of considerable importance, and frGm analysis of certain trial data, it has been found that the influence of thickness is considerably less on the bollard than free running, while the use of cast iron in place of manganese bronze, for instance, produces a considerable loss in efficiency while free running. The author puts forward a plea that further information should be given by diesel-engine builders concerning the performance of their products. The writer would like to emphasize that the provision of such data is of considerable importance, especially where trial analysis is being attempted. For instance, in m a n y cases torsionmeters are not fitted to the shafting and an indication of power m a y be obtained from enginetemperature readings--provided test-bed data for given percentage maximum torques at given percentage maximum R P M are provided b y the engine builder. Finally, some comments should be made concerning the average principal particulars given in Figs. 1 and 2. The writer has analysed details of over 30 European tugs, most of them built in the United Kingdom, and some differences are noted as compared with the author's diagrams. For instance, the curve of length against shaft horsepower is found to lie somewhere between the lines corresponding to British and American practice and is approximately parallel to the latter line. The graph for cubic number against shaft horsepower is practically coincident with that corresponding to American practice up to a shaft horsepower of about 1000. L/B ratios agree exactly with American practice over the full range of lengths shown in the chart, while L/D ratios correspond closely to the American figures up to L B P = 80 ft and then begin to follow the British line up to L B P = 150 ft. I t must be stressed, however, that it is extremely difficult to draw mean lines through the data available since there is considerable scatter of the spots, and only a very approximate indication m a y be obtained of normal trends. MR. DWIGHT S. SIMPSON, Member: I t is a pleasure to study such a paper as this. The author has assembled and well organized more information on tugboat design than can be found in any other compendium. I t is with hestitation that these few additional remarks are offered. We have heard more or less of the Hydroconic hull for several years now. This appears to be a

trade name for a well-designed double-chine hull form which has been used for m a n y years. M y own first major design of this type was built in 1938-39. While an extremely good hull of the double-chine type can be developed, I doubt if it can be made better than an equally well-designed molded form. The economics of the design depend largely on the yard in which it is built. There are m a n y small yards unequipped to bend frames or to roll and furnace plates, depending largely on burning and welding equipment. As such a yard will have a very small overhead it is reasonable to expect a considerable saving on a "straight frame" hull built in such a y a r d - - w h i c h cannot build any other kind. A few years back two designs were prepared for the same ship, one molded hull and one doublechine hull. Size, arrangement, and equipment were exactly alike. The time element threw the contract in a well-equipped yard and on a two ship contract of over half a million dollars the difference in cost of the two hull constructions was $10,000 --less than 2 per cent. As the hulls in this case amounted to about a quarter of the contract there would have been a saving of only about 8 per cent in hull costs. The use of the straight-frame form does give a greater choice of builders and in busy times might save time in delivery and, in a small yard, probably 10 per cent in cost. The Power Problem. The writer is aware of the fact that tugs, today, are bought and their services sold on the basis of horsepower. I t is, however, a question as to how many of these overpowered monsters are making full use of their horsepower and if the owner is getting his m o n e y ' s worth in performance achieved. The author suggests that one look at the lines in order to establish the largest propeller t h a t m a y be used. I t is here respectfully offered that one first look at the propeller to establish the largest diameter that should be used; then finish the lines. In many cases the actual draft of the tug is of slight importance and when it is there is a possibility that a smaller engine properly applied will deliver all the thrust that the limited propeller can absorb. A recent investigation produced the following figures for an engine of 700 bhp with reduction gear and for a towing speed of 4.5 knots:

Propeller diam 6 ft 10 in. 8 ft 0 in. 9 ft 10 in.

Propeller rpm 230 176 130

Develop thrust, tons 8.6 9.35 10.6

Per cent increase "8".'7 23.1

MODERN TABLE 5 C o n d i t i o n No. Prop. d i a m Free ( I L K ) . . . . . . . . . Towing 3.5K ....... Bollard . . . . . . . . . . . .

SHP 700 595 545

TUG DESIGN

425

COMPARISON OF D R I V E SYSTEMS

I 6 ft 5 in. Thrust, tons 4.9 5.8 6.6

SHP 700 700 660

Further, if we were limited to the 8-ft propeller and installed a 1200-bhp engine, we would use about 184 r p m and develop a thrust of 11 tons. This gain of about 19 per cent in thrust seems scarcely worth the 60 per cent additional investm e n t in the power plant and its accompanying fuel and maintenance bills. The controllable-pitch propeller improves the over-all performance considerably, providing more suitable pitch for varying conditions. True, it m a y not be the most efficient design for any given condition b u t it has a higher average efficiency than the fixed-blade propeller which can be designed for only one condition. With it our 700bhp engine again more nearly equals the 1200 bhp not so equipped. However the C P propeller adds considerably to the cost of the propulsion plant and therefore the following addition to the author's list of propulsion systems is suggested. The multispeed gear as developed b y Mr. F. Suberkrub and the G e r m a n firm of Renk has m a n y installations in European tugs, trawlers and icebreakers but has received practically no attention in this country. This gear is designed to t r a n s m i t engine power to the propeller through a choice of speed reductions, permitting the engine to develop its full R P M , power, and torque under the differing conditions of towing and free route. Usually two speeds are provided and sometimes three, especially when two engines are connected through the gear to one shaft. T h e propeller is solid, and since most modern engines are fitted with a reduction gear, with or without a reverse mechanism the additional cost of the multispeed gear is a relatively small sum. L e t us see what it accomplishes. I n considering recently the modernization of a trawler, in which the diameter of the propeller was very definitely limited to 6 ft 5 in. the figures in T a b l e 5 were obtained for a 700-bhp engine. In the table Condition I considers the engine with a standard reduction gear. Condition I I involves a two-speed gear. I t will be seen t h a t the two-speed gear gives an increase of about 12 per cent in towing and 13.5 in bollard pull.

II 6 ft 5 in. Thrust, tons 4.9 6.5 7.5

SHP 700 652 630

III 6 ft 0 in. Thrust, tons 4.9 7.5 9.3

Condition I I I indicates the results to be expected with the same engine and its standard single-reduction gear and having the propeller fitted with a K o r t nozzle. Here the predicted increase over Condition I (the same engine and gear with open propellers) is 29.5 per cent at towing speed and 41 per cent more bollard pull. These are the customary predictions as the author mentions. I believe these percentages were closely achieved on the Mississippi River towboats where the K o r t nozzle is practically standard equipment. During the past few years the K o r t nozzle has been extensively used as a rudder in addition to

its customary functibn and is said to greatly improve maneuvering ability both ahead and astern. In its fixed form it is an inexpensive means of greatly increasing propeller thrust. As a rudder considerable alteration to an existing hull is required but in a new design the additional expense is very small. In the opinion of the writer neither the multispeed gear nor the K o r t nozzle should be overlooked b y progressive owners in search of top performance. In closing let me again express thanks to the author for bringing to the tugboat the attention it deserves. MR. PETER M. K I M O N , Associate Member: The author has presented a comprehensive paper on the over-all design of tugboats. This paper has a_ wealth of information heretofore not available to the tugboat designer. I t is of great value to the student because it covers all the fundamental phases of the design and presents t h e m in a methodical and analytical way usually found in a textbook. M y comments are directed to the author's discussion of propeller design with particular e m phasis on bollard pull. Considering first the case of constant power and_ adopting the author's symbols of Appendix 1 pn2D 4 =

T

KT

Q

KQD

zt26

M O D E R N .TUG D E S I G N 3.1

J

I

Taylor - 5choen herr 4 Blades-MWR=0.20 /

~-------~..~~. 3.0

2.g

--1 0

2.8

\

ea 2.7

.®

Maximum Pifch Ra~io ~or Less fhan 2 Per Cenf Loss in Maximum 0bfainable Bollard Pull:

\

"%

~

~

J

~ x

"%%"

H "

(4 BL- MWR=O.PO

0.81

~oy.lor'-Schoenherr, .~4 BL- MW R= 0.25 0.775 13 BL- MWR =0.40 O.G9 f B a r = o.5o Gown 3 Blades ~Bar = 0.05 , l B a r = O.BO

2.4

Z.3 0.3

0.4

0.5

O.G

. FIG. 39

0.GG5

0.-/8 0.-/5

O.B o.g Pifch Rafio

0.7

O K~

D K~ . . . . . . . . . . . . .

I61

substituting 550 DHP/(27rn) for Q and eliminating n we have T = p'/~ (550~ ~/~Dv' DHPV, K r iSimpii.fying and substituting p = 1.9905 we obtain "

:

T = 24 • 80DV~ D H P 2/~ K ~K' / ~r"

......

[7 ]

.........

[8J

:From similar considerations n

\,)~p~p KoD5 _

Similar formulas based on the u -- a system of propeller cceffic[ents have been developed in the author's reference (32). As explained in the same reference because of the remaining thrust-deduction.factor 1 -- t ~ 0.965, the dcck-trial b~llard pull'will be

Thou,~rd

" =

I.O

I.I

12-

-%

1.3

1.4

1.5

H

0

VARIATIONOF I£T/I'~'02/~ AT ZEROJ WITHPITCHRATIo

or

r-

%

\

23.9DV~ D H P '/~ KK r j/.

....

[9]

Since D H P is constant, after the diameter has been d e c i d e d u p o n , only Kr/KQ~/3 needs t o . b e evaluated. The writer has prelSarea two plots of K r / K o '/3 at zero J versus pitch ratio, Figs. 39 and 40, of this discussion, of the Taylor, Troost and Gawn propeller series. Values for the Gawn propeller series were obtained from reference ~ and for the Taylor and Troost series from the author's references (31) and (37). It might be pointed cut that a certain amount of fairing was necessary to arrive at these curves. I t is realized that in the preliminary estimates of the design the pitch ratio m i g h t not be known. However, for a well-designed propeller the. pitch ratio should be only slightly larger .than the one corresponding to maximum bollard pull and therefore to maximum K r / K Q 2/~ .Acecrdingly a K r / K Q 2/~ value could be assumed, say 2 per cent less than maximum. It is also believed that these same K r / K o 2/~ curves might prove to be of some additional help to the designer as a check on the preliminary selection of propeller type and pitch. He might, for example, decide to reconsider his design towing i~ " E f f e c t of Pitc h a n d B l a d e W i d t h on P r o p e l l e r P e r f o r m a n c e , " by R. W. L. G a w n , T r a n s • I N A , - v o l . 95, 1953.

