Ship Structure

Ship Structure

Bottom Structure Originally ships were constructed with single bottoms, liquid fuels and fresh water being stored in separately constructed tanks. The double bottom structure which provides increased safety in the event of bottom shell damage, and also provides liquid tank space low down in the ship, evolved in early twentieth century. Today only ships less than 500 grt will be having the single bottom construction and large ocean going vessels are fitted with some form of double bottom.

Single Bottom Construction

Keels Keel is considered as the backbone of the ship and is located at the center line of the bottom structure. This contributes substantially to the longitudinal strength and distributes the load effectively generated while docking. The commonly found found keels are as follows:

Flat Plate Keel

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Flat Plate Keel – this type of keel are fitted in majority of the ocean going vessels.

Bar Keel

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Bar Keel – Found in smaller vessels like trawlers, tugs, smaller ferries etc. Where grounding is a possibility this type of keel is suitable with its massive scantlings, but with additional draft with out additional cargo capacity.

Duct Keel

Duct Keel – Duct Keels are provided in the double bottoms of some of the vessels.  Duct keel is a form of flat plate keel but has two center girders instead of one for a simple flat plate keel. 

Duct Keel 

This runs from the forward engine room bulk head to the collision bulkhead and are utilized to carry the double bottom piping and makes the piping accessible even when the cargo is loaded.  The entrance to the duct is provided at the aft from the forward end of the engine room.  A width of not more than 2mtrs is allowed for the duct, and strengthening is provided at the tank top and keel plate to maintain continuity of strength of the transverse floors.

Inner Bottom Plating 

The inner bottom plating is the top plating for the double bottom.  In general cargo ships the plating may be slopped towards the side for easy drainage of bilges.  They are supported by girders and floors.  At the center line of the ship the middle strake of the inner bottom plating may be considered as the flange of the centerline line docking girder.  If the grabs are used the plate thickness will be increased or double ceiling will be fitted.

Floors       

Floors are fitted to give sufficient strength to the ships structure. They form as one of the support members for the inner bottom plates. Floors can be broadly divided into: Solid floors, are fitted to strengthen the bottom transversely and support the inner bottom. Man holes for access and lightening holes are provided for the solid floors. Small air holes and drain holes may be drilled at the top and bottom respectively for solid floors inside the tanks. Plate floors are can be made water or oil tight as required by closing the openings.

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The floors run transversely from the center girder to  the margin plate on each side of the ship.  The spacing of the solid plate floors depend on the load to be supported and the local stresses experienced.  Under the engine room, boilers, bulkhead and in the pounding region plate floors are provided at every frame space.  In other places a distance of 3.05mtrs maintained between the floors, with bracket floors at intermediate frame spaces.

Solid Plate Floor

Bracket Floors

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These are skeleton floors, in which the middle part of the floor plate is omitted and replaced with a frame bar and a reverse bar with a bracket at either end.  The brackets are to be flanged at their free ands and depth should be at least ¾th depth of the center girder.

Steel Sections Used for Ship Construction 

A range of steel sections are rolled from ingot.  It is preferable to limit the sections required for ship building to those readily available that is the standard types.  If special sections are used, steel mill will have to be set up for exclusively for these sections and will not be economical.

Steel Sections Used for Ship Construction

Frames 1.  2.  

Welded frames: flat bars, bulb bars or inverted angles may be used for frames. They may be attached to the shell plating by intermittent welds, or by continuous fillet welds. Web frames: they are heavy plate frames which are not normally used as a system. They are fitted in certain parts of the ship to impart local strength. They are normally fitted in engine rooms and at every fourth frame space in “tween decks” abaft after peak bulk head.

Frames  3.

4.   5.

“Cantilever system of framing” is a modification of web frames. Deep framing: is a system in which every frame is made deeper and stronger over a given area of shell plating to provide extra strength. Frame spacing: in main body of the ship the frame spacing may not exceed 1.0 mtr. Between the one fifth length from the stem and collision bulk head the spacing must not exceed 700mm. In peak tanks and cruiser sterns it must not exceed 610mm. Numbering: frames are usually numbered from aft to forward, frame number 1 being the first one forward of the stern post.

Beams 

Functions: transverse beams have two main functions: to tie the sides of the ship together and to support the deck against water pressure and weight of the cargo.Longitudinal beams also contribute to longitudinal strength of the vessel.  Sections: flat bars, bulb bars and inverted angles are used generally.  T bars and T bulbs may be used under wood decks.  H sections or other built sections are used as strong beams.

Beams 

Transverse beams: the size of the transverse beam is governed by their unsupported span, the breadth of the ship, and in some cases, by the load which the deck has to carry.  Longitudinal beams: longitudinal beams or deck longitudinals are required under the strength deck in all ships of over 120mtrs long.  They are supported at intervals by heavy transverse beams, which must be not more than 2.5mtrs apart for the forward 7.5% of the ship‟s length or 4.0mtrs apart elsewhere.  The longitudinals are connected to the transverse beams by direct welding or by flat bars similar to those used in double bottoms.

