OPTIMIZATION OF HULL SHAPES FOR WATER-SKIING
AND WAKEBOARDING
Except where reference is made to the work of others, the work described in this thesis is
my own or was done in collaboration with my advisory committee. This thesis does not
include proprietary or classified information.
_______________________________
Robert L. Daily
Certificate of Approval:
_____________________________
Jay Khodadadi
Professor
Mechanical Engineering
_____________________________
Peter Jones, Chair
Associate Professor
Mechanical Engineering
_____________________________
A. J. Meir
Professor
Mathematics and Statistics
_____________________________
Stephen L. McFarland
Dean
Graduate School
OPTIMIZATION OF HULL SHAPES FOR WATER-SKIING
AND WAKEBOARDING
Robert L. Daily
A Thesis
Submitted to
the Graduate Faculty of
Auburn University
in Partial Fulfillment of the
Requirements for the
Degree of
Master of Science
Auburn, Alabama
December 16, 2005
iii
OPTIMIZATION OF HULL SHAPES FOR WATER-SKIING
AND WAKEBOARDING
Robert L. Daily
Permission is granted to Auburn University to make copies of this thesis at its discretion,
upon request of individuals or institutions and at their expense. The author reserves all
publication rights.
__________________________________
Signature of Author
__________________________________
Date of Graduation
iv
THESIS ABSTRACT
OPTIMIZATION OF HULL SHAPES FOR WATER-SKIING
AND WAKEBOARDING
Robert L. Daily
Master of Science, December 16, 2005
(B.S., Auburn University, 1999)
143 Typed Pages
Directed by Peter Jones
This study attempts to define an optimal hull shape for a boat towing either a skier or
a wakeboarder. Two methods for determining a free surface deformation (both hull and
wake shape) given an applied pressure disturbance are compared. The methods derive
from the same potential flow, but vary in how the pressure disturbance is defined; one
uses a Fourier type approximation, the other a piecewise constant interpolation.
Using the results from the comparison of the two methods, an approach for simulating
the wake shape given different hull shapes (via the pressure distribution) is developed.
Using this approach the hull shape is optimized based on two wake parameters: height
and slope. Examples of this optimization are presented for different towing scenarios.
The resulting hulls and wakes are then examined for realism.
v
ACKNOWLEDGEMENTS
The author would like to thank his advisor Dr. Peter Jones for his guidance through
the highs and lows of this research. He would also like to thank the other committee
members, Dr. Jay Khodadadi and Dr. A. J. Meir, for their instruction both in classes and
when problems arose during research, not to mention their patience through this process.
The author additionally wishes to express his gratitude to his entire family for their
support throughout his entire school career and for instilling a deep love for the activities
motivating this research. Also, he wishes to thank Ms. Susan Wooden for her support
and encouragement.
vi
Style manual or journal used: ASME Journal of Dynamics Systems, Measurement, and
Control.
Computer software used: Microsoft Word 2003.
vii
TABLE OF CONTENTS
List of Figures..................................................................................................................... x
List of Tables ...................................................................................................................xiii
Nomenclature................................................................................................................... xiv
1. Introduction................................................................................................................. 1
2. Mathematical Derivations........................................................................................... 7
2.1. Basic Assumptions and Relationships ................................................................ 7
2.1.1. Conservation of Mass ................................................................................. 9
2.1.2. Kinematic Boundary Condition ................................................................ 10
2.1.3. Dynamic Boundary Condition .................................................................. 11
2.1.4. Far Field Boundary Condition .................................................................. 11
2.2. Velocity Potential.............................................................................................. 12
2.3. Sinusoidal Pressure Patches.............................................................................. 14
2.3.1. Surface Integral......................................................................................... 15
2.3.2. Wave Number Integral.............................................................................. 21
2.3.3. Wave Angle Integral................................................................................. 31
2.3.4. Evaluation Grid......................................................................................... 41
2.4. Constant Pressure Patches................................................................................. 45
2.4.1. Surface Integral......................................................................................... 47
2.4.2. Wave Number Integral.............................................................................. 48
viii
2.4.3. Wave Angle Integral................................................................................. 48
2.4.4. Evaluation Grid......................................................................................... 49
2.5. Numerical Issues............................................................................................... 51
2.5.1. Sinusoidal Pressure Method...................................................................... 52
2.5.2. Constant Pressure Method ........................................................................ 59
3. Method Validation .................................................................................................... 63
3.1. Sinusoidal Pressure Basis Function .................................................................. 63
3.2. Simplified Realistic Hull Pressure.................................................................... 67
3.3. Sinusoidal Pressure Centerlines........................................................................ 69
4. Method Issues ........................................................................................................... 73
4.1. Constant Pressure Method Symmetry............................................................... 73
4.2. Sinusoidal Pressure Method Symmetry............................................................ 75
4.3. Sinusoidal Pressure Method Oscillation ........................................................... 77
4.4. Issues With Inversion........................................................................................ 82
5. Hull Optimization ..................................................................................................... 88
5.1. Event Definition................................................................................................ 88
5.2. Typical Hull Shape ........................................................................................... 92
5.3. Test Cases ......................................................................................................... 94
5.4. Jump 1 Test Results .......................................................................................... 97
5.4.1. Resulting Pressure and Hull Shape........................................................... 98
5.4.2. Resulting Wake....................................................................................... 100
5.5. Jump 2 Test Results ........................................................................................ 102
5.5.1. Resulting Pressure and Hull Shape......................................................... 103
ix
5.5.2. Resulting Wake....................................................................................... 105
5.6. Slalom 1 Test Results...................................................................................... 106
5.6.1. Resulting Pressure and Hull Shape......................................................... 107
5.6.2. Resulting Wake....................................................................................... 108
5.7. Slalom 2 Test Results...................................................................................... 109
5.7.1. Resulting Pressure and Hull Shape......................................................... 110
5.7.2. Resulting Wake....................................................................................... 111
5.8. Wakeboard 1 Test Results .............................................................................. 112
5.8.1. Resulting Pressure and Hull Shape......................................................... 113
5.8.2. Resulting Non-Optimal Pressure and Hull Shape................................... 114
5.8.3. Resulting Non-Optimal Wake................................................................. 115
5.9. Wakeboard 2 Test Results .............................................................................. 116
5.9.1. Resulting Pressure and Hull Shape......................................................... 117
5.9.2. Resulting Wake....................................................................................... 118
5.10. Results Summary ........................................................................................ 119
6. Conclusions and Suggestions for Future Research................................................. 121
6.1. Conclusions..................................................................................................... 121
6.2. Suggestions for Future Research .................................................................... 123
References....................................................................................................................... 126
x
LIST OF FIGURES
Fig. 1 Nautique Ski 196 ...................................................................................................... 2
Fig. 2 Monterey 268 SS SuperSport ................................................................................... 3
Fig. 3 Super Air Nautique 210............................................................................................ 4
Fig. 4 Coordinate Frame and Uniform Flow ...................................................................... 8
Fig. 5 Sinusoidal Pressure Patch Definition ..................................................................... 15
Fig. 6 Complex Plane and Line Integrals.......................................................................... 23
Fig. 7 Sinusoidal Pressure Hull Discretization ................................................................. 42
Fig. 8 Constant Pressure Hull Discretization.................................................................... 46
Fig. 9 Sinusoidal Pressure Combined Integrands ............................................................. 55
Fig. 10 Sinusoidal Pressure II Integrand with Slow Oscillations ..................................... 56
Fig. 11 Sinusoidal Pressure Final Integrands.................................................................... 58
Fig. 12 Constant Pressure Original Far Field Integrand ................................................... 60
Fig. 13 Constant Pressure Far Field Integrand with Slow Oscillations ............................ 61
Fig. 14 Constant Pressure Far Field Final Integrand ........................................................ 62
Fig. 15 Sinusoidal Pressure Basis Function Test Pressure ............................................... 64
Fig. 16 Sinusoidal Pressure Basis Function Test Wake.................................................... 64
Fig. 17 Sinusoidal Pressure Basis Function Test Hull...................................................... 65
Fig. 18 Simplified Realistic Hull Test Pressure................................................................ 67
Fig. 19 Simplified Realistic Hull Test Wake.................................................................... 68