MODERN

TUG DESIGN

427

3.1

B- 3,50 ~

~

3.0 4

2.9 2.8

B-4.70 ....___

~

~

B -3.65

o

e) I-4

®

. ",.Q

"

~.~

TroosfB SeriesPropellers PifchRofio ~or Less +i~an2 PerCenf Loss~nMaximum

~

Maximum

Y 2.G__

w_

Obfa~nable Bollard Pull:

~

2.5__ B-4.70 B-3.50 B-3.G5

2.4

O.'f9

~

0.305

~

B-4.55

,

B- 4.-/0 B-3.50

0.'1"15

B-3.G5 2.3

0.3

0.4

0.5 FIG. 40

0.6

0.3

VARIATION OF

0.8 0.9 LO Pii'ch Rai'i% H KT/~Q 2/~ AT

speed for m a x i m u m efficiency, if the corresponding

Kr/KQ V' is considerably lower t h a n m a x i m u m . Considering now the case of c o n s t a n t torque, and b y looking at E q u a t i o n [6] of this discussion, we see t h a t similar curves could be developed of Kr/KQ at zero J versus pitch ratio, since torque and diameter are the constants in this case. However, i t appears t h a t the a u t h o r has presented in Fig. 13 just these curves in a s o m e w h a t different form. If we substitute in E q u a t i o n [6] Q =

60 X 550 D H P 2~rN

and assuming 0.955 D H P = B H P we have T =

5250 B H P 60Kr DN 207rKQ

P r e s u m a b l y then the second term (60Kr/2OIrKQ) corresponds to the T~ value of F i g . 13. Or it could be t h a t (60Kr)/(20rrKo) has been multiplied b y a t h r u s t deduction factor and in t h a t case T~ = ( 1 ~ - t) 60Kr 207rKQ F r o m the foregoing it would also appear t h a t this

13

I.Z

1.3

1.4

1.5

ZERO J W I T H P I T C H R A T I O

formula is .useful only to the propeller design with constant torque, since the B H P - N relationship m a y not be known at other torque values. Inasm u c h as I have not been able to obtain the a u t h o r ' s reference (39) I hope t h a t the a u t h o r will clarify this point. T h e a u t h o r enumerates the opinions of several authorities as to the desired bollard pall per S H P . Examining the case of c o n s t a n t S H P and returning to F o r m u l a [9] and r e a r r a n g i n g Tbo,~ra = 23.9D2/~.K~, /WJ DHP'/~ DHP I t can readily be seen t h a t the Tbo,~rJDHP ratio is dependent on D H P and if we assume a c o n s t a n t diameter Tbollard 1 D H P ~ D H P V3 . . . . . . . . . .

[10]

I t would appear then t h a t the higher the S H P the lower the bollard pull per S H P t h a t can be obtained. This reduction in bollard pull per S H P will become even greater because of c a v i t a t i o n considerations. An increase of blade area for examPle will cause" a reduction in Kr/Ko 2/~ value.

428

MODERN TUG DESIGN

•In concluding I may suggest that Equations [6], [7] and [8] could be used in place of the corresponding formulas of the author in Appendix 1, since they are based on Kr, Ko, J rather than Ct, Cq systems and are more directly applicable to the available propeller series charts. MR. ULYSSES A. POURNARAS, Associate •femher: This paper on tugboat design is distinguished by the general approach to the subject employed by the author. The result of the simultaneous treatment of the three major classes of tugboats is that the operating requirements of each are masked. For example, it would appear that the speed-power consideration in selecting the hull form of a harbor tug is perhaps overemphasized. Should the designer concentrate on propeller-hull interaction effects at the lower speed range, the cost per pound of thrust available may be reduced to everyone's advantage. On the other hand, the free-running performance of the harbor tug appears to occupy a much more prominent position than it possibly deserves. Reduced costs, from the ship operator's point of view, depend on quickness of dispatch as well as on the tug's ability and efficiency in handling the job. In m a n y cases, a more powerful tug could very well do the job of two less powered tugs. Considering the maximum speed allowed by regulatory bodies in restricted harbor waters, it may be said that increased performance, depending to some extent on thrust available, is a better answer to modern harbor requirements. One also should keep in mind that the size of vessels requiring tug services increases as does their number. More powerful tugs eventually will be required. A very important design area mentioned in the paper is the arrangement of the towing facilities. Perhaps the ship operator is asking too much when, in some heavy-weather towing operations, he gets weary watching cables and lines snap and part. Perhaps, again, tug designers could improve this irritating situation, to say the least. In the performance analysis of propeller-driven aircraft, aeronautical engineers employ some techniques worthy of our consideration. The aeronautical take-off problem, when considered statically, resembles very much the towing operation of a tug. The techniques essentially consist of the solution, graphical in most cases, of the power available versus power required and thrust available versus thrust required energy-balance equations. A similar investigation could be carried for the tugboat at various rotational and advance propeller speeds. M a n y not-so-apparent over-all performance features of various propeller designs could be brought to light in this manner by comparative analyses.

In regard to the various rudder configurations discussed by the author, it is noted that the flapped rudder possesses a certain advantage because of its higher lift coefficient at the same rudder angle in the prestalling range, as compared with the other more conventional configurations. This advantage, however, may not be maintained at the higher rudder angles. Increased flap angles will in general reduce the positive angle stall-free range, while Reynolds number and roughness conditions also will affect both the magnitude of the obtainable lift coefficients and the extent of the stall-free range. It may be said, however, that the flapped-rudder configuration may offer the advantage of obtaining a given rudder force at a smaller angle of attack, or a larger rudder force at a given rudder area and angle. The expected increased maneuverability because of the transitional operation of the flap will be due to the higher rates of change of the rudder force, this because of the higher lift-curve slope, and therefore be limited to dynamic applications as it is correctly noted by the author. Steady-state conditions can improve only to the extent of the effect of the increased rudder force--if such is attainable--because of the camber effect of the flap. The writer wishes to express his sincere congratulations to the author for the able presentation of this paper. CDR. O. A. TEMPLETON, USN, Member: The author emphasizes in his introduction that the present trend in tugs is to increase the power without change in the over-all length of the boat. This is certainly justified in large harbor tugs handling the much larger ships being built today and where length cannot be appreciably increased without sacrificing utility in tight spots. It is not an uncommon event in Hampton Roads to see as many as a dozen tugs of the 800 to 1200-SHP variety assisting in docking the new N a v y carriers, USS Forrestal and Saratoga. The additional power is being added in modern tugs to obtain additional towrope pull, not for additional free-running speed. If the temptation to employ all this added horsepower free running is resisted, much of the difficulty with hull form caused by high speed-length ratio can be avoided. Also the danger of freak stability problems running free at full speed can be reduced. I t is recommended that large harbor tugs be designed for a speed-length ratio not exceeding 1.25. The only reason that higher speed-length ratios are encountered in new high-powered tugs is that the owner or designer has m~/de the economic mistake of calling for use of full installed horsepower in the free-running condition.

MODERN

TUG DESIGN

2000 1800 1600 1400 ~200

~: I000 ,.o 800

6OO

4O0

200

o

/

2

4.

G 8 Vk KnoUts

tO

I~

14

FIG. 41 POWERVERSUSSPEED

Fig. 41, herewith, shows a curve of S H P versus speed in knots free running for the new N a v y Y T B design, column 3 of t h e author's Table 2. This tug will now have installed 1800 shp with a C R P propeller. I t can be seen t h a t at 1350 shp a speed of 12.5 knots is attained whereas at 1800 shp speed has only increased to 12.8 knots. Thus, for every mile traveled at 1800 shp the fuel cost is about 30 per cent higher 180012"51) 1~ X 12.~ --

100

since the specific fuel consumption at 1350 and 1800 shp would be approximately the same in lb/hp-hr. On a 10-mile run at 12.5 knots the tug will arrive just a little more than 1 rain later than when traveling at 12.8 knots. Maintenance will be greatly reduced when free-running power is limited to 75 per cent (1350 shp) of the installed SHP. In this 1800 hp tug the propeller or the controls, depending on the type of propulsion, should be designed to limit the power available free running to about 75 per cent of the installed power, since the operator will most likely use all the power available and we would like to avoid this uneconomical practice. Free-running power can be limited in any of

429

the diesel-propulsion systems without any extra equipment or installation expense. In a diesel reverse-reduction-gear installation for this tug, with a propeller designed for full-power towing at 7 knots, the propeller will not absorb more than about 70 per cent of the installed power running free. Thus, we have automatically limited freerunning power. With a reverse-reduction-gear installation the propeller should be designed for the highest towing speed required (say 7 to 8 knots). A design for 4 knots, the lower end of the 4 to 8-knot range' recommended by the author, should not be used with a reverse-reduction-gear diesel installation since towrope pull above that speed will be limited and the m a x i m u m possible free route speed would be one corresponding to a speed-length ratio of about 1.0. The m a x i m u m free route power with a geared diesel C R P propeller installation can be limited b y the selection of the m a x i m u m propeller pitch. With this installation full power and 100 per cent at bollard pull might correspond to a pitch ratio of 0.70; whereas, the necessary pitch ratio for full power free route would be about 1.10. The pitch ratio corresponding to 75 per cent at 100 per cent R P M free route would be less, say 1.00. The propeller could be designed for a m a x i m u m pitch ratio of 1.00, thus limiting the free route power without any limit on towing capability. Free route power with a diesel-electric installation can be similarly limited b y providing less field weakening on the propulsion motor t h a n t h a t corresponding to 100 per cent power free route. Machinery Selection. T h e author appears to favor a diesel-electricpropulsion system, and has shown almost exclusively diesel-electric tugs in his Table 2. I t is suggested t h a t at least one modern C R P and reverse-reduction-gear tug be added to the table. Dalzell's new C R P tug, Dalzell III, developed over 50,000 lb bollard .pull on t r i a l s - higher than t h a t of any of the tugs listed. T h e diesel-electric tug is extensively used today because until recently there was no equal in maneuverability, reliability, and ability to deliver m a x i m u m thrust at all speeds. However, for a number of reasons diesel-electric propulsion will be used less frequently on future tugs just as it is rarely used on other ships today. Additional propulsion plants are available t o d a y which have all the advantages the author lists for diesel electric, except the availability of propulsion generator power for auxiliary uses, and are much lower in cost. Diesel-electric propulsion for an 1800-shp tug would cost approximately $150,000 more than diesel with reverse reduction gear or C R P propeller. Maintenance cost of the diesel-electric tug is certainly higher than for the diesel reverse-