Beams   

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At bulk heads the longitudinals may be cut and bracketed to the bulk head if the ship is less than 215mtrs. If the length is more than 215mtrs the longitudinals must be continuous. At hatch ways the longitudinals are cut and attached to the hatch end beams by brackets. Strong beams: A strong beam is a specially heavy beam which is fitted where great local strength is required. These are often fitted in engine room and boiler rooms to support deck longitudinals. Half beam: transverse beams which are cut at hatch side coamings are termed as „half beams‟. When coamings are not form the part of the deck girder, the half beams are directly welded to the hatch side coamings.

Beams 

Beam knees: beam knees are used for connecting the beams to the frames. Plate type bracket knees are normally used in ship construction.  Large knees must have flange of at least 50mm at their free edge.  When longitudinal beams are fitted , the knees at those frames where there is no transverse beam, must extend to the first longitudinal.

Water Tight Bulk Heads    1.

2. 3.

Bulk heads are an important element of transverse strength, particularly against racking stress. By dividing the ship into longitudinal subdivisions, they also give protection against fire and flooding. Minimum number of bulk heads: all ships must have at least : A collision bulkhead, not less than 5%, nor more than 8% of the ship‟s length abaft the stem at the load water line. An aft peak bulk head, to enclose the shaft tube in a water tight compartment. One bulk head at each end of the machinery space.

Water Tight Bulk Heads 5.

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Ships over 90mtrs long must have additional bulkheads spaced at reasonably uniform intervals. The number of bulkheads to be fitted depends on the length of the ship and on whether the engines are placed amidships or aft. Collision bulk head must extend up to the upper deck. The after peak bulk head need only extend up to the first deck above the load water line, if it forms a water tight flat. All other bulk heads must extend to the bulkhead deck which is usually the free board deck. Bulk heads are fitted in place of frames. They are intercostal between decks; I.E. The decks are continuous and the bulk heads are fitted in panels in between them.

Water Tight Bulk Heads 

Plating of the bulkhead may be fitted either vertically or horizontally.  But for the convenience of arranging the plates according to the thickness, increasing from top to bottom, horizontal arrangement is normally used.  Thickness is dependent on the spacing, length of the stiffeners and location of the bulk head.  The after peak bulk head plate thickness around the stern tube must be doubled or thickened to resist vibration.

Water Tight Bulk Heads 

Stiffeners may be angles, bulb angles, channels or equivalent welded sections.  Stiffeners are usually fitted vertically.  They are usually spaced about 75cms apart in all areas other than collision bulkheads and deep tank bulkheads, where the spacing is reduced to 60cms.  Stiffeners in the way of deck girders are often made heavier and are attached to the girder by deep flanged brackets.

Water Tight Bulk Heads 

Corrugated bulk heads: these are often fitted in oil tankers and are occasionally found in dry cargo ships.  The corrugations give stiffness to the plating and ordinary stiffeners are not required and slight reduction in weight is obtained in this method.  But to make a certain width of corrugated sheet more length of sheet is required compared to plane sheet and hence the actual gain in weight reduction is offset.  In some constructions widely spaced web stiffeners at right angles to the corrugations are used.

Collision Bulkhead

Corrugated Bulkhead Section

Corrugated Transverse Bulk Head

Water Tight Bulk Heads    

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In transverse bulkheads , the corrugations may run either vertically or horizontally. In longitudinal bulkheads only horizontal corrugations are allowed to give longitudinal strength. The thickness of the plate depends on the width of the „flats‟ and height of the bulkhead. Pipes passing through the bulkheads should be either welded or fastened to the bulkhead by studs or bolts screwed through the tapped holes in the plating. This is to ensure that in the event of breakage of bolt, the piece with in the plate is retained, to provide the water tightness of the bulkhead.

Pipe Passing Through the Bulkhead

Testing of Water Tight Bulkheads 

Water tight bulkheads forming the boundary of a tank may be tested by testing the tank.  Other watertight bulkheads with in the tank may be tested by filling water up to the load water line.  Other watertight bulkheads are hose tested.

Pillars 

The main function of pillars is to carry the load of the decks and weights upon the decks vertically down to the ship‟s bottom structure.  Another function of pillars is to tie together the ship‟s structure in vertical direction.  The pillar connecting ends may be of bracketed or non bracketed type, according to the indented function of the pillars.

Pillar Arrangements

Pillar Arrangements

Tension Pillar Bracketed Pillar

Pillars 

In a cargo ship the pillars inside the holds are to transmit the down load effectively to the bottom structure, where the pillars need not be bracketed.  Inside the engine room the main function pillars is to tie the structure in vertical direction, where the pillar at times may be tension, and these pillars are to be heavily bracketed.  In side tanks also the pillar can be under tension when the tank crowns are under pressure and hence heavily bracketed joints are used.  Since the location of pillars can interfere with stowage arrangements in a cargo ship, widely spaced pillars of large sections are used instead of closely spaced small solid pillars.