xi
Fig. 20 Simplified Realistic Hull Test Hull ...................................................................... 69
Fig. 21 Sinusoidal Pressure Basis Function Centerlines................................................... 71
Fig. 22 Single Constant Pressure Patch Wake.................................................................. 74
Fig. 23 Corresponding Constant Pressure Patch Wakes................................................... 74
Fig. 24 Total Constant Pressure Patch Basis Function ..................................................... 75
Fig. 25 Sinusoidal Pressure Patch Wakes......................................................................... 76
Fig. 26 Loss of Data as More Fourier Terms are Used..................................................... 78
Fig. 27 Oversampled Pressure Basis Function ................................................................. 79
Fig. 28 Oversampled Wake Basis Function...................................................................... 80
Fig. 29 Oversampled Wake Basis Function Surface ........................................................ 81
Fig. 30 Original Test Surface............................................................................................ 83
Fig. 31 Pressure Resulting From Original Test Surface ................................................... 84
Fig. 32 Pressure Resulting From Rotated Test Surface .................................................... 85
Fig. 33 Increasing Pressure Oscillation ............................................................................ 87
Fig. 34 Ski Jumping Example and Course Layout ........................................................... 89
Fig. 35 Slaloming Example and Course Layout ............................................................... 90
Fig. 36 Wakeboarding Examples...................................................................................... 91
Fig. 37 Wake Parameters .................................................................................................. 92
Fig. 38 Basic Planing Hull Shape ..................................................................................... 93
Fig. 39 Examples of Planing Boats................................................................................... 95
Fig. 40 Jump 1 Minimization............................................................................................ 98
Fig. 41 Jump 1 Pressure and Hull Shape ........................................................................ 100
Fig. 42 Jump 1 Wake ...................................................................................................... 102
xii
Fig. 43 Jump 2 Minimization.......................................................................................... 103
Fig. 44 Jump 2 Pressure and Hull Shape ........................................................................ 104
Fig. 45 MasterCraft X-Star ............................................................................................. 105
Fig. 46 Jump 2 Wake ...................................................................................................... 106
Fig. 47 Slalom 1 Minimization ....................................................................................... 107
Fig. 48 Slalom 1 Pressure and Hull Shape...................................................................... 108
Fig. 49 Slalom 1 Wake.................................................................................................... 109
Fig. 50 Slalom 2 Minimization ....................................................................................... 110
Fig. 51 Slalom 2 Pressure and Hull Shape...................................................................... 111
Fig. 52 Slalom 2 Wake.................................................................................................... 112
Fig. 53 Wakeboard 1 Maximization ............................................................................... 113
Fig. 54 Wakeboard 1 Pressure and Hull Shape............................................................... 114
Fig. 55 Wakeboard 1 Non-Optimal Pressure and Hull Shape ........................................ 115
Fig. 56 Wakeboard 1 Non-Optimal Wake ...................................................................... 116
Fig. 57 Wakeboard 2 Maximization ............................................................................... 117
Fig. 58 Wakeboard 2 Resulting Pressure and Hull Shape .............................................. 118
Fig. 59 Wakeboard 2 Wake ............................................................................................ 119
xiii
LIST OF TABLES
Table 1 Test Parameters.................................................................................................... 94
Table 2 Typical Tow Boat Parameters.............................................................................. 96
NOMENCLATURE
A
amplitude coefficient of a pressure patch (N/m
2
)
B half?beam (m)
E leading edge contribution tensor (-)
*
E final wave height basis tensor (-)
I total number of longitudinal solution points in the wetted area (-)
J total number of transverse solution points in the wetted area (-)
L wetted length of the craft (m)
M total number of longitudinal pressure patches (-)
N total number of frequency multiples / transverse pressure patches (-)
P pressure (N/m
2
)
U vehicle speed (m/s)
a pressure patch length (m)
b pressure patch width (m)
g acceleration due to gravity (m/s
2
)
i longitudinal index of solution point (-)
j transverse index of solution point (-)
k solution point index (-)
k
0
fundamental wave number (1/m)
m longitudinal index of pressure patch (-)
xiv
n frequency multiple of the pressure patch / transverse index of pressure patch (-)
p pressure strip index (-)
t time (s)
u x coordinate of velocity (m/s)
v y coordinate of velocity (m/s)
w z coordinate of velocity (m/s)
x forward coordinate (m)
y port coordinate (m)
z up coordinate (m)
?
i forward direction (-)
?
j port direction (-)
?
k up direction (-)
v total fluid velocity (m/s)
? total flow potential (m
2
/s)
? height of the fluid surface (m)
? coordinate of the pressure (-)
?
j
? wave angle (rad)
? coordinate of the pressure (-)
?
i
? fluid density (kg/m
3
)
? craft disturbance potential (m
2
/s)
xv
1
1. INTRODUCTION
Wake shape is an important factor to consider when designing recreational or
professional water skiing towboats. With the gain in popularity of wakeboards and other
water sports, the ?ideal? wake has become much more complicated. Most professional
ski events are based around objects in the water: slalom buoys, jump ramp, etc [AWSA,
IWSF]. For these events the competitor wants as little interference from the wake as
possible, i.e. a flat wake. Wakeboard events, on the other hand, rely on the wake for the
competition. Most events are based around using the wake as a launching platform for
varying jumps [WWA, WWC]. For these events the competitors want a wake shaped to
allow higher jumping. The shape of the wake is affected by several boat characteristics
and operating conditions such as trim, speed, and mean draft. The largest design variable
is the hull shape.
Most current towboats are designed based on experience and past designs. The
potential for radical improvements to boat design exists in utilizing computer simulations
to test designs before they are manufactured. There are two philosophies to designing
towboats; the first is pure design (either wakeboard or ski). These boats are designed for
the sole purpose of towing a specific piece of equipment. Other considerations such as
passenger room and comfort are secondary to towing. The boat is designed to create the
best wake for the specific equipment. A boat with this design is usually reserved for
tournament tow boats; they are too limited for general public enjoyment. An example of
this type of design is the Nautique Ski 196. Note the closed bow and limited seating.
This boat is not designed for passengers. Also notice the engine compartment in the
center of the boat; this configuration is known as an inboard engine. Directly in front of
the engine compartment is the ski pylon where the tow rope is attached. The forces from
the rope act near the center of the boat and so pull it from its course less. It is not
obvious, but the hull is also designed specifically with the wake in mind.
Fig. 1 Nautique Ski 196
The second philosophy is mixed design. These boats are for more general
recreational use. The wake produced can be used for either skiing or wakeboarding, but
is not ideal for either. An example of this type of design is the Monterey 268 SS
SuperSport. This boat has an open bow and much more seating. The engine
compartment is at the rear of the boat in what is known as an inboard/outboard
configuration. There is no ski pylon interior to the boat to attach the tow rope; instead on
the transom there is a hook. Because the rope attach point is at the rear of the boat, when
the skier swings wide it can cause the rear of the boat to also move in that direction. The
hull of this boat is designed more for passenger comfort than wake performance.
2
Fig. 2 Monterey 268 SS SuperSport
Wakes shapes for this type of boat are very hard to optimize. The main differences in
operating conditions between skiing and wakeboarding are rope length (rider placement
in the wake) and boat speed. The goal of the design is to make the wake act differently
based on these two conditions. Recently some companies have started putting water
tanks at the stern of boats such as the Super Air Nautique 210; these tanks increase both
draft and trim when full allowing the boat to create a bigger wake for wakeboarding.
Another feature of most pure wakeboarding tow boats is the tower which takes the place
of the ski pylon. The tow rope attaches much higher allowing for better jumps, etc.
3
Fig. 3 Super Air Nautique 210
A design method using computer simulations would significantly reduce the cost and
increase the performance of the hull design. The traditional method to simulate fluid
flow is computational fluid dynamics (CFD). This method involves numerically solving
the Navier-Stokes equations [Currie]:
() ()()
2
P
t
? ????
?
+????+?=+
?
u
uu
ugg
?f. (1.1)
with ? and ? viscosity parameters of the fluid. In general this is not an easy problem to
even simulate. One simplification that can be made is to assume inviscid and
incompressible flow resulting in Euler?s equations:
() P
t
? ??
?
++?=
?
?
u
u
ug
f. (1.2)
This simplification removes some terms from the differential equations, but they are
still nonlinear and without a closed-form solution. Several methods for numerically
approximating the solution to this equation are available. These methods work very well
for fluids in enclosed spaces. They provide in-depth knowledge of both the pressure and
the velocity of the fluid throughout the entire volume in question. Determining the wake
4
5
shape does not require that knowledge, however; it only requires the free surface height.
The height can be determined from the fluid pressure and velocity information but
several problems with the CFD methods exist. One problem is the boundary conditions:
to numerically approximate the solution the conditions at the boundary of the volume in
question must be known. In enclosed spaces this is generally a condition based on the
velocity of the fluid at a solid wall or a velocity or pressure profile at an inlet or outlet. In
open water this is not as easy to determine. Related to the boundary conditions is a
bigger problem, the free surface itself. One boundary of the fluid is the free surface.
CFD methods require the boundary location to be known. However, for the wake shape
problem the air-fluid boundary location is the unknown. To solve this problem using
CFD an initial free surface height must be guessed and then the solution iterated by
varying the shape and position of the free surface until all known boundary conditions are
met. This would take an inordinate amount of time, particularly because the free surface
consists of many different points each of which must be varied independently. In
conclusion, a different method needs to be employed to determine the wake shape a boat
produces.
