Robert S. Kaidy Naval Architect & CEO 360 NW ALICE AVE., STUART, FL 34994 O: 772-692-8551 E:
[email protected] www.ocean5inc.com
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 1
Presentation Goal: • Introduce design considerations for Stepped planing hull design and optimization. • Review the use of the “Wake Profile Method.” • Identify strength, weaknesses, and limits of first order approach for design and optimization. • Present examples using full size craft. • Discuss use of CFD for Design and Optimization.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 2
Wake Profile Method 1. 2. 3. 4.
Introduction Definitions Goals of Stepped Hull Design Wake Profile Method 4.1 4.2 4.3
Design Input & Output Parameters Analysis Methodology Equilibrium Solution
5. Strengths and Weaknesses of WPM 6. Example Craft 6.1 6.2
Savitsky/Morabito Validation USNA Tow Tank Validation
7. CFD 8. Conclusions 9. References & Resources
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 3
• •
Invented by Rev. Ramus of Sussex England in 1872. William Henry Fauber obtained a US Patent for hulls with multiple steps in 1908.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 4
•
Many craft were built to dominate the racing boat scene from the 1920’s, including Gar Woods.
•
Many patents exist for stepped hull technology.
•
Widely used in racing, pleasure performance craft and offshore outboard powered fishboats.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 5
Primary Advantages of Properly Designed Stepped Planing Hull over non-stepped: •
Reduced Resistance
•
Increased Speed
•
Improved Efficiency
•
Improved Seakeeping
•
Compelling Marketing / Hull Story
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 6
• Reduced Wetted Area – Reduced Viscous Resistance
• Optimal Trim Angle – Planing Surfaces Operating at Best Lift/Drag Ratio
• Higher Efficiency Planing Surfaces – Multiple high Aspect Ratio Lifting Surfaces versus One Very Low Aspect Ratio Surface
• Favorable Effects from Planing Speed to Max Speed • Trim Angle Remains Optimal at nearly all Speeds, and constant at max velocity
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 7
• Air Lubricated
• Air Bearings • Air … pretty much anything • Ram Air Lift
• Big Steps = Fast Boat • Little Steps = Fast Boat • More Steps = Fast Boat
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 8
Offsetting potential characteristics can include: • • • • • • • • •
Higher off plane trim angles and resistance Dynamic instability / Porpoising High Speed Maneuvering Instability Potential for Hooking Surge in Seaway Structural Discontinuities Potential for improper or incomplete ventilation LCG Sensitivities Off Design or poorly designed craft with higher Resistance than Conv. Hull
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 9
Wake Profile Method Numerical analysis method allows designer to: • Create New Designs without relying on anecdotal rules of thumb • Answer basic design questions: – How does the resistance or speed change if we…. – Increase/decrease the step height – Change afterbody angle – Move step fore or aft
• Optimize existing designs
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 10
•
Stepped planing hull design has classically involved the use of rough rules of thumb, guess work and costly and some dangerous experimentation to answer the most rudimentary design questions, including resistance, running trim angle, and effect of step height and geometry.
•
Existing studies and data primarily associated with the design of seaplane floats, and have been very limited utility as tools to develop new craft. (refer to reference list)
•
A new method was needed to allow small craft naval architects to directly calculate the effects of various design parameters on the overall design performance characteristics of the craft.
•
New Method for the 1st order computational analysis of stepped hulls created based on work of Savitsky-Morabito and Hadler , “Wake
•
Profile Method”
Ocean5 has developed a new method based on the synthesis of existing data and methods, to directly predict the performance of stepped hulls, and has incorporated this method into new software for the naval architect, called Virtual Seatrial - VSt
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 11
Afterbod y
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
Step
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 12
Lab= Length of afterbody aab= Angle of Afterbody measured from baseline to keel
“ab” defined as subscript for afterbody values, “fb” Forebody Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 13
Components to achieve reduced Resistance: 3.1
Optimal running trim
3.2
Reduced Wetted Area
3.3
Increased Aspect Ratio Lifting Area
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 14
3.1 Optimal Trim
Planing Hull resistance is a function of trim angle
Resistance bucket exists for all planing hulls at ~4.3 Degrees
Resistance increases rapidly at lower trim angles
Resistance relatively constant from 3.5 to 4.5 deg.