Pillars  

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The above arrangement is termed as „massed pillaring‟. The hold pillars have to take compressive stress and hence the construction should be to avoid buckling, and this depends on the length and the load carried. Normally rectangular or octagonal tubular construction is adopted and for economic reasons these may be fabricated from steel plates or suitable steel sections. The pillar ends should be connected to doubling plates with continuous weld to distribute the load effectively. In place of pillars non water tight pillar bulkheads may be fitted on the ship‟s centreline. Pillar bulkheads when fitted normally extends from transverse bulkhead to hatch coaming.

Hatch Ways      

For the ship having wide openings for cargo operations, the resistance to longitudinal stress is very much reduced. Stress concentrations cause a tendency for the decks to fracture at the hatch corners. Also there is a loss of transverse strength due to cutting of beams at the hatch coamings. To compensate for this loss of strength the deck plating should be sufficiently strengthened at hatch ways. Coamings and their connections must be sufficiently strong and rigid. For deck openings square corners are not allowed but to be rounded off.

Hatch Ways 

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The radius of the round should be 1/24th of the breadth of the opening but in any case it should not be less than 300mm. Doubling plates are not allowed for strengthening, only inserts are allowed. Coamings on the weather deck should be well extended above the deck to ensure that the water cannot enter the ship‟s hull. Height of the coaming at the weather exposed areas of the ships should be minimum 600mm with in one-quarter length from the stem. Aft of the quarter length from the stem the coamings should have a minimum height of 450mm. In rest of the places the openings are to be suitably framed.

Hatch Ways 

Welded hatch coamings may have rounded corners but due to practical difficulties for fitting hatch covers, the square cornered hatch coamings are normally fitted.  The ends of the hatch coamings must be extended beyond the hatch ends to form tapered brackets.  The deck plating must be extended inside the coaming so that it can be rounded off with a smooth edge with out any welding.  Horizontal gussets are fitted to strengthen the connection between the side coaming and hatch end beams.

Shell and Deck Plating Seams or „edge laps‟ are joints which run fore and aft that is longer edges of the plates.  Butts or „end laps‟ are joints which run in the athwartships or vertically that is along the shorter edges of the plates.  Garboard strakes are the strakes of shell plating next to the keel on either side.  Sheer strakes are the upper strakes of shell plating on either side, next to the upper deck. 

Shell and Deck Plating   

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The deck stringer is the outboard strake of deck plating, which is connected to the sheer strake. The purpose of the shell plating is to keep out the water and to tie together the ship‟s frame work. It also plays an important part in resisting the longitudinal bending stresses and hence it has to be stronger amidships particularly at deck and bottom. In long ships it is necessary to strengthen the shell plating against shearing stresses at about half depth of the ship, in the region of about one quarter of the ship‟s length from either end. Shell expansion plans are plans which show all plating on the hull drawn to the scale.

Shell and Deck Plating

Shell and Deck Plating

Shell and Deck Plating 

They also show many other details, including frames, floors, deck edges, stringers etc.  Strakes of shell plating are distinguished by the letters from the keel onwards.  Garboard strake is the first strake “A”.  The plates in each strake is numbered from aft to forward.

Shell and Deck Plating 

So D5 strake is the fifth strake from the aft and fourth strake from the keel.  Strakes on deck plating are lettered from centre line outward whilst deck plates are numbered from aft to forward.  Stealer plates: the girth of the ship decreases toward the ends and so width of the plates must be decreased in these parts.  To save making plates too narrow at the edges of the ship, it is usual to run a number of pairs of adjacent strakes into one.  This is done by means of a stealer plate.

Shell and Deck Plating

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Please note the lettering and numbering of the stealer plate that the letter is of the lower strake that runs into it and the number is one before the lower strake running into it.  Special plates:

Shell and Deck Plating 

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Shoe plates are plates that connect the stem to the flat plate keel. Coffin plates used for connecting the stern frame to the flat plate keel. Boss plates are shield shaped plates fitted over the boss of the stern frame. The deck plates are welded on their butts and seams, which gives a flush surface to the plating and also save some weight. Welded plating is more liable to crack than riveted plating, particularly in the region of sheer strake and bilge. The cracks are more liable to occur if the openings are near to the upper edge or if there are notches.

Shell and Deck Plating 

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For this openings in the region of upper edge should be well rounded off and should be kept away from the upper edge. The upper edge should be well smoothed and as far as possible other parts should not be welded to it. As a further precaution against cracking special notch resistant steels may be used. Notch resistant steels are commonly used for sheer strake, bilge and bottom plating according to the length of the ship. Connection of the sheer strake to upper deck stringer should form a „T‟ joint with a full penetration weld. Alternate method used in modern shipbuilding is to have a rounded sheer strake.