Researchers have studied the flow around planing bodies for many years. The initial
research centered on determining equations for the flow by using different simplifications
for the hull shape [Sottorf, Maruo 1951, Maruo 1967]. This research was used to
calculate the loading on the hull and thus the horsepower requirements for the boat
[Savitsky]. The research evolved to using numerical technique approximations instead of
simplifying the hull [Wang, Shen, Doctors]. Two numerical approaches evolved; the first
uses a series of vortices to represent the flow [Lai]. The more common approach is to use
6
pressure elements to represent the hull on the surface [Tong, Cheng]. This idea has
proven successful for determining lift and drag for hulls [Wellicome]; it is only recently
being used to also determine the free surface height behind the boat [Scullen].
The idea to determine the wake shape is to take a pressure distribution on a free
surface with a known wake shape. If this wake shape is linear in the pressure distribution
then many of the pressures can be combined over the wetted area of the boat. The
pressures are varied so the deformation of the surface in the wetted area matches the hull
shape. The free surface height behind the wetted area from the resulting pressures is then
the wake shape. Ideally, the method could be inverted so that a given hull shape (or
perhaps even desired wake shape) produces the pressure. This research compares two
pressure functions used as the basis for the pressure distribution. It then extends the
method into finding the wake created by using many pressure patches. The method
presented allows any pressure to be modeled and therefore any hull should be possible.
Using the developed method an optimal hull shape for both skiing and wakeboarding
given the constraints each activity is under is examined.
2. MATHEMATICAL DERIVATIONS
As the introduction mentioned, traditional computational fluid mechanics methods are
not appropriate for computing the wake a planing craft creates. Instead, a technique that
creates basis functions linear in the pressure amplitude will be utilized. These basis
functions are summed together with different pressures to create the hull shape of the
craft. The pressure that created this hull is then used to create the wake shape.
2.1. BASIC ASSUMPTIONS AND RELATIONSHIPS
To create the surface basis functions, the following assumptions are made about the
fluid and craft:
1. The craft is traveling at a constant speed (U). This assumption is equivalent to a
stationary craft in a flow with constant velocity.
2. The fluid is inviscid.
3. The fluid is incompressible.
4. The fluid has infinite depth.
5. The disturbance created by the craft is small compared to the constant velocity
flow.
The flow created under these conditions can be represented as a potential flow. The
potential of the total flow (?) is given by:
Ux??=? . (2.1)
7
where ? is the potential of the disturbance created by the craft. x ( ) is the direction the
craft is traveling, y ( ) is to the port side of the craft, and z ( ) is up.
?
i
?
j
?
k
Fig. 4 Coordinate Frame and Uniform Flow
y
x
U
From the definition of a potential flow the total fluid velocity is determined by
?? ?
uvw= ++ =??vijk . (2.2)
The following notation will be used to denote derivatives throughout the paper.
2
,
xxy
x xy
???
?= ?=
?
? ??
(2.3)
where ? is a generic scalar variable. Along with these basic assumptions, several
preliminary properties are required to derive the velocity potential.
8
2.1.1. Conservation of Mass
The first property to examine for the flow is conservation of mass [Currie],
0
V
D
dV
Dt
? =
?
. (2.4)
? is the fluid density, t is time, and V is the volume of interest. Expanding this equation
using Reynolds? Transport Theorem,
()
t
VV
D
dV dV
Dt
?=?+??
??
v , (2.5)
the conservation of mass equation (2.4) then becomes
()()() 0
t
xy z
V
uvwdV?? ? ?
??
+ ++ =
?
?
?
. (2.6)
This equation must be true for an arbitrary volume, so the integrand must always be zero,
( ) ( ) ( ) 0
t
xy z
uvw?? ? ?+ ++=. (2.7)
Expanding the derivatives in this equation gives
( )
0
txy z xyz
uvw uuu??? ??++++ ++=. (2.8)
Because the fluid is assumed incompressible ( 0
?
? = ) the conservation of mass equation
is further simplified to
0
xy z
uvw+ +=. (2.9)
In terms of the perturbation potential, the conservation of mass equation is
0
xx yy zz
? ??+ +=. (2.10)
9
2.1.2. Kinematic Boundary Condition
The next property to examine is the kinematic boundary condition. It states that the
normal velocity of the surface (
Dz
Dt
) is equal to the change in height of the surface with
respect to time (
D
Dt
?
) where ? is the height of the fluid surface. This idea is written as
()0
D
z
Dt
?? = . (2.11)
Expanding this using the definition of a material derivative,
txy
D
uvw
Dt
y
?
=?+?+?+ ?, (2.12)
the kinematic boundary condition (2.11) becomes
()()()()(
txy
D
zzuzvzwz
Dt
)
z
? ????=?+ ? + ? + ??. (2.13)
The assumption that the craft is traveling at a constant speed implies the system is in
steady state,
( )0
xy z
uvw1?? ?=? ? + ? . (2.14)
Written in terms of the perturbation potential,
0
xx xyyz z
U
z
? ??????=? + ? + ? ?
z
. (2.15)
Because of the assumption that the perturbation potential (and therefore the change in
surface height) is small, it is safe to linearize this equation. Additionally, it is assumed
that this equation applies on the undisturbed free surface (z = 0) instead of the true
surface (z = ?). Therefore, the kinematic boundary condition is
0
x
U? ?= + . (2.16)
10
2.1.3. Dynamic Boundary Condition
The next boundary condition to examine involves the conservation of momentum
(Bernoulli?s equation) applied on the surface of the fluid. The Bernoulli equation states
1
2
P
GF
?
+ ?????? = (2.17)
where F is the Bernoulli constant, G is the gravity term (Ggz=? ), and P is the pressure.
On the free surface the Bernoulli equation is,
(
222 2
1
2
2
xyz x
P
FU
)
g? ?? ? ?
?
=+ ++? + + . (2.18)
To determine the Bernoulli constant, the Bernoulli equation is evaluated far upstream
where the surface is undisturbed (? = 0) the pressure is atmospheric (P = 0) and the
velocity is constant (
?
U=?vi). The resulting Bernoulli constant is
2
1
2
FU= . (2.19)
Substituting this constant into the Bernoulli equation (2.18) and linearizing results in the
dynamic boundary condition:
0
x
P
Ug? ?
?
=? + . (2.20)
This equation will be used to determine the wave height, so the last term is left in. In all
other aspects it is assumed to apply on the undisturbed free surface, however.
2.1.4. Far Field Boundary Condition
The final boundary condition to consider is the far field boundary condition. This
condition derives from the assumptions about the fluid far away from the disturbance
created by the craft. In particular, the condition holds in two locations. The first is far
11
below the craft ( z =??); the second is far ahead of the craft ( x =?). The condition
states that the velocity perturbation will not affect the vertical velocity of the fluid
(0
z
? = ).
2.2. VELOCITY POTENTIAL
The conditions presented thus far apply to any velocity potential that meets the
assumptions presented in section 2.1. Wehausen and Laitone derive a velocity potential
for regular waves that meets these assumptions and describes the fluid behavior created
by applying a pressure to the free surface of the fluid. This potential is
()
() ()
() ()
2
2
2
020
2
3sec
0
2
2
00
11
,sec
2sec
sin cos cos sin sec
cos sec cos sec sin
kz
S
kz
ke
P
Ukk
kx ky dkd k e
kx ky d dd
?
?
?
?
?
????
?? ? ?
?? ??? ?
?? ? ?????
?
?
?
?
?
=
?
?
?
?
??????
????
?
?
??????
?
????
?
?
?? ? ?
?
g
g
(2.21)
where S is the wetted area of the applied pressure, ? and ? are the coordinates of the
pressure, ? is the wave angle, k is the wave number, and k
0
is the fundamental wave
number defined as
0
2
g
k
U
= . (2.22)
To determine the surface height, the dynamic boundary condition (2.20) is used. This
equation requires the partial derivative of the velocity potential with respect to the
longitudinal direction on the free surface. This partial derivative is
12
()
() ()
() ()
2
2
20
020
2
24
0
2
2
00
11
,
2sec
cos cos cos sin sec
sin sec cos sec sin .
x
z
S
k
P
Uk
kx ky dkd k
kx ky d dd
?
?
?
?
???
?? ? ?
? ?????
? ????
?
=
?
?
?
?
=
?
?
?
?
??+????
????
?
?
??????
?
????
?
?
?? ? ?
?
g
g ??
(2.23)
Solving the dynamic boundary condition (2.20) for surface height results in
() ()
() ()
() ()
()
2
2
2
020
2
24
0
2
2
00
11
,,
2sec
cos cos cos sin sec
,
sin sec cos sec sin .
S
k
xy P
gk
kx ky dkd k
Pxy
kx ky d dd
g
?
?
?
?
???
?? ? ?
?? ??? ?
?? ? ?????
?
?
?
?
?
?
=
?
?
?
?
??+????
????
?
?
???? ???
?
????
?
?
?? ? ?