Conv. Planing hulls trim angle decreases with speed, creating higher wetted resistance component at higher speeds
A craft with trim control at optimum trim angles would have lower resistance
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 15
3.2 Reduce Wetted Area • •
Result of trim control Result of splitting planing area into two more highly loaded areas.
V=40 knots Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
V=40 knots IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 16
4.1 Input & Output Parameters 4.2 Analysis Methodology – Use of Wake Profile Modeling 4.3 Equilibrium Solution
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 17
Other Inputs
Direct Design Input Variables After Body Parameter 1.1
Variable
Chine Beam
Bab
1.2
Step Height
hS
1.3
Length
Lab
1.4
Deadrise
bab
Characteristic
Approach
2.1
Chine Flat Width
Change Effective Deadrise
2.2
Chine Section Shape / Angle
Change Effective Deadrise
2.3
Keel Flats / Pads
Change Effective Deadrise
Outputs at Equilibrium:
2.4
N/A
Total and Component Resistance
Air Entrainment Devices & Effects
2.5
Strakes
Savitsky /Hadler
2.6
Step Planform Shape
N/A
1.5
Keel Angle
aab
Total and Component Lift
Craft Trim Angle Wetted Lengths Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 18
Other Inputs Characteristic
2.1
Chine Flat Width
2.2
Chine Section Shape / Angle
2.3
Keel Flats / Pads
2.4
Air Entrainment Devices & Effects
2.5
Strakes
2.6
Step Planform Shape
Direct Design Input Variables After Body Parameter
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
1.1
Chine Beam
1.2
Step Height
1.3
Length
1.4
Deadrise
1.5
Keel Angle
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 19
In general, solution incorporates: • •
Sum Lift, Resistance and pitching moemts about LCG & Iterate on Trim to Find Equlibrium
Solt’n
Semi-Empirical Steady State Equilibrium Solution using Savitsky Hadler Problem is broken into three parts, forebody, step, afterbody: Area
Forebody
Step
Afterbody
Inputs
Outputs
Source
•Displacement •Projected Chine Beam, •Deadrise •LCG, VCG •Speed
•Resistance •Lift •Trim Angle •Wetted Keel Length •Wetted Chine Lengths
Savitsky / Hadler
•Step Height •Trim Angle •Speed •Deadrise
•Height of Wake at any point along X •(Based on this data we can calculate afterbody keel and wake intersection point, and therefore calc. Afterbody Wetted keel length and effective trim angle based on wake slope)
SavitskyMorabito
•Wetted Keel Length, •Trim, •Projected Chine Beam, •Deadrise, •Speed
•Resistance, Lift
Savitsky / Hadler
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 20
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 21
• • • • • •
Wake Model Fundamentally Connects Forebody with Afterbody Savitsky-Morabito work defined Wake Profile curve as a function which can be readily calculated. Wake Curve can be intercepted with afterbody to determine afterbody wetting, and effective trim angle. Wake Profile work based on Tow tank testing and direct measurement of the transom wake profiles of various deadrise prismatic models. Data reduced and correlated with important design parameters to allow direct calculation of wake profile. Data interpolated to different deadrise angles based on the presented formulas and data from 10, 20 and 30 degree tow tank tests.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 22
Savitsky – Morabito provide a model for wake profile based on tank testing:
Note that K is coeff. for deadrise, interpolated from Savitsky-Morabito wake Formulas, (for beta >=20, K=2.0)
, Wake Profile at Keel
We employ the keel solution to interface with the Hadler solution to calculated the wetted keel length and planing area on the afterbody. It is possible to use the quarter chord solution to solve for the effects of step planform shape by using the resultant calculated change in wetted area. This is an opportunity for future work. Further, according to Faltinsen and Doctors, the flow separates from the step at speeds where Ds the draft at the step relative to the running waterline.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