Shell and Deck Plating 

The radius of the rounded section should not be less than 15 times the thickness of the plate.  Any opening in the shell plating must have special arrangements to preserve the strength and their corners must be rounded.  When large openings are cut they are usually framed- in by a face bar and web frames are fitted on either side of the opening, also insert plates are fitted above and below the opening or right around the opening.

Shell and Deck Plating 

Classification societies have clear rules regarding the width of the keel plates, sheer strakes, garboard strakes and deck stringers, where as for other areas of shell plating the any reasonable width.  Plating thickness vary at different areas of the shell plating.  Keel plates, sheer strakes and upper deck stringers are thicker than other strakes.

Shell Expansion Plan

Shell Plate Arrangements

Shell and Deck Plating     

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Shell plating at the bottom is thicker than at the sides. Deck platings are thinner than normal in the line of the hatch openings. Tween deck plates are thinner than the main deck plates. Shell and deck plate thickness is maintained for four-tenths of ship‟s length amidships. But is reduced slightly towards the ends except the plates covering the flat of the bottom in the pounding region. In the pounding region the plate thickness is increased.

Bilge Keels 

Bilge keels are provided to resist rolling.  The effect of direct resistance of the bilge keel with water is less.  They slightly increase the period of roll.  They upset the transverse streamlines of the ship‟s hull and thus set up eddy currents and increase the „wave making resistance‟.  They increase water pressure over a large area of the ship‟s hull and this pressure acts in such a direction to damp the rolling.  For their protection bilge keels are arranged in line with hull and bottom floors.

Bilge Keels 

If they were to project beyond these limits, they would be more liable to damage.  There are chances of bilge keels ripping off when the ship touches the bottom, but if this to happen then the ship‟s shell plating should remain intact.  In welded ships the bilge keels are attached to the continuous flat bar attached to the ship‟s hull.  The outer edge may be riveted or lightly welded so that in the event of damage the breakage will occur only at the weaker joint.  In large ships the bilge keels are very deep.

Bilge Keels 

Unless carefully designed the bilge keels can produce residual stresses at the bilge plate and this can cause cracking of bilge plate.  To prevent this the ends of the bilge keel should be tapered off gradually and should end over a floor or tank side bracket, whilst a doubling plating should be welded to the bilge plate at this point. 

Bilge Keels

Bilge Keels

Bilge Keels

Bilge Keels

Bilge Keels

Bilge Keels

Deep Tanks 

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During ballast voyage if the water is carried only in double bottom tanks the vessel tends to become stiff, so it is desirable to take ballast in one of the bottom holds, so that it can be filled water when ever it is necessary. This hold is called as a deep tank. In certain trades deep tanks are used for carrying liquid cargoes or fuel oil bunkers.

Deep Tanks 

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1. 2.

In other trades these tanks (ordinary deep tanks) are only used for carriage of dry cargo during loaded passage and it is ballasted during light ship condition. Ordinary deep tank construction is same as the other holds with some modifications for carrying water ballast. A wash plate must be fitted for reducing the free surface effect. The hatch way must be specially constructed to prevent escape of water from below.

Deep Tanks 3. 4.

5.

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Frames are to be 15% stronger than normal. Bulkhead stiffeners are to be spaced not more than 600mm apart and are to be bracketed at head and foot. The deck plating forming the top of the tank should be having 1mm thickness more than that of the boundary of the bulkheads. Beams may be normal provided that their size is not less than the bulkhead stiffeners.

Deep Tanks 7.

8. 9.

10.

Beams must be supported by inter coastal girder on either side of the centre line. Deck edge to ship side joints are to absolutely water tight. In welded construction the frames pass through the deck and the slots are made water tight by welding separate plate pieces. In an alternate method the deck plating is stopped short and separate sealing plates are welded.

Deep Tanks  1.

2.

When deep tanks are used for carrying oil fuel bunkers: The ship‟s sides and boundary bulkheads should additionally be stiffened with deep, horizontal side girders running round inside the tank and spaced not more than 3mtrs vertically apart. These girders must be stiffened on their inner edges and are connected together at the tank corners by flanged brackets.

Deep Tanks 3.

4.

5.

They are to be supported at every third frame or stiffener by brackets and are attached to intermediate frames or stiffeners. A middle line bulkhead must be fitted if the tank extends over full breadth of the ship. The bulkhead may be perforated if desired. Light inter coastal stringers are fitted in line with the horizontal girders. Quarter line girders must be fitted at the deck head and these are often formed by deepening the ordinary deck girder.

Deep Tanks  1. 2.

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When deep tanks are used for carrying oil as cargo: The cargo oil flash point should not be less than 60°C. The construction may be same as that for carrying bunker fuel, but centre line bulkhead is not necessary unless the tank is liable to be partly filled. Fuel oil, vegetable oil and water may not be carried adjacent tanks, unless these tanks are separated from each other by a cofferdam. Deep tanks may be tested by filling water to the maximum head which can come on them in practice but this should not be less than 2.44mtrs above tank top.