450

MODERN TUG DESIGN

reduction-gear tug and is also higher than for the is increased at 100 per cent R P M until maximum C R P tug where a reliable, proven design o f , C R P B M E P or maximum pitch is reached whichever propeller is used. Owners who are using C R P pro- occurs first. (In the free-route condition maxipellers of a design already "debugged" after years mum pitch would be reached first.) In moving of service in earlier applications, have mainte- the control lever astern, the same control sequence nance ccsts approaching those of geared diesel is followed. The hydraulic controls of the protugs. peller itself and the remote controls are considered The author indicates that the main disad- rugged and reliable. vantage of the controllable-pitch propeller is its As the author has stated the torque converter poor ability to deliver astern thrust. Here the with a reversing element in conjunction with a difficulty is the same as exists with fixed-pitch reduction gear can be used in lieu of the reverse wheels. T h a t is "off the shelf" C R P propellers reduction gear. However, the ahead efficiency of designed for free-running are used. This causes a this torque converter is about 92 per cent s~ that large sacrifice in bollard pull both ahead and an over-all transmission efficiency of 90 per cent is astern. C R P propeller blades, including blade all that can be expected. Thus, the main adsections, for a harbor tug should always be de- vantage over diesel electric would be in first cost signed for dead pull ahead with minimum sacrifice and maintenance rather than efficiency. General of astern thrust. This can result in about 20 per Motors has developed a Witchita reversing clutch cent improvement in dead pull astern thrust and which has been evaluated on a Great Lakes Tow10 per cent increase in dead pull ahead thrust with ing Company tug at Cleveland. This clutch with very little loss in free route efficiency. a conventional reduction gear can be used in lieu Considering that the diesel-electric propulsion of a reverse reduction gear. system has approximately 10 per cent lower transSo long as the control features of a diesel-elecmission efficiency, with the same installed engines tric tug are provided, a cheaper tug in both initial the reverse-reduction-geared or C R P propeller tug and operating cost is available which will meet all should give a thrust equal to the diesel-electric the requirements of the owner and operator. tug. Diesel-electric propulsion m a y show an adComplete control from a single bridge lever has vantage in special cases Where it is desired to have been available only on diesel-electric tugs in the large quantities of auxiliary power such as for an past. Now a single-lever bridge control, with electric towing machine. In any event several additional control stations as desired, can be propulsion systems should always be investigated provided on the diesel reverse-reduction-gear or with cost a major factor. diesel geared C R P tug. There is no difficulty in obtaining a reliable control system. A pneumatic MR. JAMES B. ROBERTSON, JR., Member: The system is usually cheapest and has the greatest author very rightly emphasizes the importance of flexibility in providing the desired control of all providing adequate stability and freeboard, watervariables. tight integrity of hull erections, and adequate A recommended control system for the C R P freeing-port area in the bulwarks. tug will be described since it is slightly more comI t is presumed that his statement concerning plex than for the reverse-reduction-gear drive. a revised Coast Guard formula for the required From the vertical or neutral position to 25 per G M for tugboats, proposed by Capt. C. P. cent travel of the control lever ahead, propeller M u r p h y refers to Capt. M u r p h y ' s remarks in dispitch is increased from zero to the pitch corre- cussion of reference (24) at the J a n u a r y 1954 New sponding to the recommended fuel-rack setting England Section meeting and published in the with the engine idling. From 25 to 75 per cent "1954 Transactions. His rather summary intertravel of the control lever ahead the engine speed pretation of these remarks is somewhat inacis increased from idle to full speed. Propeller curate. The formula referred to was not and is pitch is automatically positioned at each R P M to not the Coast Guard stability standard for tugmaintain the fuel-rack setting constant at that boats. I t is a means of approximating the GM to R P M regardless of whether towing or running limit heel to the deck edge under steady full power free route. Thus, a recommended line of B M E P at very slow speed of advance with towline leading versus R P M for the particular engine used can be athwartship. As indicated in the discussion in followed. In going from free route to a towing 1954, it had been tentatively used as a supplecondition without changing the control-lever set- mental consideration to the wind-heel criterion ting (constant R P M ) the pitch will automatically used by the Coast Guard. I t is now agreed that it decrease to maintain the same fuel-rack setting. does not give sufficient GM. Above 75 per cent travel of the control lever, pitch The problem of tug stability is a complex one.

MODERN

TUG DESIGN

While there h a v e been altogether too m a n y tug casualties indicating insufficient stability a n d / o r watertight integrity, it appears that a good m a n y of the older tugs m a y have been operating for years with less freeboard and associated G M than we now consider necessary. The skill of operating personnel h a s evidently been an important factor. In addition to the forces of wind, current, and wave and the heeling m o m e n t developed by rudder-towline interaction, tugs m a y be bodily dragged broadside b y the m o m e n t u m of heavy mass tows. The June 1956 Proceedings of the Merchant Marine Council describes a number of tugboat casualties a n d discusses their operational aspects in some detail. As that article indicates, diesel tugs are not required to be inspected b y the Coast Guard unless they are seagoing and o f over 300 gross tons. Modernization of former.steam tugs by dieselizing thus takes them out of inspection. Frequently the power is also substantially increased. Such a power increase, other things remaining equal, definitely increases the risk of capsizing. I t is recommended that any o w n e r considering such modernization have an inclining experiment performed as a basis for the assessment of stability after conversion. Because of the complex relationship of the associated elements, it is unlikely that any single simple formulation such as t h a t proposed b y the author can be a generally satisfactory tug stability index. However, such a simple formulation can certainly be very useful for preliminary assessment. The following comments relate to it so used. Since diesel-electric drive permits more effective power utilization at low speeds of advance, from the viewpoint of t h r u s t w h e n towing, it m a y be considered on a par with other forms of drive. If this is done, B H P m a y be used in lieu of SHP. M i n i m u m freeboard is frequently well abaft amidships ~nd not associated with full breadth. In extreme cases it m a y be a very poor index of righting-moment characteristics. I t is suggested t h a t a mean between the freeboard amidships and. the minimum freeboard is a better approximate index. If t h e foregoing substitutions are made, G M values given b y the author's form u l a appear to approximate minimum values found satisfactory f o r ocean service but to generally overestimate the minimum allowable values for operation on protected waters. This remark is intended to in no way deprecate.the use of a conservative standard for design purposes. In small vessels such as tugs it is all too easy for topside additions to reduce significantly the available G M and freeboard. The provision of a liberal design margin is good sense.

PROF. LAURENS TROOST, ~ I e m b e r :

431 This paper

is an important addition to the scant literature on the subject of tugboat design and will find extensive use b y naval architects interested in this subject. The following remarks are made in an endeavor to enhance the value of the paper: 1 Under the heading, "Main Propulsion Machinery," the author states t h a t with the slow turning steam engines the propeller pitch/diameter ratio can b e close to unity. I t should be made clear t h a t such a pitch ratio would be highly effective for top free-running speed, but detrimental to boUard pull. From this poin t of view a m a x i m u m pitch ratio of 0.75 for a wheel of 0150 disk area ratio seems advisable (see reference 32). 2 Under the same heading, several novel systems of propulsion are discussed. I should like to add one system t h a t is becoming increasingly popular in G e r m a n y and Belgium for trawlers and towboats; viz., multiple-gear couplings, through which one or two fast-running diesel engines operate one propeller Shaft with fixed-blade propeller. In a Sl~cial case, two 500-shp engines at 500 r p m are geared down to the propeller shaft as follows 96 (a) To 145 r p m for free-running speed at full power. (b) To 115 rpm for towing at. 31/~ knots at. full power and free running at 1 X 500 SHP. (c) To 93 r p m for towing at 3 knots at 1 X 500 S H P and for backing. This Sfiberkrfib system claims high flexibility and propulsive efficiency at a price considerably lower than diesel-electric and adjustable-pitch propeller systems. 3 Under the heading, "Propellers and Propeller Design," there is some discussion on the relative merits of Taylor (ogival sections), Robertson (symmetrical seci:ions), and Troost (aerofoil inner sections and ogival outer sections) wheels. There is a reference to A. J. C. Robertson's (25) and H. F. N0rdstr6m's work (20) on the reverse action Of propellers. (The Nordstr6m propellers are almost identical to those of Troost.) For a fair comparisgn of ahead and astern bgllard thrust it should not be overlooked t h a t the Robertson symmetrical section wheel makes 10 per cent more revolutions a{ equal D H P than the corresponding ogival-section wheel of equal pitch r a t i o = 0.75. For equality of R P M , the pitch ratio of the former should be corrected to 0.85 and the bollard-thrust values accordingly. Following values for Kt/Ko --- T D / ~ are found in Table 6 for the bollard thrust condition. t6 For particulars refer to Jahrbuch der Schiffbautechnischer Gessellschaft, 1954, p. 233.

