Advanced Topics in Stepped Hull Design - Robert Kaidy - IBEX Session104_1

8/18/2019 Advanced Topics in Stepped Hull Design - Robert Kaidy - IBEX Session104_1

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IBEX 2013 – SESSION 302

Adv. Topics in Stepped Hull DesignPage 1

Robert Kaidy - Naval Architect

OCEAN5 NAVAL ARCHITECTS

Robert S. KaidyNaval Architect & CEO

360 NW ALICE AVE., STUART, FL 34994

O: 772-692-8551 E: [email protected]

www.ocean5inc.com


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Presentation Goal:

• Introduce design considerat ions for Stepped planing hull design

and optimization.

• Review the use of the “Wake Profile Method.”

• Identify strength , weaknesses, and l imits o f f i rst order

approach for design and optimization.

• Present examples using full size craft.

• Discuss use of CFD for Design and Optimization.


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Wake Profi le Method

1. Introduction

2. Definitions

3. Goals of Stepped Hull Design

4. Wake Profile Method4.1 Design Input & Output Parameters

4.2 Analysis Methodology4.3 Equilibrium Solution

5. Strengths and Weaknesses of WPM

6. Example Craft6.1 Savitsky/Morabito Validation

6.2 USNA Tow Tank Validation

7. CFD8. Conclusions

9. References & Resources


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• Invented by Rev. Ramus of Sussex England in 1872.

• William Henry Fauber obtained a US Patent for hulls with multiple steps in 1908.


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• 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.


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Primary Advantages of Properly Designed

Stepped Planing Hull over non-stepped:

• Reduced Resistance

• Increased Speed

• Improved Efficiency

• Improved Seakeeping

• Compelling Marketing / Hull Story


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• 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


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• Air Lubricated

• Air Bearings

• Air … pretty much anything

• Ram Air Lift

• Big Steps = Fast Boat

• Little Steps = Fast Boat

• More Steps = Fast Boat


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Offsetting potential characteristics caninclude:• 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 thanConv. Hull


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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


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• 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 stepheight 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


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Afterbody Step


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Lab= Length of afterbody

aab= Angle of Afterbody measured from baseline

to keel

“ab” defined as subscript for afterbody values,

“fb” Forebody


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Components to achieve reduced Resistance:

3.1 Optimal running trim

3.2 Reduced Wetted Area

3.3 Increased Aspect Ratio Lifting Area


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3.1 Optimal Trim

Planing Hull resistance is afunction of trim angle

Resistance bucket exists for allplaning hulls at ~4.3 Degrees

Resistance increases rapidly at

lower trim angles

Resistance relatively constantfrom 3.5 to 4.5 deg.

Conv. Planing hulls trim angledecreases with speed, creatinghigher wetted resistancecomponent at higher speeds

A craft with trim control atoptimum trim angles would havelower resistance


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3.2 Reduce Wetted Area• Result of trim control

• Result of splitting planing area into two more highly loaded areas.

V=40 knots V=40 knots


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4.1 Input & Output

Parameters

4.2 Analysis

Methodology – Use ofWake Profile Modeling

4.3 Equilibrium Solution


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After Body Parameter Variable

1.1 Chine Beam Bab1.2 Step Height hS1.3 Length Lab1.4

Deadrise ab1.5 Keel Angle ab

Characteristic Approach

2.1 Chine Flat WidthChange Effective

Deadrise

2.2Chine Section

Shape / Angle

Change Effective

Deadrise

2.3 Keel Flats / PadsChange EffectiveDeadrise

2.4Air Entrainment

Devices & EffectsN/A

2.5 Strakes Savitsky /Hadler

2.6Step Planform

ShapeN/A

Other InputsDirect Design Input Variables

Outputs at Equilibrium:

Total and Component

Resistance

Total and Component Lift

Craft Trim Angle

Wetted Lengths


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After Body Parameter

1.1 Chine Beam

1.2 Step Height

1.3 Length

1.4 Deadrise

1.5 Keel Angle

Characteristic

2.1 Chine Flat Width

2.2Chine Section Shape /

Angle

2.3 Keel Flats / Pads

2.4Air Entrainment Devices &

Effects

2.5 Strakes

2.6 Step Planform Shape

Other Inputs

Direct Design Input Variables


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In general, solution incorporates:

• Semi-Empirical Steady State Equilibrium Solution using Savitsky Hadler• Problem is broken into three parts, forebody, step, afterbody:

Solt’n Area Inputs Outputs Source

S u m L

i f t , R e s i s t a n c e a n d p i t c h i n g

m o e m t s

a b o u t L C

G

& I t e r a t e o n T r i m t o

F i n d

E q u l i b r i u m

Forebody

•Displacement

•Projected Chine Beam,

•Deadrise

•LCG, VCG

•Speed

•Resistance

•Lift

•Trim Angle

•Wetted Keel Length

•Wetted Chine Lengths

Savitsky /

Hadler

Step

•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)

Savitsky-

Morabito

Afterbody

•Wetted Keel Length,

•Trim,

•Projected Chine Beam,

•Deadrise,

•Speed

•Resistance, LiftSavitsky /

Hadler


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• Wake Model Fundamentally Connects Forebody with Afterbody

• Savitsky-Morabito work defined Wake Profile curve as a function which can be readilycalculated.