Peaks and Panting Arrangements Peaks are those parts of the ship‟s hull which are forward of the collision bulkhead(fore peak) or abaft the after peak bulkhead, excluding the overhang of the stern (the after peak).  They are usually constructed so that their lower part forms a tank, which is known as fore peak tanks.  Cellular double bottoms and longitudinal frames, if they are fitted, usually extend only from after peak bulkhead to collision bulkhead.  In the peaks a system of deep „open floors‟ is fitted with transverse frames and beams. 

Fore End Structure

Fore End Construction Details

Fore End Construction Details

Fore End Construction Details

Fore End Construction Details

Peaks and Panting Arrangements      

The floors extend from side to side in one piece and are stiffened by a flange or face bar on their upper edges. The centre girder is made inter coastal between the floors, but usually extend for a few frame spaces into the peak. Special strengthening is required in peaks to enable the shell plating to resist panting stresses. When peaks are used as tanks, rules for the construction of the ordinary deep tanks are to be followed. Heavy deck girders are not required, but wash plate need to be fitted at the centre line. They are tested as the same as the deep tanks.

Peaks and Panting Arrangements 

Dry peaks are tested by filling water up to the water line.  Tiers of panting beams are fitted forward of collision bulkhead below the lowest deck.  These are similar to ordinary deck beams and are connected to the frames by beam knees, but are only fitted at alternate frames.  The tiers of beams are fitted 2mtrs apart vertically and are supported by either wash plates or pillars.  Panting stringers are fitted on each tier of the beam and their inner edges are shaped or gusset plates are fitted to stiffen the joints.

Peaks and Panting Arrangements 

At intermediate frame spaces where there is no panting beam, the panting stringer is supported by a beam knee of half its depth.  At their fore ends the stringers are joined by flat plates by the name ‘breast hooks’.

Aft End Structure The ship‟s stern is designed and constructed in order to improve flow pattern around the propeller.  Two types of stern constructions are generally used for construction; Cruiser stern and transom stern.  A cruiser stern presents a pleasant look and is hydro dynamically efficient.  Transom stern construction offer more deck area at the stern and easy to construct. 

Aft End Structure 

Transom stern also offers improved flow around the stern.  Since the overhang for cruiser stern construction is more and hence large slamming forces, substantial construction with adequate strength is required.  Solid floors are fitted at every frame space, and a heavy centre line girder is fitted right aft at the shell and deck.  A number of „cant frames‟ are fitted abaft the aftermost transverse frame.  The cant frames should extend to the first transverse frame.

Cruiser Stern

Cruiser Stern Construction

Longitudinal Cut Section

Aft End Structure 

Transom stern construction is similar to cruiser stern except that the cant frames are replaced with a flat plate called „transom‟  In the transom stern construction the cant frames are not required.  As the stern is offering a flat surface the same be stiffened with vertical stiffeners.  Deep Floors and centreline girder are provided at the lower region of the transom stern construction.

Transom Stern

Transom Stern Construction

Stern Frames 

The form of stern frame is dependent on the shape of the stern, propeller size and the rudder type.  To prevent serious vibration at the after end sufficient clearance should be provided between the propeller and the stern frame.  The stern frame of a ship may be cast, forged or fabricated from steel sections and plates.  Normally on large sea going vessels it either cast or fabricated.  If fabricated the job is done by specialist work shops out side shipyard.

Stern Frames 

To avoid casting and transport difficulties when cast in single piece, the cast frames are cast in different sections and then welded together in shipyard.  Fabricated stern frames are mostly cast in shipyard itself.  Both cast and fabricated sections are supported by horizontal webs.  The stern frame connection to the hull structure has to be substantial to prevent serious vibrations set up by the stern frame during propeller rotation.

Stern Frames 

The rudder post is extended and welded to the transom floor.  The propeller post is extended and welded to the deep floor.  The lower sole piece is extended forward and welded to the keel plate.  The side shell plates are directly welded to the stern frame.

Stern Frame Construction

Rudders 

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Many forms of rudders are available and the type and form fitted is indented to give the best manoeuvring characteristics. Originally rudders consisted of a single plate with supporting arms riveted on either side. In modern ships double plate fabricated stream lined rudder is used. The shape of a rudder plays an important part in its efficiency. The area of the rudder is approximately 2% of the product of the length of the ship and the designed draught.

Rudders 

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Since the vertical dimensions of the rudder are somewhat restricted due to the area constraint as mentioned above, the fore and aft dimensions are increased. Again due to this increased dimensions the torque necessary to turn this rudder is overcome by fitting balanced or semi balanced rudders. The rudder may be balanced or semi balanced. They may be hinged on pintles and gudgeons, or they may turn about an axle which passes through the rudder. The rudder stock, which turns the rudder, passes vertically upwards to the steering gear through a gland at the shell plating, or a water tight rudder trunk.