with
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 23
LIMITS OF WAKE PROFILE METHOD:
DEADRISE LIMITED – WITHIN BOUNDS OF TYP CRAFT TRIM ANGLES LIMITED – WITHIN BOUNDS OF TYP CRAFT ENSURES WETTED CHINE SOLUTION ONLY, PREVENTS CASES WHERE SPRAY JETS WET AFTERBODY (‘W AFTERBODY WETTING)
SETS LOWER BOUNDS TO PREVENT SPRAY JET WETTING OF AFTERBODY (‘W AFTERBODY WETTING’) BE CAREFUL HERE, THIS IS BEAM SPEED COEFF., = V/(qB)^1/2, & DATA & EXPERIENCE SHOWS FOR Cv BELOW 4.0 IS NOT ACCURATE. THIS APPROACH MAY NOT WORK FOR VERY LONG, NARROW CRAFT…SUCH AS CATAMARANS, BUT WE HAVEN’T YET TRIED IT
Further, according to Faltinsen and Doctors, the flow separates from the step at speeds where Ds the draft at the step relative to the running waterline.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
with
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 24
LIMITS OF WAKE PROFILE METHOD:
1. Solution Highly Dependent on Trim Angle 2. High Trim Angles Produce Erroneous Results 3. Deadrise highly dependent on appendages, such as pads and lifting strakes 4. May need to create a pseudo deadrise if appendages are significant 5. Method Cannot predict simple potential problems – Too Short Steps, Improper Ventilation Paths
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 25
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 26
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 27
Strengths: –
Predicts Resistance, trim and lift forces based on primary design parameters
–
Allows incorporation of conventional appendages, including strakes, trim tabs, etc.
Limits and Weaknesses: – –
Limited by Experimental Data Set Many factors not considered in solution: • • • •
Off-Design Low Speed Resistance and trim Maneuvering Transverse Stability Seakeeping
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
–
Other Aspects not Considered in Solution: •
Planform Shape of Step
•
Step Inlet geometry
•
Ventilation systems/methods
•
Edge Treatments
•
Step Outer Wetting (“W Wetting”)
•
Porpoising / Dynamics
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 28
Case
Description
Type
1
Savitsky /Morabito Validation
Wake Profile Validation
2
Garland Validation
Stepped Hull Tow Tank Testing Displacement
Semi-Planing
Model Trim vs. Velocity
Planing
7
6
Trim (degrees)
5
4
3
Savitsky Prediction
2 Unstepped Hull Zero Step
1
Step Depth =6% Chine Beam Step Depth = 4% Chine Beam Step Depth = 2% Chine Beam
0 0
5
10
15
20
25
30
35
Velocity (ft/s)
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 29
6.1 Example Craft – Savitsky/Morabito Validation Design Particulars – Savitsky/Morabito Validation Case LOA Bab Bfb bab bfb
= = = = =
32’ 7.8’ 7.8’ 12.5 deg 12.5 deg
Displ.= LCG = Hs = Lab = aab =
10 KIP 1.9’ Fwd. Step=15.4’ 5% Bab = 0.39’ 13.5’ 0.5 deg
Cv V
= =
4.3 46 MPH = 40 Knots
Results
Conclusions • Wetted Length aligned well 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 TRIM
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
• Vessel Trim aligned well • Afterbody Lift overpredicted by WPM versus Example • Overall WPM appears to produce results in good alignment with validation case • Validation case example only and not tank model. AB WETTED KEEL LENGTH
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 30
6.2 Example Craft – USNA Garland Validation Design Particulars – Garland Validation Case LOA Bab Bfb bab bfb
= = = = =
4.8’ 1.5’ 1.5’ 15 deg 15 deg
Displ.= LCG = Hs = Hs = Lab =
57.45 # 0.3’ step = 1.97’ FWD. 2%, 4% & 6% Bab 0.03’, 0.06’, 0.09’ 1.67’
aab
=
0.0 deg
Cv V
= =
varies varies MPH
Notes: 1. Tow Tank Model 2. Analysis Run at model size 3. Potential problems with scaling
Conclusions
Results 5
8.5
8.5 0% STEP HT. - O5 0% STEP HT. USNA
Trim Angle (Degrees)
4.5
7.5 4% STEP HT. - O5
2% STEP HT. - O5 7.5
4% STEP HT. - USNA
7.5
2% STEP HT. - USNA
6.5
6% STEP HT. - O5 7 6% STEP HT. - USNA
6.5
6.5
5.5
6