?
g
g
(2.24)
An equation for wave height has now been developed; all that remains is to define the
pressure acting on the surface and evaluate the integrals.
The double integral over the surface does not have an analytical solution for all
pressures. The solution could be solved numerically at this point, but that would involve
solving four nested integrals numerically for each point in a solution grid. This is very
expensive computationally. To reduce the computation time pressures are chosen that
result in an analytical solution for the double integral over the surface. Two forms of
pressure functions will be presented. Both are linear in the pressure amplitude
coefficients. The first method creates a pressure that varies sinusoidally in the transverse
direction (y, ?) and is piecewise constant in the longitudinal direction (x, ?). The second
method is piecewise constant in both the longitudinal and transverse directions.
13
2.3. SINUSOIDAL PRESSURE PATCHES
The sinusoidally varying pressure method utilizes pressure patches that vary as sine
waves in the transverse direction to approximate the true pressure [Cheng]. Multiple
pressure patches over the same surface with different sine frequencies are added together.
This is similar to a Fourier approximation of a function. The total pressure is piecewise
constant in the longitudinal direction. In general, the form of the pressure is
() ()
1
(,) sin
2
N
n
n
n
PA BB
B
?
?? ? ? ?
=
??
+?
??
??
?
B?. (2.25)
The frequency of the sine wave is modified by changing n; A
n
(?) is the amplitude
coefficient of the pressure patch, B is the half-beam, and L is the wetted length of the
craft. With this pressure, the surface equation (2.24) becomes
()
() ( )
() ()
() ( )
0
1
1
0
1
1
,,,
2
;
1
sin,
2
1
,,, ;
2
B
N
nn
n
BL
N
n
n
B
N
nn
n
BL
ASxy dd
g
B yB
n
Ax yBxy
gB
yB
ASxy dd
Byg
?????
??
?
?
?
?????
??
=
??
=
=
??
?
?
?
? ??
?
? ??
?+=
?
??
??
?
?
??
?
?
?
?
?
??
?
?
??
(2.26)
with
() () ()
() ()
()
2
2
2
020
2
24
00
2
2
0
1
,,, sin cos cos
2se
cos sin sec sin sec
cos sec sin .
n
nk
Sxy B kx
Bkk
ky dkd k k x
ky d
?
?
?
?
?
? ?? ?
??
?
? ?? ? ??
????
?
?
?
?
???
=+ ?? ?
?
? ???
?
??
?
?
?+ ??? ?
?? ?
?
?
???
?
??
?
?
??
?
g
g
?
?
(2.27)
14
Fig. 5 Sinusoidal Pressure Patch Definition
x = -L
x = 0
x = -a
m = 1
m = 2
m = M
y = B y = 0
y,?
x,?
a
The first step in determining the wave height is to evaluate the double integral over
the wetted area.
2.3.1. Surface Integral
The pressure is piecewise constant in the longitudinal direction, i.e. A
n
(?) is piecewise
constant in ?:
15
a
() ()
,1
,2
,
,1
,
0; 0
;0
;2
;1
;2
;
0;
n
n
n nm
nM
nM
Aa
a
A Aamam
ALa La
L
?
?
?
? ?
?
?
?
?
<
?
?
?<<
?
? ?< 0), the limit evaluates to the l
2
component:
()
()
()
21
4
0
2
12
1
ikj
j
l
ik e
FQdQ d
sk k
?
?
?
?
?
??
=
?
??
. (2.69)
25
This integral is solved as
()
()
()
21
4
2
12
1
ikj
j
l
ik e
FQdQ
sk k
?
?
?
?
?
=
?
?
. (2.70)
Written in terms of sine and cosine functions the integral is
()
()
()
()()
4
2
21 21
12
1
sin cos
j
j
l
k
FQdQ k i k
sk k
?
?
??
?
?
?
? ?
=?+
? ?
?
?
. (2.71)
If the singularity is negative (k
2
< 0), the limit evaluates to
( )
4
0
j
l
FQdQ
?
=
?
. (2.72)
Therefore, the total imaginary component of this integral is
()
()
()
()
4
2
21 2
12
2
1
cos ; 0
Im
0;
j
j
l
k
kk
FQdQ
sk k
k
?
?
?
?
?
?
??
>?
??
=
?
???
?
??
<
?
?
. (2.73)
To simplify evaluation of this function, the inequality condition is converted into terms of
wave angle over which this function will be integrated. From equation (2.51) the
singularity being positive (k
2
> 0) implies
(1) 0
2sin
(1)
0.
sin
j
j
n
B
?
?
?
? ?>
?
<
(2.74)
j is given, so this function solved explicitly in terms of the wave angle is
1;0
2
2; 0.
2
j
j
?
?
?
?
=<<
= ?<<
(2.75)
26
The singularity being negative (k
2
< 0) implies
(1) 0
2sin
(1)
0.
sin
j
j
n
B
?
?
?
? ?<
?
>
(2.76)
This function is also solved explicitly in terms of the wave angle as
1; 0
2
2;0 .
2
j
j
?
?
?
?
= ?<<
=<<
(2.77)
The next integral to consider is the l
6
integral. The following parameters define the
line to integrate over:
6
:, 0
2
ii
lQRe dQiRed R
??
?
??== ???. (2.78)
The integral therefore becomes
() ()
()
1
6
2
10 2
lim 1
i
iRe
i
j
i
j
i i
R
l
Re e
FQdQ iRed
Re k sRe k
?
??
?
?
? ?
?
?
?
??
=??
? ?
??
. (2.79)
Written in terms of sine and cosine functions this integral is
() ()
()
1
6
sin cos2
2
12
0
lim 1
Ri
i
j
j
R
ii
l
ie e
FQdQ d
kk
se e
RR
????
?
??
?
?
?+
?
??
=??
????
??
????
????
??
. (2.80)
Much of the integrand is simplified by taking the limit, but one term remains:
() ()
()
1
6
2
sin cos
0
1lim
Rij
j
R
l
i
FQdQ e d
s
?
???
?
?
?+
?
??
=? ?
??
. (2.81)
27
This term depends on the value of
1
?
?
. If
1
0?
?
= the integral is solved as
()
()
6
1
2
j
j
l
i
FQdQ
s
?
?
??
=
?
. (2.82)
On the other hand, if the integral evaluates to
1
0?
?
?
( )
6
0
j
l
FQdQ
?
=
?
. (2.83)
Therefore, taking the total imaginary component of this integral leads to
()
()
6
1
1
1
;
Im
2
0;
j
j
l
FQdQ
s
?
?
?
?
?
?
?
??
??
??? 0
0
=
=
???
?
??
?
?
?
. (2.84)
Once again, it is simpler to write the condition in terms of the wave angle. From
equation (2.39), this condition becomes
( ) ( )
() ()
()
1
1cos sin
01c
1
tan .
xam yB
xam yB
xam
yB
? ??
? ?
?
?
=+ ? +??
??
=+ ? +??
??
?+ ?
=
m
m
m
(2.85)
Solving this condition explicitly for the wave angle produces
( )
1
1
tan
xam
yB
?
?
?+ ???
=
?
??
m
?
. (2.86)
The last integral to consider is over the l
7
segment. The following parameters define
this segment:
7
:, 0lQidQid? ??= =?
,
. (2.143)
Substituting back into the total wave height (2.142) produces
. (2.144)
*
,
11
NM
i j nm nmi j
nm
gAE??
==
=
??
This equation is linear in the pressure coefficients. It can be written in matrix form by
introducing two new indices: p corresponds to the pressure strip and frequency and k
corresponds to the location on the surface, i.e.
( )
()
1
1.
p mN
ki Jj
n= ?+
=? +
(2.145)
44
The wave height equation then becomes
45
pk
(2.146)
*
,
1
MN
k
p
gAE??
=
=
?
which is written in matrix form as
*
g??= AE . (2.147)
In summary, the integrals in equation (2.132) are numerically integrated over a grid
of points for the leading edge of the first pressure strip. The grid is then shifted to
generate the entire basis function tensor. Using the basis function tensor and pressure
coefficients the wave height is determined.
2.4. CONSTANT PRESSURE PATCHES
A second method to define the pressure patches is to assume a constant pressure over
the entire patch [Scullen]. Multiple pressure patches with varying pressures are then
created over the wetted area of the craft. The wave height is linear in each of the
contributing heights, so the heights from the individual pressure patches are added
together to generate the total wave height, i.e.
() (
,
11
,
NM
nm
nm
)x y??
==
=
??
xy. (2.148)
Scullen and Tuck present the following wave height
() ()
,
,
,
,
nm
nm
nm z
S
P
0x yGxyd
g
? ???
?
=??
??
? (2.149)
created by such a pressure patch, where
()
( )
2 cos sin
20
020
,, Re sec
2se
ik x y kz
k e
Gxyz dkd
kk
? ??
?
2
c
? ?
??
? ?++
?