Rudders 

The rudder stock is connected to the rudder by bolted joints for the convenience of removing the rudder for maintenance.  The centres of pintles or axles must be in the same line as the centre of the stock, to enable the rudder to turn.  The weight of the rudder may be taken by a bearing pintle, or by a bearing at the rudder head, or by a combination of both.

Rudders 

Unbalanced rudder is a rudder with their whole area abaft of the rudder stock. Water pressure tries to force the rudder amidships causing considerable stresses on rudder stock and steering gear.  Balanced rudder is a rudder having a part of its area forward of the rudder stock, so that the water pressure acting on this area counter balances the pressure acting on the portion aft of the stock. As the pressure acting on the rudder varies with rudder angle, complete balancing over the entire rudder angle is not possible. Normally the pressure acting is fully balanced at an angle of 15°, with one fourth area of the rudder forward of the rudder stock.  Semi-balanced rudder will be having a very small area forward of the rudder stock, which is too small to give full balancing effect. They are often found in twin screw ships.

Balanced Rudder

Semi Balanced Rudder

Unbalanced Rudder

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In double plate type rudders the frame work may be fabricated or cast steel.  When the rudder is fully fabricated type, they are fabricated from steel plates and the plate sides are thickened by internal webs.  One side of the plate is prepared and the vertical and horizontal stiffening webs are welded to this plate.  The other side plating the „closing plate‟ is welded to the internal framing from the exterior.  This is achieved by welding flat bars to the webs prior fitting and then slot welding the plates.

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Normally a drain plug is provided at the bottom.  A lifting hole with a pipe insert is often provided for lifting purpose.  To prevent internal corrosion the internal surface is suitably coated or in some cases the interior is filled with an inert plastic form.  Completed rudder assembly is pressure tested by immersing the rudder under water having a height of 2.45mtrs above the rudder assembly.

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Pintles on which the rudder turns in the gudgeons are tapered and bearing bush made of lignum vitae or synthetic materials.  The lubrication of bearings is provided by water in either of the cases.  A number of large vessels are now fitted with oil lubricated oil lubricated metal pintles.  Rudder stock is the connection link between the rudder and the tiller.  The rudder stock passes through the rudder trunk.

Rudder Pintles Bearings

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The rudder stocks are made of cast or forged steel the diameter of the stock depends on the torque required to turn the rudder. Normally the water side end is formed to a flange and and is bolted to the rudder for the convenience of removal for maintenance. The weight of the rudder may be carried partly by the lower pintle and partly by the rudder bearing within the hull. Rudder bearing assembly is provided with a water tight gland at the upper end. Most of the rudder‟s weight may come onto the rudder bearing if excessive wear down of the lower pintle occurs. This causes drop in the rudder and same may be measured and corrected during dry docking.

Rudder Bearing and Stuffing Box

Rudder Bearing and Stuffing Box

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The rudder trunk through which the rudder stock passes is made as short as possible to reduce the length of the rudder stock.  The rudder trunk need not be water tight at the lower end.  The trunk is made in the form of a box with a transom forming one forward side.  An inspection opening is provided with a water tight door.

Stern Tube     

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The stern tube form the after end bearing of the propeller shaft. It incorporates a water tight gland to prevent ingress of water. There are two types of stern tubes fitted on to the ships. The first one is having the water lubricated bearings, with after end open to the sea. The bearings are made of lignum vitae linings or composite materials with a lining of brass on the shaft. The second type will be having water tight sealing at both the ends and will be having oil lubricated white metal bearings.

Oil Lubricated Stern Tube

Oil Lubricated Stern Tube

Hatch Way Closing Appliances 

Any opening on a ship where there is chances of water ingress when at sea should have a water tight closing arrangement to maintain the water tight integrity of the vessel.  This is tested during load line surveys.  The height of hatchway coamings on freeboard decks shall be at least 610 mm above the deck; the height of coamings on superstructure decks shall be at least 610 mm above the deck if situated within a quarter of the ship‟s length from the stem, and at least 460 mm if situated elsewhere.

Hatch Way Closing Appliances 

Modern ships have steel hatch covers and most of them are patented.  The most popular of them is MacGregor type.  In general any of them consist of steel covers called „pontoons‟ and are stiffened by webs or stiffeners internally and water tightness is obtained by gaskets and clamping devices.

Hatch Way Closing Appliances   

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Securing cleats or cross joint wedges, together with suitable jointing material are to be fitted. At least two cleats per panel on either side are to be provided. These panels may be of different types like fore and aft single pull, folding, piggy back, pontoon or side rolling. Each pontoon is connected to the one ahead and astern of it by a chain on each side to ensure that the pontoon will get pulled or pushed by the adjacent panel in a gang fashion. Vertical stowage of panels is at one end of the hatch has advantage as the space for stowage is very less.