432

MODERN

TABLE 6 K t / K q = T D / ¢ VALUES Ahead Ogival sections (OS) . . . . . . . . . . . . . . 9.21 Aerofoil sections (AS) . . . . . . . . . . . . . . 9.70 Symmetrical sections (SS) . . . . . . . . . . 8.13

TUG DESIGN

Astern 6.91 6.85 8.13

From the table we see that with OS the astern bollard thrust is 0.75 of the ahead thrust, with AS 0.71; with SS they are equal, of course. AS gives 5 per cent more ahead thrust than OS, and 13 per cent more than SS. As to the astern thrust, the AS and OS are equal within 1 per cent. They are inferior to the SS to the amount of 15 per cent. Since even for harbor tugs the ahead thrust is to be weighted heavier than the astern thrust, the cornbination of inner aerofoil sections with outer ogival sections seems to be indicated. In Europe, the great majority of modern tugs use this sort of propeller. This is supported by work of Conn,17 and of Gebers. is At free running speeds, the AS will also be 2 to 3 per cent more efficient than the OS and SS propellers. 4 The most serious disadvantage of the controllable-pitch propeller as compared to the fixedblade wheel, is quoted to be its po~)r ability to deliver astern thrust. By measures with respect to the initial pitch distribution of the controllablepitch propeller as recommended in reference (42) at the conclusion of the appendix, it is possible to improve the astern thrust very considerably without hampering the ahead performance. Careful design should make it possible to restrict the loss in kstern bollard thrust, compared to a fixed-blade propeller designed for towing speed, to within 6 per cent. 5 W{th regard to the author's discussion of the application of a Kort nozzle, mention should be made of the use of this nozzle as a highly efficient rudder in addition to its beneficial effect on towing-speed propulsive efficiency. There is a 70-ft diesel tug operating in the harbor of Rotterdrm with a turnable nozzle, the maneuverability of which, ahead, as well as stern, is second to no other tug in that region. 19 The tug has a MAN-Diesel engine of 325 shp at 325 rpm, directly coupled to a 5-ft two-bladed adjustable-pitch propeller placed within the Kort-nozzle rudder. The ahead bollard pull registered at 310 shp and 325 rpm is 13,650 lb or 44 lb/shp. MR. C. B. HORTON, JR., M e m b e r : The author has presented a stimulating and much needed review of the important aspects of tug design. I t is very true that this whole subject has been neg17 " B a c k i n g of Propellers," Trans. I n s t i t u t i o n of Engineers and S h i p b u i l d e r s in Scotland, 1934-35, p. 27. 18 S c h ~ r b a u , 1933, p. 235. 19 S c h i # e n IVerf, Dec. 5, 19,52, p. 560.

lected in the literature. The highly competitive situation in both the design and construction aspects of these vessels, and the empirical, attitudes of most owners are main causes of this situation. The author deserves congratulations for contributing to a more scientific approach. I t is not quite true as implied in this paper that many aspects of tug design are so vaguely understood by those actually designing and building tugs. I should like to clear up some of the doubts indicated concerning one subject, the Kort nozzle, together with flanking rudders, which are used for added maneuverability. The Kort nozzle, having been a patented device until fairly recently, is not well understood by those who have not been concerned with its design, testing and use. This is being remedied by extensive model testing and study in most of the major model basins of the world, in particular by the publications of systematic tests conducted by the Netherlands Ship Model Basin in Wagen!ngen and reported by Dr. van Manen. Soon it should be possible for any naval architect to use the information now being made available to evaluate, a Kort nozzle for any suitable application, with the same confidence as any propeller design. An evaluation of our own extensive work in this field, going back to 1936 and covering hundreds of applications to both tugs and river towboats, as well as other uses, shows without exception gains in performance obtained economically and, for tug type operation amounting to a saving of 20 to 40 per cent in horsepower. Mistakes have been made over the years in this work, leading to difficulties with maneuverability and maintenance on Kort-nozzle boats, but these mistakes have been recognized and corrected by further experience. In addition, recent scientific work and model testing have improved the performance of K o r t nozzles, especially in the higher speed region. This recent work has been proved in practice on river towboats which are the most powerful towing vessels ever constructed, 6000 hp on twin screws on 8 ft 9 in. draft. To summarize the present situation, on Kort nozzles for tugs there should be no doubt concerning the improvement in efficiency obtainable, without fear of undesirable side effects. The misgivings expressed in the paper about attachment of the nozzle to the hull are groundless. The Kort nozzle as applied is much stronger than any normal stern frame or skeg, and provides great protection for the propeller. The statements about reduced backing power, and lack of gain if modified nozzle profiles are used to increase backing power are both incorrect. Nozzle tugs consistently show improved back-

MODERN

TUG DESIGN

ing power, and Kort-nozzle river towboats, where backing power is a prime consideration, are the best backing screw boats we have been able to find. We always "modify" our K o r t nozzles to improve backing power because it does not involve appreciable loss in ahead performance. The suggestions t h a t the performance of the K o r t nozzle depends on close and impractical tip clearance is not borne out by experience. We have experimented widely with tip cl_~arance, and confirm the desirability of small tip clearance, but most of our experience has been with clearances t h a t are completely practical. We have found t h a t very close tip clearances of the order suggested by Dr. van Manen are practical also if sufficient care is taken in construction, and we are using them. An interesting point is t h a t no harm is done if tile clearance is reduced in service b y extreme bearing wear since the blade tips are harmlessly worn away. One error in past practice with K o r t nozzles on single-screw tugs was the use of the fixed K o r t nozzle without astern steering rudders. A tug so built loses the bias which gives a poor but predictable astern control to tile normal tug, and the erratic astern motion leads to an unhandy and sometimes dangerous vessel'. For single-screw tugs, either the movable or steering nozzle should be used, especially for the small sizes, or for a rugged and superior system the fixed nozzle with three rudders is best. Both are equal in efficiency despite appearances, due to the scientific study given to designing the two astern steering rudders ahead of the propeller as integral parts of the K o r t nozzle and making them act as very effective guide vanes. Such a tug offers a new kind of control, steering astern as well as ahead, superior in efficiency ahead a a d astern and offering the kind of control t h a t is necessary to allow the adoption of push towing on a large scale. I t is perhaps unfortunate t h a t the developments I have described m u s t remain in the unpublished category deplored by the author. Without some commercial incentive, however, the extensive and expensive development t h a t has resulted in the present successful K o r t nozzle would rarely be performed. A tug is now being developed which will serve as a demonstration. The general principles are available, considerable technical detail is published with more coming soon, and certainly no tug owner, designer, or builder can longer ignore performance and maneuverability advantages which can demonstrably be achieved b y the K o r t nozzle. CDR. RICHARDS T. MILLER, USN, Member: As a sometime naval architect it is always a pleasure to read a paper on the m a n y facets peculiar to

435

the design of a particular ship type. I t has been a special pleasure to read this excellent paper on the design of m y second love--tugs. Some m a y wonder why m u c h design effort should be expended on so prosaic a craft as a t u g - indeed m a n y of the tugs plying our water-ways today show every evidence of having "growed like T0psy." A careful analysis of their operating ledgers by the owners should sfiow the economic fallacy of the " T o p s y " approach. A review of the Coast G u a r d ' s casualty reports will convince anyone of the danger. M a n y who will concede the wisdom of designing before attempting to build a quarter to half million dollar ship will question tile lack of standardized designs. "A tug is a tug is it not?" The answer to this of course is, " N o ! " There are shiphandling tugs for New York h a r b o r - - a n d shiphandling tugs for the N a v y with reduced superstructure height to permit working under the overhangs of our great modern aircraft carriers. There are tugs handling coal barges t h a t have long deckhouses for roomy accommodations because they seldom tow over the stern; tugs handling car floats t h a t have skyscraper wheel houses so the master can see over the top of his cargo of freight cars; and, tugs frequenting canals and inland waterways t h a t have squat wheel houses for easy passage under the m a n y bridges. There are direct-reversing, controllable-pitch-propeller and diesel-electric tugs. You, the architect or builder propose a design, and tile owner would like something a little different. I vividly recall raising the stack in at least four increments on the drawings of the first tug design with which I was associated. T h e owner held fond memories of the old steam tugs of his first fleet, and I must admit there was no mistaking the house letter mounted on the stack of this early-modern diesel-electric deep sea tug. This is as it should be for variety lends spice to life. A world of standardized cars, houses, skyscrapers, ships or even tugs would imply standardized thinking b y a standardized people, no progress, and a dreary existence. There are basic principles, however, which apply to any good ship design regardless of variations in arrangements or details. In his very thorough discussions of design formulas and criteria for powering, stability and maneuverability in tug design, tile author has performed a fine service for our profession. With your indulgence I shall add a few comments of m y own.

As a proponent of low prismatic Coefficients for tug hulls, I was much impressed with the contour curves of residual-resistance coefficients for high displacement-length ratios t h a t were appended.

434

MODERN TUG DESIGN

The loss of superiority of a fine hull at the higher so attractive. There has been a movement to-~ speed/length ratios is much more apparent in ward the use of controllable-pitch propellers in these curves than in. Taylor's curves. This is some recent tug designs. M a n y successful apparticularly significant in the design of a modern plications of this type of propulsion have been high-powered tug whose owner is particularly in- made on European ships. Unfortunately m y perterested in high free route performance. I t is, of sonal contact with controllable-pitch propellers in. course, of less significance under the reduced the Mine Force has not been happy. Alteraspeeds of towing conditions when a full, free run tions currently being made to the propeller pitchof water to the propeller is a primary concern and control systems of minesweepers are expected to the finer form is preferable. The author's recom- • improve the situation, but the reliability of the mended compromise of a Cp of 0.57 to 0.60 is a propellers has yet to be proved. Tug operations good one. with a controllable-pitch propeller that I have obThe problem of air-drawing by the propeller served failed to exploit all of the advantages of is a very real one, particularly when backing down. these propellers. Pitch control was used simply Loss of available backing power can be as high as as a backing device. Pitch and engine R P M were 90 per cent in severe cases. Tests in a circulating not adjusted for optimum engine performance at water channel of a shallow-draft design with each condition of loading, nor was the necessary which I was associated vividly illustrated the effect data for such adjustment in evidence in the pilot as well as the air-bubble enshrouded propeller house. I might add that in the opinion of at that was the cause. Modification of the lines to least one ship operator of m y acquaintenance a give better cover of the propeller provided a cure. naval architect in each pilot house would be a I cannot concur too strongly with the author on prerequisite of such an ideal operation. the need for meeting the most stringent criteria of Regarding the use of flanking rudders on cons t a b i l i t y and reserve buoyance in tug designs. ventional tugs, I was associated with a qualitative Coast Guard casualty reports cite numerous cases model test that tried such a scheme. No signifof lost equipment and lost lives from tug disasters. icant improvement 'in backing maneuverabihty In nearly every instance it is the same story of a was shown. Although a straight course could be tug-tripping tow, a heavy list, a foundered hulk. maintained for a slightly longer period of time beOne has only to observe the decks-awash craft on fore the inevitable slump into a turn, once into the the intra-coastal waterway to wonder that the in- turn no amount of rudder would straighten the cidence of these accidents is not greater! course. The results should not b e construed to To the discussion of hull structure and scantdepreciate the possibilities of a ship-handling or lings it is of interest to add a note on the difference general-purpose harbor tug with an afterbody in N a v y .and civilian practice in keel design. specifically designed t o incorporate both a Kort N a v y ships will not ground. Also, the Navy, nozzle and flanking rudders in either a single or being concerned with docking many of its ships, is twin/screw configuration. Based upon the outparticularly cognizant o f the !ncompatability of standing maneuvering and powering characterisbar keels and long keel-block-cap life. Ergo tics of r i v e r towboats so fitted, such an arrangeevery N a v y designed tug I have studied has a ment should be investigated further. flat-plate keel instead of the bar keel prevalent in The author is very kind to add m y name to Mr. commercial craft. Benson's method of computing available propeller In the matter of powering, several items are thrust up to 100 per cent slip. M y rearrangement brought to mind by the author's general discus- of the m e t h o d to fit Taylor's notation and availsion. Direct-connected air-reversing diesels are able propeller characteristic curves was done a economical to install and relatively simple to number of years ago for m y first mentor, Mr. maintain. Hence, they offer an attractive plant Richard Cook, in the gratification of professional for the operator predominantly in the long haul (as distinguished from academic) naval architectowing business; but, their reversing response is ture. The method h a s been in continuous use by much too slow for a ship-handling tug. The N a v y Mr. Cook since that time. has employed such tugs in ship-handling of necesIn conclusion I Wish to thank the author for the sity, and has not a few dented sides to show for it. time and effort that he put into this mgst interestTo m y mind a diesel-electric plant offers the finest ing and valuable paper. control and, with two prime movers, the most flexible power arrangement available for tug operaMR. JAMES J. TURNER, 2° Visitor: I t is a privition. Maintenance, however, does require ex- lege to comment on such a fine paper. The few perienced mechanics. Therefore, operations away 20 Bureau of Ships, Code 436, D e p a r t m e n t of the N a v y , W a s h i n g from well-established ship repair facilities are not ton, D. C.