=
?
??
. (2.150)
The partial derivative of this equation with respect to height, evaluated on the undisturbed
free surface (z = 0), is
()
( )
2 cos sin
20
22
020
,,0 Re sec
2s
ik x y
z
k ke
Gxy dkd
kk
? ??
?
ec
? ?
??
? ?+
?
=
?
??
. (2.151)
Therefore, the wave height due to a single constant pressure patch is
()
() ()
,
2
,0 2
, 22
020
cos sin
,Resec
2
.
nm
nm
nm
ik x y
S
Pk
k
xy
gk
e dddkd
?
?
?? ??
??
sec? ??
? ??
?
?
?? +???
??
=
?
??
??
g
(2.152)
Fig. 8 Constant Pressure Hull Discretization
x = -L
x = 0
x = -a
m = 1
m = 2
m = M
b
y,?
y = 0y = B
i = 1
x = x
1
i = 2
x = x
2
i = I
x = x
I
j = 1
y = y
1
j = 2
y = y
2
j = J
y = y
J
n = N n = 2 n = 1
y = b
x,?
a
As with the sinusoidal pressure method, a double integral over the wetted area of the
craft must be evaluated to determine the surface height.
46
2.4.1. Surface Integral
From Fig. 8 the surface integration is
() () () ()
()
( )
,
1
cos sin cos sin
1
nm
am
bn
ikxy ikxy
Sambn
edde
?? ?? ?? ??
d? ??
??
?? +? ?? +??? ??
?? ??
??
=
?? ? ?
?. (2.153)
To evaluate this integral two variable substitutions are made:
*
*
.
x
y
? ?
? ?
= ?
= ?
(2.154)
The surface integral evaluates to
( ) ( )
()() ()()
()() ()( )
,
cos sin
cos sin cos sin
22
cos sin cos sin
22
sin cos sin cos
.
sin cos sin cos
nm
ik x y
S
ik x am a y bn ik x am a y bn b
ik xam ybn ik xam ybnb
edd
ee
kk
?? ??
? ??
?? ? ?
??
?? ??
?
?
?? +???
??
?+? +? ?+? +?+???
???
?+ +? ?+ +?+???
???
=
+?
??
?
?
?
?
(2.155)
The single patch wave height (2.152) becomes
()
()
()() ()()
()() ()( )
2
2
,
0
, 2
2
200
cos sin cos sin
cos sin cos sin
sec1
,Re
2sincossec
.
nm
nm
ik x am a y bn ik x am a y bn b
ik xam ybn ik xam ybnb
P
k
xy
g kk k
ee
?
?
dk
? ??
?? ? ?
?
?
?? ? ?
?
?
?
?
?
?+? +? ?+? +?+???
???
?+ +? ?+ +?+??? ?
??? ?
=?
?
?
?
?
?
?
??
g
?
?
(2.156)
The surface integration is accomplished. Two integrations remain in this method, the
wave angle and wave number integrations. The first one to evaluate is the wave number
integral.
47
2.4.2. Wave Number Integral
To simplify the single patch wave height, a new function is created based on the four
terms of the surface integral. Essentially, a height component must be found for each of
the four corners of the pressure patch. The wave height equation becomes
() ()(
()( )
,
,0 0
00
,,,
,,.
nm
nm
P
)x y x am a y bn x am a y bn b
g
xamybn xamybnb
?? ?
?
??
= +??? +??+?
?
?+ ?++ ?+?
?
(2.157)
where
()
()
()
2
2
cos sin
0
0
2 2
200
sec11
,Re
2sincos sec
ik x y
k
x ye
kk k
?
??
?
?
dk? ?
??? ?
?
?+
?
=?
?
??
. (2.158)
Evaluating the wave number integral produces
()
( ) ( )
2
0 2
2
glog2Hsin
1
,
2sincos
TT TT
x yd
?
?
?
? ?
???
?
+?
=?
?
(2.159)
with
2
00
sec sec sinTkx ky? ?=+ ?, (2.160)
g the even auxiliary function defined in equation (2.91), and H the Heaviside step
function, defined as
()
0; 0
H
1; 0
z
z
z
<
?
=
?
>
?
. (2.161)
2.4.3. Wave Angle Integral
As with the sinusoidal pressure patches, the wave angle integral must be evaluated
numerically. To simplify this integration the corner integral (2.159) is divided into two
parts, a local and a far field.
48
( ) ( ) ( )
0
,,
LF
,x yxyx???=+y (2.162)
The local field corresponds to the area directly under the craft and does not contribute to
the wake created by the craft; it does have an effect near the craft, however. This term is
()
( ) ( ) ( )
2
2
2
glog2HHsin
1
,
2sincos
L
TT TxT
x yd
?
?
?
? ?
???
?
+? ???
??
=?
?
. (2.163)
The far field on the other hand contains the craft wake, but the integrand is much simpler:
()
( )
2
2
H
sin
,
sin cos
F
x
T
x y
?
?
d? ?
? ??
?
=
?
. (2.164)
The total wave height is found by summing the far field component at each corner of
the pressure patch and accounting for the local field contribution which will be discussed
in section 2.5.2.
2.4.4. Evaluation Grid
As with the sinusoidal pressure patches, the wave angle integral is solved
numerically. Therefore, a grid over which to solve the integral is created. The same grid
as the sinusoidal pressure method is used with one difference. Previously, n
corresponded to the frequency of the pressure; now it corresponds to the transverse
pressure patch. The location is discretized in the same way as in the previous method
(2.134). Similar to the sinusoidal pressure method, a basis tensor consisting of the
contributions of each pressure point at each index location is found:
()( )
)
,,, 0 0
00
,,
nmi j i j i j
ij ij
E x am a y bn x am a y bn b
x amy bn x amy bn b
??=+???+??+
?+ ?++ ?+.
(2.165)
This method requires a wave height contribution from each of the corners of the pressure
patch. Each of these contributions is identical except for the offset. This offset is
49
()
()
1
2
1
2
1
1.
i
i
j
j
xama i am
x am a a i am a
ybnbj bn
ybnbbj bnb
??
+= ?+
??
??
??
+ ?= ?+ ?
??
??
?= ??
?+= ??+
(2.166)
These can also be written in terms of the index as
1
1
.
iim
i
jjn
xamx
xamax
ybny
ybnby
?
i? +
?
? +
+=
+?=
?=
?+=
(2.167)
The basis tensor (2.165) then becomes
()( ) ( )(
,,, 0 1 0 1 1 0 0 1
,,,,
nmij im jn im jn im jn im jn
Exyxyxyxy?? ??
?+ ? ?+ ?+ ? ? ? ?+
=? +
)
. (2.168)
As with the previous method, only the first pressure patch contribution needs to be
calculated. The other patches are identical but offset. Substituting this basis tensor back
into the single pressure patch wave height equation (2.157) produces
( )
,,
,
nm i j nm nmi j
gxyPE?? =
,
,
. (2.169)
The total wave height (2.148) therefore becomes
. (2.170)
,,
11
NM
i j nm nmi j
nm
gPE??
==
=
??
The system is linear in the pressure coefficients and is written in matrix form similar the
sinusoidal pressure method, equations (2.145), (2.146), and (2.147).
In this method equation (2.164) must be numerically integrated. The integration is
performed over a grid of points to produce the contribution of one corner of a pressure
patch. The total patch contribution is found by shifting the first corner to the other three
50
51
corners and summing the results. This first patch is then shifted to produce the entire
basis function tensor. The pressure amplitudes together with this tensor determine the
total wave height.
2.5. NUMERICAL ISSUES
Both the sinusoidal and the constant pressure methods involve a numerical integration
of the wave angle integral. These integrals all contain features such as singularities and
rapid oscillations which make the numerical integration difficult to perform.
2.5.1. Sinusoidal Pressure Method
The integrals for the sinusoidal pressure method (2.132) are
()
( ) ( )
()
()
32
2
01 0 1
,1
2
2
0
1
1
2
,2
2
0
0
1
1
,3
2
0
sgn sec sec
1
sec sin ( 1)
2
sec
sgn
2 sin 2 sin
(1)
sec sin ( 1)
2
sec
sgn
2 sin 2 sin
(1)
sec sin (
j
j
j
j
j
j
j
j
kfk
el d
n
k
B
nn
f
BB
el d
n
k
B
nn
f
BB
el
k
?
?
?
?? ??
?
?
??
????
?
??
?
?
??
????
?
??
??
??
?
?
?
?
?
?
?
?
=? ?
+?
??
??
??
=?
+?
??
??
??
=?
+
?
?
()
() ( )
() ()
()
() ()
0
2
32
2
101
,1
2
2
0
1
112
1,2
2
0
0
11
2,2
1)
2
1 sgn sec cos sec
1
sec sin ( 1)
2
sec
sgn sgn cos
sin 2 sin
;1
sec sin 1
2
sec
sgn sgn cos
sin
j
j
j
j
j
d
n
B
k
ef d
n
k
B
n
B
ef d j
n
k
B
n
ef
?