Hatch Way Closing Appliances  

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The space the pontoons are stowed is called hatch recess. In closed position the pontoons interlock each other and weather tightness is achieved by the rubber gasket provided all around the pontoon. To provide sufficient compression for the rubber gasket to enable make the joint weather tight, the rail section, where roller rests in closed position is lowered down, so that the by the weight of the pontoon itself the rubber packing is compressed. The covers are secured to one another and to the coamings by cross wedges on the top side and side cleats on the sides. Alternately folding type arrangements are also can be made, where suitable hydraulic cylinders are provided at the leading panels.

Special sealing Arrangements

Manholes and Covers 

Manhole covers do not vary much in design, their shape however are sometimes different for different places.  When fitted outside a tank they may be either circular or elliptical.  But when fitted inside they are almost always elliptical to facilitate their removal.  Usual size openings vary between 450mm to about 600mm.

Chain Locker Arrangements 

Chain locker is arranged in the forward of the collision bulkhead below main or second deck.  The chain locker dimensions are governed by the dimensions of the anchor cable.  The chain locker is divided into two compartments by the centreline bulk head.

Chain Locker Arrangements 

Port and Starboard chains are stowed in separate compartments inside the chain lockers.  The bitter ends of the chains are secured to the bottom of the centre line bulkhead and arrangements are often made to slip the cable from outside chain locker , to release the anchor along with chain from the ship, in case of an emergency.

Chain Locker Arrangements

Chain Locker Arrangements

Chain Locker Arrangements 

A perforated false bottom is provided above the bottom plating with sufficient height between them so that the space is used for collecting mud and water carried in during anchor operations.  The bottom is made with inward slanting towards the centreline bulkhead to ease in drainage.  Stiffeners for the bulk heads are provided from outside to prevent damage to these members by the anchor chain.

Bulwarks 

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Bulwarks fitted on weather decks are provided as protection for personnel and are not intended as a major structural feature. They are therefore of light scantlings and their connections to the adjacent structures are of some importance to avoid undue stresses on bulwarks. Bulwarks must not be welded on to the sheer strake within half length of the mid ship, as this is liable to cause plating to crack. To overcome this difficulty floating or “loose” bulwarks may be employed. Bulwarks in exposed positions must be 1mtr high. Where mooring pipes are fitted, the plating is to be doubled or thickened around them.

Floating Bulwark

Rails and Freeing Ports

Hinged Freeing Port

Bilge and Ballast Piping Arrangements 

All cargo ships are provided with pumping and piping arrangements so that any water tight compartment or water tight section of a compartment can be pumped out when the vessel has list up to 5°, and is on an even keel.  For a passenger ship the above areas should be able to pump out in any conditions.  In machinery spaces the bilges should be able to be pumped out through the main bilge line and by means of an emergency bilge suction and through a large capacity pump.

Bilge and Ballast Piping Arrangements 

Normally the emergency bilge suction will be connected to the main sea water pumps.  To enable the suction to be taken in listed position, the bilge suctions are to be fitted in both port and starboard, except for narrow spaces like duct keel, shaft tunnel etc.  As the vessels will be having a slight trim by stern, the bilge suction may be provided only in the aft region, but where the single hold length exceeds 33.5mtrs, suctions are also arranged in the forward of the hold.  On many vessels a sloping margin plate is fitted and bilge suctions can be conveniently be placed in this recess.

Bilge and Ballast Piping Arrangements 

But if the tank top extends straight to the sides, bilge wells of min 0.17 m3 capacity to be provided.  At open ends of bilge suctions in holds and other compartments, out side machinery space and shaft tunnel, a strum box is provided.  The strum box is a perforated plate box welded to the mouth of the bilge line which prevents debris being sucked into the bilges lines.  The perforations in strum box do not exceed 10mm in dia.

Bilge and Ballast Piping Arrangements 

The total cross sectional area should be at least twice the cross sectional area of the bilge pipe.  Strum boxes are arranged at reasonable height from the bottom of the bilge space for easy access and cleaning.  In machinery spaces and shaft tunnels the bilge suction is through mud boxes, which are accessible for regular cleaning.

Strum Boxes

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Bilge suction pipes run fore and aft along the bilges.  They may run through the lightening holes of the tank side brackets or run along on top of the bilges.  They must not pass through deep tanks or double bottom tanks or oil fuel bunkers unless it is unavoidable.  To prevent accidental flooding of the tanks and holds non return valves are provided.  The suction pipes to the tanks in the fore end of the ship are led through double bottom, or through duct keel.

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The fore peak suction must have a screw down valve fitted on to it forward of the collision bulkhead, and capable of being operated from above the load water line. Deep tank suctions must be arranged so that they can be fitted with blank flanges to prevent water from passing in or out. When carrying solid cargo the ballast lines are blanked and bilge suction is kept open. When carrying ballast the bilge lines are blanked and ballast line is kept open. In the aft end they run through the duct keel. If peak tanks are used for ballast they must have suction lines leading to them.