MODERN

TUG

remarks which I have are limited to tbe machinery portion of the paper. The author states that the tarclue converter shows promise of incorporating all the advantages o.f diesel-electric drive without the heavy transmission losses associated therewith. Later i~ the paper the author gives an over-all transmission efficiency of 95 per cent for a transmission system using a torque converter with a c~nventional gear. This is a significant gain over the diesel-electric plant. If all the speed reduction was taken in the torque converter itself the efficiency obtained would approach that of the diesel-electric plant. This would indicate that the torque converter could be used to its best advantage where it replaced a hydraulic or electric coupling (with a maximum reduction ratio of about 1.2). In the author's discussion of novel systems of propulsion, he states that the particular advantage of the free-piston gas generator would be its low weight-to-horsepower ratio. I believe a more imp2rtant advantage of this system would be its ability to give high thrust at low per cent of rated R P M and still give rated H P at any propeller RPM. Its propeller-thrust capabilities are close to that of a diesel-electric plant. In tile author's discussion of controllable-pitch propeller, he lists as a disadvantage its poor ability to deliver astern thrust; however, from a crashstop-reach consideration the ability to get astern thrust quicker may overcome its poor ability to deliver astern thrust. I agree with the author that its poor ability to deliver astern thrust could be a serious objection to the installation of a controllable-pitch propeller in a harbor tug but would like to point out that the U. S. N a v y specifies a controllable-pitch 15ropellet for its latest harbor tug design. The specification requirements for propeller thrust indicate that the limitation is fully recognized. M y personal preference for a harbor tug would be a combination of torque converter, reverse reduction gear and a fixed-pitch propeller. MR. WILLIAM B. MORGAN, Associate Member: The author has presented a very interesting paper on the design and propulsion of tugs. The writer has a few comments and questions about propeller design as discussed in the paper. Fig. 13 is used to estimate the propeller performance at bollard pull. Are the results obtained from this figure and the equations applicable to both constant-power and constant-torque installations? Assuming that T, -- K t / K q and that Tr = Kq, where Kt and Kq are at J = O, an equation for bollard pull can be derived from the equations given in Appendix 1

DESIGN

455

I3,

0.I3

ii

~To

O. I I

"~ 9

fO.50--

/

0,09 ~-

c 13

0.08

7 #

0.07

~

.

~ ///

~4 3

o.o~

0.04

/

0.03

t

0.5

0.G

FIG. 42

07l

0.8

0.9

. 1.0

I,I

1.2

Pifch / Diome÷er Ra~tio

1.3

0.01

1.4

TROOST 3 - B L A D E D B - S E R I E S P R O P E L L E R PERFORMANCE AT BOLLARD P U L L

{ D H P F ~ = D H P C~ X 550 T = \ nD / nDCq X 27ra = 60 X 550 D H P K, 2 R P M D Kq 7r when Kt and Kq are at J = 0, this becomes the equation for bollard pull Bollard pull =

5252 DHP0 Y¢. . . . . [11] RPMoD

Likewise, the R P M at bollard pull when constant torque is being developed can be derived from Appendix 1

60 ( DI-II'oV'

R P M = 60n = - D ~ \ u - ~ /

=

( 33000 DHPo V'

60 \ 2 ~ R P - M ~ 0 ~ - K ~ )

"" [121

and when Kq is at J = 0 /2700 X D H P 0 ) ' / ' R P M (bollard) = 60~. K-P1V~ ~ ] From these results it would seem that the equations given in Fig. 13 can be strictly applied only to constant-torque installations. Also, it would seem that B H P is assumed to be equal to D H P unless there is a correction factor applied to Tc and Tr. I t also would be interesting to know if Fig.

436

MODERN

TUG DESIGN

14

0.14

13

13

0.13

12

12

r-I0

~

0.12.-

II

0.1l

~I0

Expc~nded AreaRa~ib

0. I3 0.1~

\ v

0.11

0.I0

9

\

Expanded

Area Ra÷io

/

/

#

0.09

,---

0.45--...

c 9 2

o

to s

o.os ~

~ 7

o

L9

F

7

0.07 ~

~

tr

G

/-",/

t-

2i

iI

0.S

0.6

FIG. 43

0.'/

0.8 0.9 1.0 1.1 1.2 P{tch/Diaroe~er Ratio

1.3

o.o5

si '

0.04

3

0.03

z f

0.02

I 0.5

0.01 1.4

o.o, L

0.03

~T~ 0.6

FIG. 4 4

0.7

0.02 0.8 0.9 1.0 1.1 1.2 Pitch / Diame÷er Ralqo

1.3

0.01 1.4

TROOST 5-BLADED B-SERIES PROPELLER PERFORMANCE AT BOLLARD PULL

TROOST 4-BLADED B-SERIES PROPELLER PERFORMANCE AT BOLLARD PULL

13 was derived from open-water tests of propellers or from actual bollard pull tests. Results in reference (7) seem to indicate t h a t estimates of bollard pull from open-water tests are up to 10 per cent too high. When constant power is being developed, the bollard-pull equation remains the same except t h a t the R P M is at the bollard-pull condition instead of at the design condition. Using the equation in Appendix 1, the bollard-pull R P M for constant power is given b y R P M (bollard) = 6 0 ( ~

- - ~ t t~ l - / J - - -/\

'/~

= 6o(55o \ ~ p ~D ¢ ~ ]PoV, ..

[13]

Diagrams h a v e been prepared, similar to Fig. 13, for 3, 4 and 5-bladed Troost propellers with different expanded-area ratios, Figs. 42, 43 and 44 of this discussion. When constant torque is available, Equations [11] and [12] are to be used and when constant power is available Equations [11] and [13]. For constant power the R P M for Equation [11] must first be calculated from Equation [13]. These diagrams are replots of data given in reference (31) and since they" are based on open-water tests, they tend to overesti-

mate the bollard pull a few per cent (up to 10 per cent). T h e author gives a brief discussion on the astern performance of different propellers. For ahead efficiency a propeller should develop most of its thrust from camber. When a propeller with cambered sections is reversed in rotation to give thrust in the astern condition, the camber of the sections tends to decrease the astern thrust. To overcome this effect of camber, the sections m u s t be operated at very high angles of a t t a c k which results in a poor backing performance. This is the reason a Troost wheel gives less astern efficiency than a propeller with symmetrical sections. In the case of a controllable-pitch propeller, the blades are turned through a certain angle to given astern thrust instead of the rotation being reversed. N o t only is the camber in the wrong direction when the blades are turned but the radial pitch distribution is radically c h a n g e d . For example, a propeller blade w th a pitch ratio of 0.75 and constant radial pitch has an angle of 38.52 deg at the hub (0.3 radius), 18.83 deg at 0.7 radius, and 13.43 deg at the blade tip. When this blade is turned through a n angle of - 3 7 . 6 6 deg, the pitch ratio at the 0.7 radius is 0.75 in the astern direction and the angles of the blade are 0.86 deg at the hub, -- 18.83 deg at the 0.7 radius, and .--24.23 deg at the tip. From these results it

MODERN

TUG DESIGN

I

437

2.00 .

.oo

/

I

VKTowing FIo. 45

can be seen t h a t i n the astern condition, the angle of attack of the blade at the root is negative relative to the flow while at the tip it is very high. This poor distribution of angle of attack causes an increase in drag of the blade. For this reason the controllable-pitch propeller usually gives the poorest astern performance. However, the follow ing reference shows t h a t at bollard pull the controllable-pitch propeller m a y perform better than a fixed-pitch propeller 21 In reference to the K o r t nozzle, a recent article 22 discussed the use •of a rotating K o r t nozzle. This system h a s m a x i m u m power available in any direction and might be worth considering for tugboat application. The powers available and relative cost of this device are not known. The author states t h a t a 4-bladed propeller will excite less hull vibration than a 3-bladed propeller of equal diameter. The magnitude of the exciting force is important, but more important is whether the frequency of this force coincides with the natural frequency of the system (or of some com.ponent part). PROF. L. A. BAmR,-Mernber: The author prepared the groundwork for this paper while taking his graduate work at the University of Michigan and we are proud of his presentation before the Society. Obviously the writer's pertinent comments have been discussed with the author prior to publication b u t one or two suggestions might be added. An alternate plot to Fig. 12 is added as Fig. 45 since it proved useful in the past. T h e pounds pull per B H P for the tug can be estim a t e d b y the author's methods or approximated from the table for any tug power. In connection with modern form design Mr. 21 " C o n t r o l l a b l e - P i t c h Propellers," b y L. A. R u p p , Trans• S N A M E , vol. 56, 1968, pp. 272-358. 22 Maritime Reporter, A u g u s t 15, 1957.