• 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.


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Savitsky – Morabito provide a model for wake profile based on tank testing:

Further, according to Faltinsen and Doctors, the flow separates from the step at speeds where with

Ds the draft at the step relative to the running waterline.

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 inwetted area. This is an opportunity for future

work.

Note that K is coeff. for deadrise, interpolated from

Savitsky-Morabito wake Formulas, (for beta >=20, K=2.0)

, Wake Profile at Keel


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LIMITS OF WAKE PROFILE METHOD:

Further, according to Faltinsen and Doctors, the flow separates from the step at speeds where with

Ds the draft at the step relative to the running waterline.

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


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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


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– Limited by Experimental Data Set

– Many factors not considered in

solution:

• Off-Design Low Speed Resistance and

trim

• Maneuvering

• Transverse Stability

• Seakeeping

– 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

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:


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Case Description Type

1 Savitsky /Morabito Validation Wake Profile Validation

2 Garland Validation Stepped Hull Tow Tank

TestingModel Trim vs. Velocity

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35

Velocity (ft/s)

T r i m (

d e g r e e s )

UnsteppedHull

ZeroStep

Step Depth=6% ChineBeam

Step Depth= 4%ChineBeam

Step Depth= 2%ChineBeam

Savitsky Prediction

Displacement Semi-Planing Planing


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6.1 Example Craft – Savitsk y/Morabito Val idat ion

Results Conclusions

• Wetted Length aligned well

• 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.

LOA = 32’

Bab = 7.8’

Bfb = 7.8’

ab = 12.5 deg

fb = 12.5 deg

Displ.= 10 KIP

LCG = 1.9’ Fwd. Step=15.4’

Hs = 5% Bab = 0.39’

Lab = 13.5’

ab = 0.5 deg

Cv = 4.3

V = 46 MPH = 40 Knots

Design Particulars – Savitsky/Morabito Validation Case

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

TRIM

0

0.5

11.5

2

2.5

3

3.5

4

4.5

5

AB WETTED KEELLENGTH


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6.2 Example Craft – USNA Garland Val idat ion

Results Conclusions• 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

LOA = 4.8’

Bab = 1.5’

Bfb = 1.5’

ab = 15 deg

fb = 15 deg

Displ.= 57.45 #

LCG = 0.3’ step = 1.97’ FWD.

Hs = 2%, 4% & 6% Bab

Hs = 0.03’, 0.06’, 0.09’

Lab = 1.67’

ab = 0.0 deg

Cv = varies

= varies MPH

Notes:

1. Tow Tank Model

2. Analysis Run at model size

3. Potential problems with

scaling

Design Particulars – Garland Validation Case

2.5

3

3.5

4

4.5

5

2.5 3.5 4.5 5.5

T r i m A

n g l e ( D e g r e e s )

Cv (non-dim. speed)

0% STEP HT. - O5

0% STEP HT. -USNA

2.5

3.5

4.5

5.5

6.5

7.5

8.5

2.5 3.5 4.5 5.5

Cv (non-dim. speed)

4% STEPHT. - O5

4% STEPHT. - USNA

2.5

3.5

4.5

5.5

6.5

7.5

8.5

2.5 3 3.5 4 4.5 5

Cv (non-dim. speed)

2% STEPHT. - O5

2% STEPHT. - USNA

4.5

5

5.5

6

6.5

7

7.5

2.5 3.5 4.5

Cv (non-dim. speed)

6% STEP HT.- O5

6% STEP HT.- USNA


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7.0 Compu tat ional Fluid Dynam ics

• What is CFD: – Employs numerical solution to the Navier Stokes Equations bydiscretization 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


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7.0 CFD

• Why is it important:

– Permits simulation of effects that cannot be modelled using Semi-Empirical 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


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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


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1) Wake Profile Method for the design of stepped

hulls can be used effectively to reliably predict theperformance of the craft based on a basic design

parameters.

2) Higher order methods, such as CFD, must be usedto further optimize and investigate additional

detailed design elements, and performance

characteristics such as maneuvering, pre-planing

regimes and shape of steps in planform.


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References:

1. Savitsky, Daniel and Michael Morabito. “Surface Wave Contours Associated with the ForebodyWake 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: CambridgeUniversity 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 19489. Mssrs. Morabito & Savitsky, Personal Communications via Email, Summer 2010


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Thanks fo r Your Interest

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Advanced Topics in Stepped Hull Design - Robert Kaidy - IBEX Session104_1

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