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Air pipes must be fitted to the tanks for air to escape when the tank is filled.  They must be fitted at the outboard corners of the tank, at the opposite and from the filling pipes, or at highest point of the tank top.  The total area of the pipes must normally be equal to that of the filling pipes.  Air pipes must extend above the load water line.  If they are to extend above free board deck, they are to be at least 760mm above that deck or 450mm above other superstructure decks immediately above the free board deck.

Air Pipe and Sounding Pipe Arrangements

Striking Plate Arrangement

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In order to ascertain the depth of water in the tanks and bilges, sounding pipes are fitted. These usually consist of straight vertical pipes and they must extend to above the bulkhead deck, except the pipes to the tanks below engine room spaces. They must be of at least 32mm inside dia and must be placed as near as to the suction pipes. Doubling plates often called as „striking plates‟ are usually arranged at the plate at the facing end of the bottom of the sounding pipe. This is required to prevent sounding bob wearing down the bottom of the tank. Alternately, the end of the pipe may have a plug screwed into it and holes cut to allow the water to enter.

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Various patented tank sounding devices are available and same can be fitted instead of a conventional sounding pipe as long as they satisfy the requirements of the classification society.

Bilge and Ballast Lines

Deck Fittings 

Various fitting on the deck and deck edges are provided to assist in safe and efficient mooring operations and provide clear run or lead for the mooring ropes and warping wires.  Bollards or mooring bits are provided for mooring the ships once it is along side.  These are welded to the deck or to a box type structure which is welded to the deck.  Additional supports by means of stiffeners are provided directly underneath the deck.

Mooring Bitts

Fairleads 

Fairleads are used to guide the hawsers or mooring wires to the bollards or mooring winches.  Fair leads are attached to the deck, a raised seat or the deck and bulwarks.  The normally found fairleads on board merchant ships are multi-angled fairlead, the pedestal fairlead, the roller fairlead and the panama fairlead.  Multi-angle Fairlead: This consists of two horizontal and two vertical rollers.  The wire passes through the central gap between these four rollers.

Fairleads

Fairleads 

The multi angle fairlead is fitted at the deck edge and reduces the number of guide rollers or other fairleads required to give a clear lead of wire to the winch.  Pedestal Fairlead: A pedestal fairlead consists of a single vertical or horizontal roller mounted on a raised pedestal or seat and often termed as „Deadman‟ or „Old- man‟.  They provide a direct lead of mooring line to windlass, winch or capstan.

Fairleads

Fairleads 

The pedestal fair lead leads the wire across the deck to the winch clear off any obstructions.  Roller Fairlead: A roller fairlead is one or more vertical rollers fitted on a steel base which may fasten directly to the deck or to the deck and bulwarks.  Roller fairlead is used at the deck edge to lead in the mooring and warping wires.

Fairleads

Fairleads 

Panama type fairlead- A no-roller almost an elliptical opening fairlead mounted at the ships side and enclosed so that mooring may be led to ashore with equal facility either above or below the horizontal and strictly pertains only to fairleads complying with panama regulations but often applied to any closed fairlead or chock.  A panama fairlead is so named since they were mostly used in the Panama Canal.

Fairleads 

The ship is hauled by small locomotives and the wires are sent out through these leads – they are of adequate strength to prevent the metal being cut open by the wires.

Fairleads

Typical Mooring Arrangement

Deck Fittings  

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Where it is welded to the deck the plate thickness is increased. Where it is fitted onto the bulwark support for the bulwark at this region is increased substantially compared to else where. Additional stiffeners are provided at this region. On the deck where the deck machineries are fitted a lot of local stresses are imparted to the structure. To resist these stresses by distributing them effectively to other portions of the structure, additional strength members are fitted directly below these equipments‟ foundations in the form of pillars and also thick plate inserts are welded.

Superstructure and Deckhouses 

A superstructure may be described as an erection above upper deck, extending from side to of the ship forming a part of the main hull.  A deck house may be described as a comparatively light structure more or less resembling a steel box, and used mainly for accommodation and similar purpose.  It does not extend to the full width of the ship and is placed on ship‟s main hull as distinct from forming an integral part of it.

Superstructure and Deckhouses 

The most important structural member of this is the bridge front bulk head as it has to withstand force of the seas.  The area where great care has to be exercised is where these structures terminate abruptly and large stress concentration will develop locally.  Long super structures exceeding 15% of the ship‟s length and exceeding 50% of the ship‟s length amidships should be given special attention as they also contribute to the longitudinal strength of the ship.

Superstructure and Deckhouses 

In passenger ships where long super structures are fitted, the bending stress distribution is more or less linear and the strength deck is shifted above upper deck and will be in the way of the superstructure deck.  This type of superstructures are termed as effective superstructures as they contribute to the effective strength of the hull.