?
?
?
?
?
?? ??
?
?
??
???
??
??
?
?
??
?
??
?
?
??
?
?
?
+?
+?
?
??
?
??
=? ?
+?
??
??
?
??
??
??
==
+?
??
?
??
=
?
?
?
()
1
0
2
2
0
2sin
;2
sec sin 1
2
j
B
dj
n
k
B
?
??
?
?
?
??
?
?
??
??
??
=
+?
?
.
(2.171)
It should be noted that the denominator is identical in each of these integrands:
2
0
sec sin ( 1)
2
j
j
n
Dk
B
?
??=+?. (2.172)
52
The integrands also contain only a few common numerators:
( ) ( )
()
() ( )
() ()
32
01 0 1
1
1
32
101
1
11
1sgnsecsec
sec
2sgn
2 sin 2 sin
1 1 sgn sec cos sec
sec
2 sgn sgn cos .
sin 2 sin
LN k f k
nn
LN f
BB
FN k
n
FN
B
?? ??
????
?
??
?? ??
???
??
? ?
?? ?
?
??
?? ?
?
+?
=
??
=
??
??
??
=?
??
??
=?
??
??
(2.173)
To cut down on the number of calculations performed during the numerical integration,
the leading edge wave height equation (2.131) is divided into three terms:
() () (
,,0,
,, 2
44 8
nm nm nm
nmL
AAkAn
)I II III
B
?
??
=+ + . (2.174)
The first term in this equation is
() ( ) ( )
() () ()
1,1 1,1 1,2 1,2 1,3 1,3
2,1 2,1 2,2 2,2 2,3 2,3
11 1
nn n
Ielel elel elel
el el el el el el
+? +? +?
+? +? +?
=? ? +? ? ?? +
+? ? +? ? ?? + .
(2.175)
This is written in terms of the numerators and denominator above as
() ()
() ()
() ()
() ()
0
12
2
2
10
2
11 1 12 2
11 1 12 2
11 1 12 2
11 1 12 2
.
nn
nn
nn
nn
LN LN L N L N
I
D
LN LN L N L N
d
D
LN LN L N L N
D
LN LN L N L N
d
D
?
?
?
?
+? + ?
?
+? + ?
+? + ?
+? + ?
?
??+??
= ?
?
?
?
?? + ?? +
+ ?
?
?
?
???
+ ?
?
?
?
?? + +? ?
+ ?
?
?
?
?
(2.176)
53
The second term is
() ()
1,1 1,1 2,1 2,1
1
nn
IIefef efef
+ ?+
=? ? +? ?
?
; (2.177)
or in terms of the common numerators and denominators
() ()
2
122
11 1 11 1
nn
FN FN FN FN
IId
DD
?
?
?
+? +?
?
??
???
=+??
??
?
. (2.178)
The third term is
1,2 2,2
III ef ef= + ; (2.179)
or in the common numerators and denominators
2 0
1202
2FN FN
III d d
DD
?
?
? ?
?
=+
??
. (2.180)
Combining the integrals in this way cuts down on the number of times the integration
routine calculates common terms. It also allows the integration routine to only be more
accurate where needed and reduce the number of integrating points thereby speeding up
the calculation. Typical combined integrands are shown below.
54
-pi/2 -pi/4 0 pi/4 p i/2
-30
-20
-10
0
10
20
30
I
?
?? ??
0
? 2?
-pi/2 -pi/4 0 pi/4 p i/2
-30
-20
-10
0
10
20
30
II
?
??
??
0
? 2?
-pi/2 -pi/4 0 pi/4 p i/2
-30
-20
-10
0
10
20
30
III
?
?? ??
0
?
2?
Fig. 9 Sinusoidal Pressure Combined Integrands
k
0
= 3.08, x = 0.25, y = 0.14, m = 1, n = 1, a = 0.1, B = 0.5
There are still a few issues preventing accurate numerical integration of these
functions. The first is the rapid oscillation in II as it approaches the integration limits.
To aid in the integration of this oscillation a variable substitution is made:
tant ?= . (2.181)
This substitution applied to the II integral (2.178) leads to
() ()
** **
12
11 1 11 1
nn
FN FN FN FN
IId
DD
+? +??
??
??
???
=+??
??
?
t (2.182)
55
with the numerator and denominator being
( ) ( )
()()
*2
10
2
0
1
2
11sgn 1cos1
1(1)
2
1
.
1
j
j
FN t k t
n
Dkt t
B
xam yBt
t
2
1
? ?
?
?
??
?
???
=? + +
???
=++?
+?+
=
+
m
?
?
?
(2.183)
This new integrand is shown below.
-5 0 5
-5
0
5
II
t
Fig. 10 Sinusoidal Pressure II Integrand with Slow Oscillations
k
0
= 3.08, x = 0.25, y = 0.14, m = 1, n = 1, a = 0.1, B = 0.5
The oscillations are much more reasonable. There is a new problem, however, the
integration limits are now positive and negative infinity. The integrand rapidly
approaches zero as t approaches positive or negative infinity. The integration is safely
truncated with minimal inaccuracies in the result.
Another problem exists; the integrand of each of the functions becomes infinite at
some points. To find where these singularities are, the value of the wave angle that sets
the denominator equal to zero needs to be found:
2* *
0
sec sin ( 1) 0.
2
j
j
n
Dk
B
?
??=+?= (2.184)
56
Instead of solving this equation directly, it is easier to solve it in terms of the transformed
variable used to stretch the II integrand (2.181):
**2
0
1(1)
2
j
j
n
Dkt t
B
0
?
= ++? =. (2.185)
The solution where this singularity occurs is
2
*
0
11
1
22
n
t
Bk
?
??
=? ? + +
??
??
. (2.186)
The wave angle of the singularity is therefore
2
*1
0
11
tan 1
22
n
Bk
?
?
?
? ?
??
? ?
=? ? + +
??
? ?
??
? ?
? ?
. (2.187)
By inspection, when j = 1 in the denominator the sign of the wave angle must be positive
and when j = 2 the sign must be negative to make the total denominator zero.
The location of the singularity has been found, now the singularity must be dealt with.
To remove the singularity from the integration, the integrands are modified. In general,
()
22
**
**
11
*
2
**
1
ln
NN
NN
dd
DD DD
??
?? ??
????
????
? ?
?
? ???
==
==
??
??
?
??=? +
?
??
??
??
??
?
. (2.188)
The second term in the integrand effectively cancels out the singularity of the first term.
To not add any net change to the equation, the integral of the second term is added back
to the total equation. This technique is used to remove the singularity from each of the
integrals in both the original (wave angle) and transformed variable. To apply this
technique the derivative of the denominator must be computed. In the original variable
the derivative is
57
( )
23
0
2cos secDk
?
? ?=? ; (2.189)
in the transformed variable the derivative is
( )
2
0
2
12
1
kt
D
t
?
+
=
+
. (2.190)
The modified integrands with the singularities removed are shown below.
-pi/2 -pi/4 0 pi/4 p i/2
-10
-5
0
5
I
?
?? ??
0
? 2?
-10 -5 0 5 10
-4
-3
-2
-1
0
1
II
t
-pi/2 -pi/4 0 pi/4 p i/2
-2
0
2
4
6
III
?
?? 4??
0
? 2?
Fig. 11 Sinusoidal Pressure Final Integrands
k
0
= 3.08, x = 0.25, y = 0.14, m = 1, n = 1, a = 0.1, B = 0.5
The integrands still contain discontinuities, but they are now either due to different
integration ranges, i.e. I and III contain two integrals one for negative wave angles and
the other for positive wave angles, or to the sign function contained in the integrands.
58
The first discontinuity type is not an issue because over each integration range the
function is continuous. The sign discontinuity could be removed by splitting the
integration into two separate integrals at this point. This introduces several coding issues
though; locating the discontinuity and determining which segment the singularity is in to
name a couple. Because of these issues the sign discontinuity is resolved by simply using
many integration points. While this is not the most elegant method, it does not introduce
significant integration errors.
2.5.2. Constant Pressure Method
The constant pressure method has two terms to evaluate, equations (2.163) and
(2.164); they are
()
( ) ( ) ( )
()
()
2
2
2
2
2
glog2HHsin
1
,
2sincos
H
sin
,
sin cos
L
F
TT TxT
x yd
x
T
xy d
?
?
?
?
?
? ?
???
??
???
?
?
+? ???
??
=?
=
?
?
(2.191)
with
2
00
sec sec sinTkx ky? ?=+ ?. (2.192)
The local field integral is treated specially. As Scullen and Tuck show, the local field
contribution to the surface height is mainly directly under the hull. It appears as a
discontinuity in the far field contribution that is constant in the longitudinal direction. It
also contains the even auxiliary function (2.91) which must be computed either from an
interpolation or a series expansion. Therefore, instead of taking the computational time
and effort, the local field contribution is approximated as the value of the far field at the
leading edge of the wetted area. The total wave height at this location is effectively zero.