4.5
5.5
3.5
5
4
5.5 3.5 4.5 3 3.5
2.5
4.5
2.5 2.5
3.5
4.5
Cv (non-dim. speed)
5.5
2.5
2.5 2.5
3
3.5
4
4.5
5
• Garland concluded that 4% step height optimum for resistance. • Garland concluded that ventilation by natural means sufficient and does not affect resistance. • Vessel Trim aligned well within applicable Cv Range • Craft Lift and Resistance did not align with WPM due to scaling effects
3.5
4.5
Cv (non-dim. speed)
5.5
2.5
3.5
4.5
Cv (non-dim. speed)
Cv (non-dim. speed)
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 31
7.0 Computational Fluid Dynamics
• What is CFD: – Employs numerical solution to the Navier Stokes Equations by discretization of a fluid volume within and around a solid shape. – Non-Linear Solution can solve for steady state or dynamic system in time – Reynolds-average Navier-Stokes includes Turbulence Modelling – Various Turbulence Models and other features can be included – Full Navier Stokes solutions can include free surface and mixed flow/multi-phase – Volume Air Fraction used to understand Mixed Air / Water System
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 32
7.0 CFD • Why is it important: – Permits simulation of effects that cannot be modelled using SemiEmpirical or Analytical Methods
• What Can it do: – Model the Steady State of the Craft Operating in the Water and Air – Aero and Hydro Effects – Model Dynamic, time Varying Effects,..ie porpoising – Model Small Features like Pads, Strakes – Measure Pressures, flow velocities and vectors, Spray Shapes – Measure Rigid Body Forces
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 33
7.0 CFD
Limits & Challenges to CFD for Stepped Hulls: – – – – – –
Meshing Detail around small details Time Step and Number Iterations / Step & Convergence Validation, Validation, Validation Alignment with Seatrials “What’s Real Dilemma” or the curse of the management plot Garbage In / Garbage Out • • • • • • • •
Validation Weights Centers Model / Mesh Quality Propulsive Forces / Prop. Model (Lift Forces) Time Varying Forces Modelled Damping Aero Model
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 34
1) Wake Profile Method for the design of stepped hulls can be used effectively to reliably predict the performance of the craft based on a basic design parameters. 2) Higher order methods, such as CFD, must be used to further optimize and investigate additional detailed design elements, and performance characteristics such as maneuvering, pre-planing regimes and shape of steps in planform.
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 35
References: 1. Savitsky, Daniel and Michael Morabito. “Surface Wave Contours Associated with the Forebody Wake of Stepped Planing Hulls.” Marine Technology Vol. 47, No. 1, pp. 1-16 (2010). 2. Savitsky, Daniel. “Hydrodynamic Design of Planing Hulls.” Marine Technology (1964). 3. Garland, William R., Midshipman First Class, “Stepped Planing Hull Investigation.” Senior Paper, United States Naval Academy 2010 4. Clement, Eugene P. and Joseph G. Koelbel. “Optimized Designs for Stepped Planing Monohulls and Catamarans.” High Performance Marine Vehicles (1992): PC35-43. 5. Faltinsen, Odd M. Hydrodynamics of High-Speed Marine Vehicles. New York: Cambridge University Press, 2005. 6. Smyth, Pete, “Stepping in the the Future,” Professional Boatbuilder , Number 5, June/July 1990. 7. Hadler, J.B., “The Prediction of Power Performance on Planing Craft.” SNAME Transactions 1966 8. Milwitzky, B. ,”A General Theoretical and Experimental Investigation of Motions and Hydrodynamic Loads Eperienced by V-bottom Seaplanes during Step Landing Conditions.” NACA TN 1516 Wash. DC 1948 9. Mssrs. Morabito & Savitsky, Personal Communications via Email, Summer 2010
Robert Kaidy - Naval Architect OCEAN5 NAVAL ARCHITECTS
IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 36
Thanks for Your Interest
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IBEX 2013 – SESSION 302 Adv. Topics in Stepped Hull Design Page 37