T h o m a s D. Bowes has refined the run and counter in his fireboats and tugs to the i m p r o v e m e n t of T R H P and propeller efficiency, and I hope he will add to the discussion. The recent New York fireboat John D. McKean was relieved of serious stern vibration and the performance improved b y the addition of inexpensive vertical flow control fins designed b y the writer in 1955. Referring to Fig. 20, the author might add to his bibliography a paper given b y Mr. F r a n k Vibrans before the A S M E in Lotiisville, Ky., M a y 19-25, 1957 which amplifies diesel-engine performance at other than rated R P M . Another interesting article worth listing is b y T. E. Hannan entitled " T h e Pulling Power of Tugs," which was published in the August issue of Ship and Boat Builder 1957. PROF. HARRY BENFORD, 3/Iember: A year ago, in commenting on Professor Ridgely-Nevitt's trawler paper, Professor Owen pointed out t h a t the author had done his graduate study (some sixteen years before) under Society scholarship. This was rightfully held up as a good example of what m a y happen when we cast our bread upon the waters. I should like to point out t h a t Mr. Argyriadis also did his graduate study under Society scholarship. In point of fact, he was one of last year's recipients and wrote this informative and useful paper while still at school. This surely sets a new record for rate of return on investment and offers convincing proof of the Society's enlightened selfinterest in granting funds for educational purposes. No biographical information on the author appears among the footnotes, which is unfortunate because t h a t in itself is an interesting story. Mr. Argyriadis was born and raised in Greece. He came to Ann Arbor in 1948 to study naval archi-

438

MODERN

TUG DESIGN

tecture. His p a t h was not easy. He was inexperienced in the English language, he had to take on outside Work for financial support and his studies were interrupted to serve time in the Greek N a v y . Despite these handicaps, when he graduated in 1952 he stood third in his class of 21 which was an above average group and included two other eventual winners of Society scholarships. After about a year's employment in the field, Mr. Argyriadis was required to serve in the U. S. Army. This proved a boon because he not only found himself doing naval architecture for the Transportation Board at Fort Eustis b u t also was granted U. S. citizenship without further delay. Upon completion of his milita~7 service a Society scholarship allowed him to enter graduate school and the rest of the story is evidentl I am sure the other members of the Society are as gratified as I to have played a part in advancing Mr. Argyriadis' formal training. We wish him good luck and continued success.

the stern frame. The steering gear must be of the quick-acting ram type operated b y releasing the exhaust and must give 45 deg on the rudder. Some of the guards are of wood with faee irons; others are channel bars integral with the h u l l - fore and aft preferred. The top of the stem is rounded by plate or casting to hold bow fenders. Another type of New York Central tug is for handling large carfloats short distances across the river from New York to New Jersey. This type must be powerful enough to handle 366-ft earfloats--1000 tons light and 2000 tons l o a d e d - - a n d to cut across tide to enter a slip and to back fast enough to avoid hitting the bridge for loading cars. The characteristics of this type tug are: Length 92 ft overall, beam 23 ft, depth 12 ft 6 in. Engine is noncondensing, 20 in. X 26 in. stroke, 400 ihp; propeller diameter 8 ft 6 in. b y 11 ft pitch; width of blade 33 in., weight 2950 Ib cast iron. A bar keel is fitted to prevent sliding when hauling floats from the bridge, assisted b y a hardMR. ANDREW J. BIRCH, Member: First it is over rudder at 45 deg to prevent the tow from important to determine the type of work to be drifting with the tide before getting turned around performed b y a tug. For New York Central so t h a t the toggle end of the carfloat will be facing tugs there are three such types. toward the rack and bridge on the other side of the In the first class is the shifting tug for moving river. Here again the pilot house m u s t be high and placing barges for loading and unloading at the enough above the top of the platform on the carpiers. The barge must be placed with doors op- float so t h a t the captain m a y have a clear view posite the cuts in the piers for gang planks. This all around. The engine m u s t respond quickly tug operates more going astern than ahead and from full ahead to full astern., m u s t act quickly to avoid damage to the barges The third New York Central type is designed around the pier and bulkhead. This type has for the long tow from the West Shore on the New characteristic measurements as follows: 80 ft Jersey side to Long Island and Staten Island. overall, 21 ft beam, 10 ft 6 in. depth. Engine is This tug has the same d u t y of putting carfloats in 18 in. X 24 in. stroke, 250 hp, 130 rpm. Pro- the racks and bridges at both ends of its run. peller diameter 7 ft 4 in., pitch 10 ft, hub 14 in. The principal characteristics are: Length 105 long b y 12 in. diam, width of blade 31 in., weight ft, beam 24 ft 6 in., depth 12 ft 6 in. Engine is 2650 lb. Bunkers 22 tons, water 12,600 gal, water compound, 20 in. X 40 in. X 26 in. stroke; steam consumption 800 gph. The engine is H P and pressure 135 psi developing 500 ihp, 165' psi deexhausts to atmosphere. The reason for this is veloping 750 ihp at 130 rpm. Propeller 8 ft 9 in. diam, 12 ft 3 in. pitch, hub 17t/2 in. diam, width t h a t the engine works mostly in mud and silt. The pilot house has to be high to see over the of blade 38 in., weight 4175 lb, cast iron. Bunkers tops of the barges, with the searchlight on top 31 tons, water tanks 16,000 gal, water consumpoperated from inside the house. The large bitt tion 1000 gph. shown in Fig. 11 of the paper on the forward deck Pilot house high enough to see over carfloat has proved to be dangerous. Tow ropes break platforms. This tug handles a carfloat on either and the crew members get hurt. On New York side and must be strong enough to stand the Central tugs side bitts are used to replace the strain without being crushed. For this condition centerline bitt. 3 or 4 web frames with strong beams are fitted to T h e rudder is made as large as possible with a the forward quarter. slight balance resting on the shoe of the stern frame. The pintle is as large as possible with a AUTHOR'S CLOSURE steel liner and a steel bushing in the s h o e - - b o t h arranged for easy removal. The stern frame DOROS ARGYRIADIS: The response of the disshould be scarphed just below the boss so that a cussers to this paper is very gratifying and the broken shoe can be replaced without dismantling author would like to thank all those who have

MODERN

TUG DESIGN

participated in it. C o n t r a r y to the limited information available in the technical literature on tugboat design, the number of persons qualified to discuss these unique boats is impressive and the discussion, I am sure, adds a considerable amount of very useful information for the interested designer and broadens the scope of this study. Several discussers have pointed out t h a t some of the approximate methods presented in the paper for the preliminary evaluation of the particulars a n d / o r performance of a tugboat can be further refined. While there is no doubt in the author's mind t h a t any one formula or any one approximation can be refined, one also should keep in mind t h a t most of the formulas and approximations presented in this paper are m e a n t to apply to the preliminary design of a tugboat only and should be used as such. I t i s realized t h a t the final characteristics of the boat m a y v a r y somewhat from the ones predicted in the beginning, but the author believes t h a t if judgment and -good sense are used in the preliminary stages of the design the formulas and approximations presented in the paper will prove useful to the designer. T h e writer wishes to t h a n k Mr. Tomalin for his lengthy and interesting discussion, but cannot agree with him t h a t tip clearances between propeller and hull are not i m p o r t a n t if one considers t h a t the worst wake distribution, a major factor in ship vibrations, is to be found close to the hull of a tug. Perhaps this difference of opinion arises from the fact t h a t Mr. Tomalin refers to experiments conducted on some 255-ft Coast Guard cutters. I t is entirely possible t h a t the wake distribution perpendicular to the plane of the propeller on these cutters is such as to make tip •clearance unimportant, but, unfortunately, tugboat forms have nothing in common with Coast Guard cutters. The wake distribution in tugboats is probably the worst to be found in any commercial boat. Those of us who have participated in the design of the 95-ft U. S. Coast Guard patrol boats are thoroughly familar with the careful and scientific way in which propeller design is approached b y the United States Coast Guard. Unfortunately, this does not apply to the commercial small-boat designer as well, and in m a n y cases the design of propellers for small or medium-sized commercial boats is rather crude. How else can one explain the installation of stock propellers to m a n y fishing •or other small commercial craft, or the poor bollard pull exhibited by some tugboats ? Mr. Tomalin compares the propeller design of tugboats, towboats and Coast Guard cutters. This is unfortunate, since the three different types

439

have very little in common, except size. While towboats have been using successfully propeller tunnels and K o r t nozzles due to their specific utilization, single-screw tugs have such a hull form as to make th~ experience gained with towboats practically useless. On the other hand, cutters are high-speed vessels in comparison with tugs and their form is so different that, again, no useful comparison between the two types can be made. Mr. Flipse's discussion revolves around a very interesting s u g j e c t - - t h e use of hydrofoil sections in rudder design. Unfortunately, the figures presented in Mr. Flipse's discussion concern hydrofoils operating in low wake velocities and the author is not sure t h a t a fully flapped hydrofoil rudder operating in the high wake velocities often found around tugboats is a practical answer to the tugboat steering problem. The 60 per cent figure as a per cent increase due to a fully flapped hydrofoil sounds good, but, as Mr. Flipse suggests, it is highly impractical. At the same time, as the discusser also points out, it sould be unreasonable to have the fully flapped hydrofcil rudder operating only from 18 deg port to 18 deg starboard. For these reasons it is believed that an adjustable-flap hydrofoil might be more desirable than the one suggested by Mr. Flipse. Mr. T a g g a r t seems to prefer tug resistance plots based on volumetric coefficient rather than dis.placement/length ratios. The use of different coefficients in certain plots is a m a t t e r t h a t need not be argued since different individuals prefer different coefficients. VI'he writer believes t h a t most n a v a l architects are far more familiar with displacement/length ratios than with volumetric coefficients, although s o m e hydrodynamicists have been switching to volumetric coefficient in the past few years. I t should be pointed out t h a t certain curves of the plots appearing in Appendix 3 of the paper have been refaired from the original David Taylor Model Basin data to m a k e them agree more closely with other tugboat test data, such as data received from Japan, the Netherlands a n d the Teddington National Physical L a b o r a t o r y in England. These changes appear mostly in the lower speed/length-ratio values. Mr. T a g g a r t suggests t h a t the formulas presented in this paper, such as the one for economic speed, should be refined b y including additional factors such as displacement/length ratio, and so on. However, as mentioned previously, some of the formulas presented in the paper are m e a n t to apply for preliminary design only. As such, it is believed t h a t the introduction of additional factors would only complicate matters. On the other hand, Mr. T a g g a r t is somewhat inconsist-