59
Therefore, the local field must cancel the far field out. The local field is assumed
constant in the longitudinal direction for the length of the wetted area.
All of the actual wave height in the constant pressure method comes from the far field
contribution. This integral must still be computed. The far field function is much
simpler than the previous integrals. It does not have any complex functions in the
integrand and only contains a single integral to evaluate. It does have the same issues as
the sinusoidal pressure integrals. The integrand oscillates as the wave angle approaches
the integration limits and it also contains a singularity. These problems are solved using
the same methods developed for the sinusoidal pressure method.
-pi/2 -p i/4 0 pi/4 p i/2
-20
0
20
40
?
F
?
?? 4??
0
? 2?
Fig. 12 Constant Pressure Original Far Field Integrand
k
0
= 3.08, x = 0.25, y = 0.14
The integrand oscillation is countered by a variable transformation (2.181). The far
field integral then becomes
()
( )H sin
,
F
x
T
x y
t
?
?
?
??
=
?
d (2.193)
with
60
(
2
0
1Tk txyt=++). (2.194)
This integrand is shown below.
-10 -5 0 5 10
-10
-5
0
5
10
?
F
t
Fig. 13 Constant Pressure Far Field Integrand with Slow Oscillations
k
0
= 3.08, x = 0.25, y = 0.14
This integrand still has a singularity at t = 0. The singularity is removed using the
method from the sinusoidal pressure patches (2.188). Using this method the far field
integral becomes
()
( ) ( )
0
Hsinsin
,
F
x Tkx
x y
t
?
?
?
??
?
=
?
dt. (2.195)
The second term in the integrand is odd so its integral is zero and nothing needs to be
added to the integral to cancel it out. This integrand is shown below.
61
-100 -50 0 50 100
-1
-0.5
0
0.5
1
?
F
t
Fig. 14 Constant Pressure Far Field Final Integrand
k
0
= 3.08, x = 0.25, y = 0.14
The singularity has been removed; due to the scale of the integration limits the oscillation
is more pronounced. Many integration points must be used, but this function can be
accurately numerically integrated.
62
3. METHOD VALIDATION
To validate the methods presented in the previous section several tests are performed.
These tests compare the sinusoidal and constant pressure methods derived above and
where possible include or reference outside results.
3.1. SINUSOIDAL PRESSURE BASIS FUNCTION
The first test is taken from Cheng and Wellicome?s paper presenting the sinusoidal
pressure method. The test utilizes the simplest pressure function available to this method,
a single sine wave with unit amplitude,
1
sin
2
P? y
? ?
?
=+
?
?
?? ?
??
? ?
. (3.1)
The beam and length of this ?hull? are both one. The pressure is constant in the
longitudinal direction, so only one pressure strip is used, as shown in figure 1.
63
0 0.2 0.4 0.6 0.8 1
-0.5
0
0.5
x
y
0.2
0.4
0.6
0.8
Fig. 15 Sinusoidal Pressure Basis Function Test Pressure
The fundamental wave number, representing craft speed, is 3.0779. The density of
the water and acceleration due to gravity are both set to one. This test is shown in figure
4 of Cheng and Wellicome?s paper. Both the sinusoidal and constant pressure methods
are implemented to attempt to reproduce this figure.
-2 0 2 4 6 8 10
-5
0
5
x
y
Constant Pressure M ethod
Sinusoidal Pressure M ethod
-1
-0.5
0
0.5
1
Fig. 16 Sinusoidal Pressure Basis Function Test Wake
The gross aspects of the two methods match each other quite well and also match the
results from Cheng and Wellicome. The results are very similar in the ?wake? region of
the surface. The peaks and valleys of the oscillations occur in the same places and are
approximately the same amplitudes. On the other hand, there are significant differences
64
in the two methods. It is expected that the sinusoidal pressure method would perform
better in this test because the input pressure is exactly one of its basis functions.
However, that method has significantly more noise both ahead of the applied pressure
and between the pressure and the limits of the wake. Ahead of the craft the surface
height should obviously be zero because the craft?s speed carries the disturbance
downstream. Similarly, the double rows of spikes spreading out at nearly 45 degrees
behind the pressure disturbance occur ahead of the predicted wake and are noise created
by this method. This noise does not create a problem using this method though because it
is outside of the area of interest and does not corrupt the key results. The constant
pressure method has no problem with noise in its results.
A much larger problem exists with the sinusoidal method directly under the applied
pressure, where the ?hull? is. The sinusoidal pressure method predicts that the water
immediately rises as a pressure is applied to the surface as seen in Fig. 17.
0 0.2 0.4 0.6 0.8 1
-0.5
0
0.5
x
y
Constant Pressure M ethod
Sinusoidal Pressure M ethod
-1
-0.5
0
0.5
1
Fig. 17 Sinusoidal Pressure Basis Function Test Hull
This rise is quite dramatic when viewed in the total wake surface, Fig. 16. This rise
cannot happen physically. Approximately half way along the length of the pressure the
water surface drops to match the surface predicted by the constant pressure method. This
65
66
result is not predicted in the Cheng and Wellicome paper and must be a numerical
anomaly in the calculations, but no reason has been found for it. This result is much
more significant than the noise of the sinusoidal method. The goal of this research is to
match a hull shape to a predicted wake. If the hull the sinusoidal method predicts is
incorrect in this simple example, then the method is not a valid model for predicting more
complex hull shapes.
Another point that should be mentioned is the time to compute the results using each
of these methods. The sinusoidal pressure method takes orders of magnitude longer to
complete the calculations than the constant pressure method. This timing makes sense
because the sinusoidal pressure method computes many more integrals than the constant
pressure method. Additionally, the integrals in the sinusoidal method often require an
additional integration at each point in the integrand. Even though this interior integral is
accomplished with an interpolation, it still slows the integration down.
Overall, both methods predict very similar wake shapes. The sinusoidal pressure
method has more numerical integration noise, but the noise is in parts of the surface that
do not interfere with the hull and wake surfaces. The sinusoidal pressure method also
takes significantly longer to compute. The major downfall of the sinusoidal pressure
method, though, is the rise in the wave height near the forward section of the hull. This
rise does not match any physical phenomenon (such as a bow wave) in either size or
shape; additionally it does not match the results in Cheng and Wellicome. The rise
makes this method unreliable for predicting a hull shape given a pressure definition.
3.2. SIMPLIFIED REALISTIC HULL PRESSURE
Scullen and Tuck also present a test case for a pressure distribution. This test case
represents a simplified function for the pressure a planing hull creates. The pressure is
given by
()
2
2
141P
x
??
=??
??
??
y. (3.2)
This ?hull? has a length of two and a beam of one.
0 0.5 1 1.5 2
-0.5
0
0.5
x
y
1
2
3
4
Fig. 18 Simplified Realistic Hull Test Pressure
For this test the fundamental wave number is one. As with the previous test
gravitational acceleration and the density of the water are both set to one.
67
-2 0 2 4 6 8 10
-5
0
5
x
y
Constant Pressure M ethod
Sinusoidal Pressure M ethod
-10
-5
0
5
Fig. 19 Simplified Realistic Hull Test Wake
Once again, the major aspects of the results from both the constant and sinusoidal
pressure methods match. They both show a series of peaks and valleys in the
approximate shape of an actual boat wake. The area inside the major wake has smaller
disturbances. These results are very promising for the goal of using these methods to
predict an actual wake. The sinusoidal method has several of the same issues that
plagued the sinusoidal pressure basis function test. The noise ahead of the boat is not as
apparent, but the double row of spikes behind the applied pressure is present as before,
Fig. 16, and in this test intersects the actual wake making the issue more problematic.
The rise in the surface under the applied pressure is also still present. This rise matches
the characteristics of the previous test, a rise in the front half of the pressure becoming a
depression further along the pressure.
68
0 0.5 1 1.5 2
-0.5
0
0.5
x
y
Constant Pressure M ethod
Sinusoidal Pressure M ethod
-10
-5
0
5
Fig. 20 Simplified Realistic Hull Test Hull
The area directly under the applied pressure using the constant pressure method
matches the results obtained by Scullen and Tuck, figure 9. As with the sinusoidal
pressure basis function test, the constant pressure method completed the calculations
many orders of magnitude faster than the sinusoidal pressure method.
The results of this test are essentially the same as the previous test. The constant
pressure method performs quite well and the sinusoidal pressure method has several
problems that make it an unreliable candidate to predict a hull and wake shape. An
additional promising result is the wake behind this pressure is beginning to resemble a
boat?s wake.
