Performance Prediction and
Design Optimization
Midn 1/c Jon P. Silverberg
Performance Prediction
• The velocity of a ship is inherent to its
mission effectiveness
• Three methods of determination
– Hydrodynamic Tank Testing
– Parametric Analysis
– Computational Fluid Dynamics
Mk II Navy 44
Sail Training Craft
• Designer David Pedrick supplied:
– Lines Plan
– Sail Plan
– Parametrically predicted speeds from the
IMS Velocity Prediction Program (VPP)
• Sailing craft complexities
– Six Degrees of Freedom
– Propulsion Systems
– Lifting Surfaces
Performance Prediction
Process
• Performance predicted for both motoring
(upright) and sailing conditions
• Three methods of hydrodynamic analysis
– Tank Testing
– “FKS” Computational Fluid Dynamic (CFD) code
– “SPLASH” CFD code
• Upright analysis use only hydrodynamic data
• Sailing conditions necessitate aerodynamic
data and use of a Velocity Prediction Program
Initial Parametric Analysis
• Needed to determine sailing conditions and
sail forces which the IMS VPP did not provide
• Predictions were
performed with
University of
Michigan’s
“PCSail”
developed by
David Martin
• Derived
conditions to test
in the tow tank
Basic Hydrodynamics
• Two main components of Resistance
– Wavemaking – determined experimentally
– Viscous (friction) – calculated
• Speed of the model is scaled to reproduce
wavemaking characteristics
• Turbulence stimulators are added to
reproduce viscous flow conditions
• Wavemaking results scaled to ship size
• Viscous resistance calculated for ship size
Tow Tank
Testing
• Data Recorded
–
–
–
–
Drag
Lift
Yaw Moment
Pitch
• Tests included
• Upright Conditions
• Sailing Conditions
• Viscous Corrections
• Performed more than 570 runs
FKS – CFD
• Developed by Dr. Noblesse at Carderock NSWC
• Calculated the far-field wave spectrum
– Far-field waves are the totality of wave interactions
– Wavemaking resistance is calculated through the
energy required to create the far-field waves
• Hull had one degree of
freedom (forward)
• Lift was not calculated
• Simulations were fast
• FKS was best suited for
upright calculations
• Initial investigation
made on the Wigley hull
Wavemaking Comparison
Scale effects
in tank testing
Spray and pitch
Viscous
interaction in
wavemaking
SPLASH – CFD
Development was by
Bruce Rosen of South
Bay Simulations and
Joe Laiosa from Navair
at Patuxent River
• Calculated inviscid
fluid velocities across
a discretized hull
• Hull had six degrees
of freedom
• Calculations included
upright testing and
sailing conditions
• SPLASH ran on a
high resource system
All Upright Data
Upright Resistance of Mk II Navy 44 STC
5,000
4,500
Tank
FKS
SPLASH
4,000
Resistance [lbs]
3,500
Pitch Effects
3,000
2,500
Different method
of viscous
calculations
2,000
1,500
1,000
500
0
0
2
4
6
Velocity [knots]
8
10
12
Tank
Testing
Sailing
Hydrodynamic
Data
SPLASH
Utilized MATLAB and
MAPLE to interpolate
through sailing
matrices
Mk II Navy 44 VPP
• Aerodynamics
– Analyzed sail forces using basic wing theory
– Force coefficients from sails derived from IMS VPP
• Solution
– Draghydro= Driveaero
– Lifthydro= Sideforceaero
– RightingMomenthydro =
HeelingMomentaero
– YawingMomenthydro =
YawingMomentaero
Custom VPP had a complex
hydrodynamic model and
solution but a simplified
aerodynamic model
• Custom VPP written in Excel using Visual Basic
• Created three VPPs from Tow Tank data, SPLASH
data, and FKS data (which proved unusable)
• Predicted speed for any wind condition and angle
Solved by simultaneous
equations using finitedifference iteration
Performance Predictions
Low wind speeds
showed variable
results due to
simplifications in the
aerodynamic model
All other wind speeds
showed excellent
correlation between
all velocity predictions
Performance Prediction Conclusions
• Tow tank method was limited by
scale factors at low model speeds
• FKS was limited to upright testing
• SPLASH proved a valuable tool
when its accuracy was increased
with tank data
• IMS and PCSail VPP’s provide
reliable trends based on
extensive hydrodynamic data
• Custom VPP provided best
results using complex
hydrodynamic models
• Polars constructed as best fit of
all VPP data
Rudder Design
• Designer David Pedrick
provided an unfinished
rudder design for the Mk II
Navy 44
• Three design comparisons
– Size (Tow tank and SPLASH)
– Planform (SPLASH)
– Location and Depth
(SPLASH)
Size Comparison
• Comparison of
the Pedrick and
“Beaver” rudders
• Beaver rudder
provided better
turning ability
and upwind
ability in high
wind speeds
• SPLASH and Tank testing results were similar
• SPLASH was used for the rest of the testing
based on its accuracy and precision
Planform Comparison
• Picked different shapes to determine the
resulting flow patterns
• Each shape was analyzed in SPLASH in
under two hours
Baseline
Bulge
Elliptical
Tip
Zoid
Planform Results
• Picked different shapes to determine the
resulting flow patterns
• Each shape was analyzed in SPLASH in
under two hours
Baseline
Bulge
Elliptical
Tip
The Tip rudder showed a drag
reduction of 0.8% in turning
and 0.2% while sailing upwind
• The Tip rudder was
the most efficient
rudder
• The Tip moved the
induced vortex
away from the main
lifting surface
Location and Depth Comparison
• Compared rudders
– Moved forward 1.6 ft
– Moved forward 3.2 ft
– Increased size to
maximum draft (85%
total draft)
• Forward movement
interfered with wake from
the keel
• Increased depth interfered
with keel vortex
• No rudder proved better
than the baseline
• Forward movement thought to reduce drag
caused by wavemaking
• Size increase thought to increase efficiency
by reducing the relative induced drag
Rudder Design Conclusions
• The Beaver rudder provided better
turning and upwind ability, but was
slower downwind
• Moving the rudder forward or increasing
its maximum draft increased drag
• By adding a tip onto the Baseline
rudder, the overall performance of the
Mk II Navy 44 would be improved
• CFD was more effective and efficient at
redesigning appendages than tow tank
testing
Future Work
• Integrate the rudder results into the custom VPP
• Larger models should be tested to validate tow
tank results
• Viscid CFD codes should be used to evaluate the
rudder comparisons
• An improved Aerodynamic model should be
used in the custom VPP
• Full-scale testing would validate all of the data
once a prototype is built
Questions?