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ent in his discussion. At one point he is willing to refine one formula, and then he would like to simplify Admiral Simpson's beam formula by introducing approximations for the distance of the center of buoyancy below the waterline. Clearly, judgment should be used in all cases as to how far the simplification can and should be carried out. One of the assumptions that Mr. Taggart makes in trying to simplify Admiral Simpson's beam formula is that the m-coefficient used in the paper is 0.09 in all cases. This is not entirely correct, since m is equal to i/Cb where the coefficient i depends again on waterplane coefficient. The author would like to thank Mr. Taggart for his discussion and it is hoped that by cooperation among interested designers, a standard set of formulas and assumptions eventually will be used for the design of tugboats to the benefit of one and all concerned. Mr. Wheeler presents in his discussion a plot of steel weight parameters versus length for tugs. I t is believed that this applies to the European type of tugboat and the designer should be careful and use judgment in applying these data to American designs for the reasons explained in the main body of this paper. Mr. Wheeler also brings up the point of optimum LCB location and introduces a formula for its approximation. This formula seems to be of the experimental type and the author, having never used it, is not familiar with its capabilities and limitations. Some recent experiments performed at the University of Michigan's Naval T a n k by the writer on the influence of displacement and trim on resistance of conventional merchant marine hulls have shown that the LCB location has a decisive influence on the resistance of a ship. Nevertheless, no method of plotting could be found whereby the LCB location influence could be reduced to a useful plot or to a workable formula. The experiments were of a limited nature to be true, and it is hoped that eventually they will be expanded so that definite results m a y be announced at a later date. Nevertheless, from the original meager data, it appears , that the half angle of entrance of the waterline is much easier to handle and, in conjunction with other parameters, m a y result in a useful plot or formula which will enable the naval architect to predict the increase or decrease of resistance that m a y be expected from a difference in displacement or trim. The author wishes to thank Mr. Wasmund for his discussion on electric drives for tugboats as well as for the correction of an obvious mistake in the body of the paper, where it is stated erroneously that " . . . reversing is obtained by reversing the field of the motor" a practice fairly corn-

mon in Europe, but not in this country. Obviously, this sentence should read " . . . reversing is obtained by reversing the field of the generator." Captain Brown points out that statical and dynamical stability criteria should not be used interchangeably. While no one will argue the correctness of this point, the author believes that at least in the preliminary stages of the design one easy stability criterion should suffice to ensure the safety and seaworthiness .of the boat. Cross curves of stability are very useful, but a considerable amount of work is required to produce a set of these curves and the necessity of them at the early stages of the design is doubted. The author would wholeheartedly agree with Captain Brown that cross curves of stability are an absolute necessity in tugboat design. The only question is as to when these cross curves should be produced. In the opinion of the author, it would be useless to work on the cross curves of stability until the tugboat form has been finalized to the point where the designer can be sure that major changes will not occur. On the other hand, the formula given in the paper m a y be used at the preliminary stages of the design to compare three or four different tugboat hull types in order to make a decision as to which one has the best stability characteristics. To those of us who have had the opportunity to work on the YTB design, Captain Brown's comments are very gratifying. We believe we have taken great pains in producing a good tugboat hull form and I am sure that we are all anxiously awaiting to hear what the actual performance of this boat will be. While the author agrees with Captain Brown that a diesel-electric installation will normally have a higher first cost than any other rrethod of propulsion in a tugboat, it is felt that maintenance costs of electric motors and associated equipment is sometimes overemphasized. At the same time, the perfect control obtainable with an electric drive is, in the opinion of the author, superior to anything else available in the field and m a y justify a higher initial investment in m a n y cases. In view of the strong objections that Captain Brown and other N a v y personnel voice to the recommendation of introducing a bar keel whenever possible to minimize danger to the hull from grounding, the author would like to withdraw this recommendation. Mr. Taplin comments on several points of rudder design, and asks several questions of interest. In reply to his questions, the following can be said: (a) The reason for the inferiority in the astern operation of a flat-plate rudder with fishtails as compared to a streamlined rudder is the fact that in this condition the former presents a perpen-

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dicular rectangle to the water, thus disturbing the flow and reducing lift. (b) The evidence asked for t h a t fishtails are useful on tug rudders is overwhelming. Several of the tugboats operating in the New York H a r b o r area, for example, are equipped with fishtails and could not hope to do their work as well without them. (c) Practical and worth-while rudder angles depend, of course, on the type of the rudder employed and the speed of advance of the vessel. Angles from g5 to 40 deg are useful, provided conventional types of rudders are used. If a type (c) rudder with an adjustable flap is used, the maxim u m rudder angle can be reduced to 20 or 25 deg, without reducing the steering ability of the boat. Mr. Taplin points out correctly t h a t a flapped rudder will probably result in poor astern controllability. For this reason, the author believes t h a t it might be useful to provide an adjustableflap rudder with a lock of some type t h a t would keep the flap on the centerline when the vessel is backing. In this way, the full advantages of the adjustable-flap rudder when going ahead can be realized, while the known advantages of the symmetrical airfoil section also can be retained when the vessel is operating in the astern condition. T h e author is indebted to Dr. Corlett for his excellent discussion of a novel form of tugboats t h a t are apparently very successful abroad. I t is perhaps high time t h a t we in the United States start really designing small boats and try to o1~: rain the best possible hull for the job in question. I t is obvious t h a t Dr. Corlett and his associates have put a lot of thought into the design of the Hydroconic type of hull. Since this paper ,vas written, the author has had a chance to become more familiar with the Hydroconic concept and is now convinced t h a t the .principles and methods used in this type of design are such as to make H y droconic tugboats, in all probability, the best tugs in the world. Dr. Corlett's figures are v e r y interesting, especially the ones about the large reduction of labor costs in the Hydroeonic type of construction. Since the United States is indeed a high-labor-cost country, this; design m a y prove very'successful on this side e f the Atlantic as well. However, not only is this type suited to economic construction, but also through careful design, the chines themselves can actually produce appreciable hydrodynamic advantages through their potential effect on controlling the direction of water flow around the hull. Because of the existence of this influencing ability, it is possible, after some experience with the type, to obtain o p t i m u m flow patterns within close limits, mainly b y positioning the

441

chines. The ultimate acceptance of Hydroconic design principles, as applied to the design and construction of small vessels, seems inevitable. I t is only hoped t h a t a Hydroconic type of tugboat will soon be available on this side of the Atlantic so t h a t exact comparisons between the H y droconic and the conventional hulls can be made. The author must disagree with Dr. Corlett on one item; the astern pull characteristics of the Troost-type propeller as compared to the T a y l o r wheel. Of course, the exact performance of the different propellers will depend upon the design speed and the difference will decrease with decreasing design speed. Perhaps the best possible solution to the problem is the special design of a screw with a combination of inner airfoil sections and outer ogival sections. Dr. Leathard's discussion concerning propeller design methods is very interesting. There is no doubt t h a t b y mathematical analysis of systematic series data for a particular propeller type, a pitch ratio will be found t h a t will give m a x i m u m ahead pull for any given design speed. Unfortunately, and in order to obtain a complete picture of the situation, a lot of work needs to be done. Different types of propellers m u s t be considered and their characteristics for different design speeds must be plotted and compared before a final answer can be given. Although the use of a multispeed gear box might at first seem to complicate the design problem involved, further investigation will show t h a t the availability of at least two or perhaps three different R P M simplifies the design considerably by allowing not only a good bollard pull, b u t also a reasonable free-running speed, especially if the engine manufacturer permits a certain percentage of overspeed when running free. Dr. Leathard seems to disagree with some of the ratios plotted in Figs. 1 and 2. I t should be emphasized t h a t these figures were obtained b y reducing some 40 or 45 different Europe~/n tugboats to single points, and then trying to place a mean line through these points. I t might be t h a t the data obtained were mostly older than the data used b y Dr. Leathard, in which case it is very possible t h a t British practice differs much less t o d a y from the American practice than it did a few years back. On the other hand, the original points were so widely scattered, t h a t the mean curve could easily be pulled one way or another in accordance with the judgment of the person doing the plotting. Mr. Simpson's comments on the Hydroconic hull design have been answered ably b y Dr. Corlett. Mr. Simpson also suggests t h a t the design of the boat should be centered around the pro-

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peller and the author wholeheartedly agrees with his comments. In fact, Mr. Simpson's c o m m e n t that " . . . a smaller engine (at times) properly applied will deliver all the thrust t h a t the limited propeller can absorb" should be underlined and never forgotten by any tugboat designer. Tile authors is also thankful to Mr. Simpson for his discussion of the multispeed gear, a developm e n t used extensively in Europe, but unknown on this side of the Atlantic. Mr. Kimon's discussion centers around propeller design and bollard pull. I t is certainly true t h a t the higher the shaft horsepower, the lower the bollard pull per shaft horsepo~'er t h a t can be obtained, a fact t h a t can be clearly seen from Fig. 15 of the paper. The discusser's formulas [6], [7] and [8] can very well be used in place of the • corresponding ones presented in Appendix 1, but the choice is a difficult one to make and will depend entirely upon the propeller-series charts the designer chooses to use. There is no doubt t h a t formulas similar to the ones presented in Appendix 1 of the paper for Ct and C~ can be developed for other systems, and Mr. Kimon's contribution to the value of the paper is distinguished b y the fact t h a t he has given us some valuable formulas for the determination of bollard thrust and rpm b y using Kt and Kq coefficients. To clarify the Tc coefficient used in Fig. 13 of the paper, it should be stated t h a t it can be used only when constant torque is being developed, unless one is willing to use a trial-and-error method in establishing a B H P - n relationship. Furthermore, the T c v a l u e corresponds to the 3 X KdTrKq suggested b y Mr. Kimon and the curves have been obtained by observing and plotting actual boat data. Mr. Pournaras suggests t h a t tugboat design is not a scientific process and t h a t methods and means used in other industries should and could very well be employed in tug designs. His remarks are absolutely correct and the author would be more than h a p p y to see some time and m o n e y spent in this field of naval architecture for research and experimentation. The only question t h a t arises is the one of obtaining the necessary money and it is v e r y doubtful t h a t any investor would be willing to gamble his hard-earned cash on the v e r y marginal tugboat operation. Mr. Pournaras mentions t h a t the ship operator m a y be asking too much when, in some heavyweather towing operations, he gets weary watching cables and lines snap and part. An obvious solution to this problem would be the installation of a constant-tension winch and the incorporation of such a winch in the towing bit of the tugboat.

Again the question of first cost comes immediately into mind. I t is well known t h a t constant-tension devices are rather expensive and it is doubtful t h a t the expense of such a device would be justified for the few times a modern harbor tugboat would have to assist a disabled ocean-going vessel in heavy weather and tow her into port. On the other hand, there is no question t h a t such a constant-tension device m a y be very useful and indeed a necessity in an ocean-going or salvage tug. The author believes t h a t C o m m a n d e r Templeton is absoltttely correct in forecasting t h a t the power of modern tugboats will increase without any appreciable increase in the over-all length of the boat. His recommendation that large harbor tugs be designed for a speed-length ratio not to exceed 1.25 is very well taken. However, even if the designer calls for reduced power at free-running conditions, it is doubtful t h a t the skipper will not call for full installed horsepower when running free. C o m m a n d e r Templeton recommends t h a t at least some controllable-pitch-propeller tugboats be included in Table 2 of this paper. He also points out t h a t most of the tugs in Table 2 are diesel-electric tugs. This is true due to the fact that. most of the modern tugboats, until recently at least, have been of the diesel-electric type. As the controllable-pitch-propeller tugboats enter the field, they should certainly be included in the table, and if the particulars of the Dalzell I[1 were a~cailable at the time the table was written, it would have been included in it. C o m m a n d e r Templeton also points out t h a t Dalzell I I [ showed a bollard pull of about 50,000 lb. However, in the interest of justice, it should be pointed out t h a t this boat is equipped with a higher horsepower engine than the ones listed in Table 2. If, instead, of the total bollard pull used in Commander Templeton's discussion, the bollard pull in pounds per shaft horsepower was indicated, one would find out t h a t the Dalzell I I I showed about 29 to 30 lb of thrust per shaft horsepower which is approximately equal to an average diesel-electric boat. The author is indebted to C o m m a n d e r Templeton for his thorough discussion of bridge-control systems. There is no d o u b t t h a t a good set of engine controls at the bridge is almost as import a n t as a good propeller or a good rudder and contributes greatly in making a tugboat efficient and economical to operate. The-writer also is indebted to Mr. Robertson for his timely and extensive remarks on the stability of tugboats. His remarks on repowering of tugs and the associated dangers should be printed in italics and an inclining experiment, as suggested

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b y the discusser, should follow every conversion. As far as h-values for use with the G M formula presented in the paper are concerned, some representative figures for the boats in Table 2 are as folio-cos : Y T B Design . . . . . . . . . . . Grace Moran . . . . . . . . . . . . H e l e n L . Tracy . . . . . . . . . N a n c y Moran . . . . . . . . . .

h = 13 ft h = 13 ft h = 12 ft h = 12 ft

3 in. 0 in. 11 in. 9 in.

Mr. Robertson suggests t h a t a mean between the freeboard, amidships and the minimum freeboard is a better approximate index for use with the G M formula than the minimum freeboard in itself. This is a good suggestion, and the author will be more than h a p p y to comply with it. I t is hoped t h a t other designers will also take up Mr. Robertson's suggestion whenever they find it necessary to use the G M formula supplied in this paper. Professor Troost has been kind enough to contribute his unique propeller-design knowledge to this paper. With regard to the astern thrust characteristics of the controllable-pitch propeller, the testimony of several tugboat skippers in the New York H a r b o r area might be of interest. Several of these men, well qualified to offer a qualitative opinion, having served on both fixed-pitch and controllable-pitch tugboats, have been shocked b y the poor astern thrust of controllablepitch propellers. Of course, it is very possible. t h a t these propellers have been designed for freerunning speed rather than bollard pull or other towing conditions. This would, of course, cause a large sacrifice in bollard pull, both ahead and astern. The trouble with controllable-pitch propellers is t h a t they are mostly designed b y the manufacturers and there is no doubt t h a t a qualified designer, such as Professor Troost, could improve on the astern-thrust characteristics without impairing the ahead efficiency. If, for example, the controllable-pitch-propeller blades would be designed for dead pull ahead with minimum sacrifice of astern thrust, then the astern thrust of such a propeller might increase as much as 20 per cent over the one designed for free-running speed. Nevertheless, the author recalls t h a t some 2 years ago one of the major controllable-pitch-propeller manufacturers in this country would guarantee for a certain propeller 22,400 lb (10 tons) of astern thrust, while a conventional Taylor wheel of the same specifications should deliver a b o u t 32,000 lb (14.3 tons) in the astern condition. In order to do justice to ail concerned, some clarification of Professor Troost's figures of the R o t t e r d a m tugboat with a turnable K o r t nozzle should be made. I t is noted t h a t this tugboat has

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an engine rated at 325 shp at 325 rpm directly coupled to an adjustable-pitch propeller placed within a Kort-nozzle rudder. Professor Troost further mentions t h a t the ahead bollard pull registered at 310 shp was 44 lb' per shp. This would seem to be exceedingly high, and although there is no doubt t h a t the K o r t nozzle did improve somewhat the thrust obtained, it should be kept in mind t h a t this boat is u n d e r p o w e r e d - i n - c o m p a r i s o n with the boats given in Table 2 of the paper. Since now t h e pounds of bollard pull per shaft horsepower will increase as the shaft/h'6rsepower decreases, the 44 lb per shp pull exhibited b y this tug should not be considered unusual. In point of fact, I recall that Dr. Corlett mentioned to me t h a t one of his Hydroconic tugs, the Sydney Cove, exhibited more than 40 lb pull per shp while th~engine was developing slightly over 1000 shp. Mr. H o r t o n ' s remarks should help considerably in convincing some doubtful naval architects that a K o r t nozzle is worth while investigating in tugboat design. As Mr. Horton points out, it is unfortunate t h a t the developments he has described m u s t remain, at least for the time being, in the unpublished category. ' Mr. H o r t o n ' s remarks as to tip clearances between propeller and the K o r t nozzle do not seem to agree with Dr. Van M a n e n ' s findings. In his recent paper before the Society, Dr. Van Manen indicated t h a t close tip clearances were quite important in obtaining m a x i m u m possible t h r u s t addition from a K o r t nozzle. C o m m a n d e r Miller's discussion presents vividly the problems of the tug designer and illustrates the difficulties arising when the owner wishes one thing a n d the designer knows something else is better. His remarks' are even more valuable since he is very well. known in the tugboat design field and has contributed greatly in designing bo/its of exceptional performance. Mr. Turner correctly points out t h a t a free-pi ston gas generator would have the advantage of being able to give high thrust at low per cent Of rated R P M and still give rated horsepower at a n y propeller R P M . However, one must not disregard the fact t h a t a free-piston gas generator has found a fairly new field of application in the m a r i n e field. While the advantages of this system are quite obvious and a thorough investigation of its capabilities should prove to be very in-. teresting, it is believed t h a t further experimentation and tests will be necessary before a free-piston gas generator can be applied successfully and economically to a tugboat. Mr. Turner points out t h a t his personal preference for a h a r b o r t u g main propulsion system would be a combination of a torque converter, a

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reverse reduction gear, and a fixed-pitch propeller. This again involves the case of the unknown, if one considers the torque converter as an unproven piece of machinery, at least for the horsepo~-ers under consideration. Torque converters of much lower power have been used successfully for m a n y years, but torque converters able to handle 1500 to 2000 or more shaft horsepower have not been developed to the point where they can be used without any fear of introducing an item that will require serious maintenance. In addition to that, it should be pointed out that the 95 per cent overall transmission efficiency quoted in the main body of the paper for a transmission system employing a torque converter was based on preliminary data supplied to the author by one of the torque-converter manufacturing concerns. I t now appears that the manufacturers might have been somewhat optimistic in evaluating the efficiency of their equipment. In view of recent results, it is now estimated that the over-all efficiency of a transmission system using a torque converter would be closer to 92 or 93 per cent. Mr. Morgan asks if Fig. 13 can be used to estimate bollard pull for both constant-power and constant-torque installations. The answer to that is, of course, no. Unfortunately, it was not made clear at the time, but Fig. 13 should be used only for the estimation of constant-torque installations. Furthermore, it should be stated that Fig. 13 was derived from actual bollard-pull tests. Mr. Morgan's comments are similar to the ones that Mr. Kimon presented previously and the reply to Mr. Kimon's discussion should apply to Mr. Morgan's as well. We are, nevertheless, all indebted to Mr. Morgan for his work in develop-

ing performance characteristics of the Troost 3, 4 and 5-blade propeller series at bollard pull. These figures should add considerably to the value of the paper and should prove very helpful to the interested designer. Professor Baler's comments are indeed welcome and the author would be happy to substitute Professor Baler's Fig. 45 for Fig. 12 of the paper. Professor Baier's way of plotting barge resistance in pounds of pull per brake horsepower versus towing speed is valuable, since it indicates at a glance how nmch of an advantage a more powerful tug would have. The author would like to thank Professor Benford for his kind words and to point out that without his help and encouragement, the publication of this paper would have been impossible. Mr. Birch's comments on the types of tugs that the New "York Central System is using are quite interesting. His discussion goes to prove that it is useless to talk about the best type of a tug available when one particular company has three different types operating, all of them designed for specific tows or pulls. Mr. Birch's discussion should make it clear that a thorough co-operation between the owner and the naval architect is not only desirable, but an absolute necessity in order to come up with a successful tugboat design. In closing, the author would like to thank once more all those who contributed to the discussion of this study. There is no doubt that the value of the discussion surpasses that of the paper and it is encouraging to see that so m a n y well-known designers and naval architects are interested in small boat design and see it fit to contribute their knowledge and experience to this field.