3.3. SINUSOIDAL PRESSURE CENTERLINES
The previous tests explored the general shape of the wakes generated using the two
pressure methods. Cheng and Wellicome also present centerlines of the surfaces
produced by the sinusoidal pressure basis function test run at different beam to length
ratios. The pressure for these tests is the same as in the sinusoidal pressure basis function
test (3.1) except the beam is varied in each test, i.e.
69
1
sin
2
y
P
B
?
? ?
?
=+
?
?
?? ?
??
? ?
(3.3)
where B is the beam. The length and speed in this test are also the same as the sinusoidal
pressure basis function test. Cheng and Wellicome present the results of these tests in
figure 3. Their results are also presented in the Fig. 21. The most obvious feature of
each of these plots is the pressure rise in the sinusoidal pressure method at the leading
edge of the pressure. In each test, the surface this method predicts rises dramatically
before dropping to a more reasonable value halfway along the pressure, x = -0.5. Other
details are also different in the three methods. The first test, B = 0.4, is inconclusive.
Neither method matches Cheng and Wellicome?s results. The basic form of the three
results is similar; a depression under the ?hull? followed by a rise and second depression
aft of the boat. The details of the three are quite different, however. The second test,
B = 1, provides more insight. Again, the basic forms of the results are similar. The
amplitudes, though, of both the sinusoidal and constant pressure methods are not as high
as Cheng and Wellicome?s results predict; they are approximately equal to each other,
however. This test corresponds to the sinusoidal pressure basis function test presented
above. In the sinusoidal pressure basis function test, the amplitude of the waves is
around one. This is also true in Cheng and Wellicome?s results for this test. Their
centerline, on the other hand, has an amplitude of over two. This implies the amplitude
of their centerline tests may not be accurate. The other aspects of this test match Cheng
and Wellicome?s results well. The location of the second peak does not exactly match
the peak of Cheng and Wellicome, but it is close in each method. The last test, B = 10,
has similar results. The Cheng and Wellicome amplitude is higher, but the peaks line up
70
better than the second test. Once again, the sinusoidal and constant pressure methods
results match quite well.
From Cheng and Wellicome
Sinusoidal Pressure
Constant Pressure
-3 -2 -1 0
-1
0
1
2
x
z
B = 0.4
-3 -2 -1 0
-2
-1
0
1
2
3
4
x
z
B = 1
-3 -2 -1 0
-2
0
2
4
6
x
z
B = 10
Fig. 21 Sinusoidal Pressure Basis Function Centerlines
Overall, the results from the methods tested do not match Cheng and Wellicome?s
results exactly. It is expected that the sinusoidal pressure method would. Some
difference is anticipated and acceptable in the constant pressure method. In each test, the
71
72
amplitudes do not match those predicted by Cheng and Wellicome. The sinusoidal and
constant pressure method amplitudes do match each other reasonably well, however. The
problem may be in the amplitude predicted by Cheng and Wellicome. Each method
predicts approximately the same frequency for the waves; wave peaks are close enough
to be acceptable. The surface rise in the sinusoidal pressure method, as in the previous
tests, is a fatal problem. The constant pressure method is a valid method to predict hull
and wake shapes given a pressure distribution.
73
4. METHOD ISSUES
In addition to the test case inaccuracies discussed in the previous chapter, there are
several other issues which may complicate using each of the methods to compute a boat
wake. Each of these issues deals with the different pressures the method is based around
and involves symmetry and oscillation frequencies.
4.1. CONSTANT PRESSURE METHOD SYMMETRY
The first issue involves the fact that boats are generally symmetric. The symmetry
assumes that the port and starboard sides of the hull are identical and also that the center
of gravity is directly over the centerline of the hull. These two assumptions lead to a
symmetric pressure distribution. Because the pressure is symmetric it is safe to only
analyze one side of the wake response and assume the other side is identical. The
constant pressure method utilizes constant pressure elements as the basis of the applied
pressure. Multiple pressure basis functions (at different locations) are added together to
produce the total pressure. A problem arises when only computing the response of half
of the pressure patches, however. The response of each pressure patch is symmetric
about the center of the pressure patch, but the pressure patch is offset from the centerline
of the hull making the pressure (and the response) non-symmetric about the centerline of
the hull (Fig. 22). Because this response is not symmetric it is no longer accurate to only
examine one side of the wake.
0 2 4 6 8 10
-6
-4
-2
0
2
4
6
x
y
Fig. 22 Single Constant Pressure Patch Wake
One solution to this problem is to not take advantage of the symmetry and compute
responses from pressure patches over the entire hull. This method must be employed if
the hull is not symmetric for any reason. On the other hand, if the hull is symmetric there
are corresponding pressure patches on either side of the centerline. These two patches
can be analyzed to produce a combined response.
0 2 4 6 8 10
-6
-4
-2
0
2
4
6
x
y
Fig. 23 Corresponding Constant Pressure Patch Wakes
This response is now symmetric; however, the advantage in time saving from only
computing responses for one half of the pressure patches has been lost. The response
needs to be calculated for only one half of the wake, but for each of the pressure patches.
74
Another solution is to only use the pressure patch on one side, and reflect the wake
created across the centerline back into the side being calculated. This reflection actually
comes from the corresponding symmetric pressure patch. The response from the actual
pressure patch that is lost across the symmetry line is identical to the response from the
corresponding symmetric pressure patch. This modified height basis function is now
symmetric about the centerline and can be safely used while still only using the pressure
patches on one side of the hull and computing the wake over the same half of the
centerline.
0 2 4 6 8 10
0
1
2
3
4
5
6
x
y
Fig. 24 Total Constant Pressure Patch Basis Function
4.2. SINUSOIDAL PRESSURE METHOD SYMMETRY
A similar symmetry condition exists for the sinusoidal pressure method; the
symmetry is easier in this case though. The pressure basis functions in this method are
defined as existing over the entire width of the hull. This means they can be inherently
symmetric without reflecting the response of another pressure patch. From (2.25) the
pressure is defined as a series of sine waves with increasing frequency. Essentially the
pressure is approximated with a simplified Fourier series. This pressure is symmetric if
75
only odd terms are used for n. To verify this symmetry condition a sinusoidal pressure
patch is simulated without the symmetry assumption. As can be seen in Fig. 25, the
resulting surface disturbance is indeed symmetric. It is valid to only compute half of the
wake.
x
y
0 2 4 6 8 10
-5
0
5
Fig. 25 Sinusoidal Pressure Patch Wakes
To strictly satisfy the derivation all Fourier terms are used to define the pressure, but
because of symmetry if the hull (and thus pressure disturbance) is symmetric the
coefficients corresponding to even n are zero. Computation time can be saved, however,
by simply discarding these terms and only computing the response from the odd terms of
n. To take this into account the index for the pressure terms is modified.
*
2nn1= ? (4.1)
where . This modified index is always odd. It is used in place of the old
index in each of the sinusoidal pressure method basis functions. Now the pressure basis
function is guaranteed to be symmetric and the resulting wake will be as well. This
means that once again only half the wake has to be simulated. Also, only half the basis
*
1, 2, 3,n = L
76
functions need to be determined because each of the asymmetric pressure basis functions
is ignored.
4.3. SINUSOIDAL PRESSURE METHOD OSCILLATION
The fact that the frequency is increasing in the sinusoidal pressure method brings up
an interesting facet of this method. To accurately capture the characteristics of a sine
wave a minimum of two points per period of the sine wave must be known. This is the
minimum frequency (Nyquist frequency) at which a signal must be sampled in order to
reconstruct it using a Fourier series [Franklin]. In practice, it is desirable to sample at
more points per period than the Nyquist frequency suggests because a Fourier series is
not generally used to reconstruct the signal; a simpler method (e.g. linear interpolation) is
used in reconstruction. These simpler methods perform better with more than two
samples per period.
The number of points this method is allowed is limited, however. In order for there to
be as many pressure coefficients as there are calculated wave heights under the hull the
number of points from the centerline to the beam must equal the number of pressure
terms. The number of samples per period is obviously lowest with the highest frequency.
The worst case number of samples per period can therefore be calculated by comparing
the number of points per half-beam to how many periods there are in a half-beam at the
highest frequency. This ratio is
worst
case
#
21
samples N
period N
=
4
?
(4.2)
77
where N is the number of pressure terms. In the limit as more pressure terms are used,
the number of samples per period approaches the Nyquist limit.
0 5 10 15 20
2
2.5
3
3.5
4
Number of Pressure Terms
M
i
ni
m
u
m
N
u
m
b
e
r
of
S
a
m
p
l
e
s
/
P
e
r
i
od
Fig. 26 Loss of Data as More Fourier Terms are Used
It does not take many pressure terms before there are essentially two samples per period
in the highest frequency pressure basis function. This means the higher frequencies are
in danger of loosing data due to not sampling often enough. One method to examine the
impact of this problem is to oversample the system and compare the results to the results
produced by the actual sampling.
The first function to study is the pressure basis function. The lowest and highest
frequency pressures are shown along with where they are actually sampled and a linear
interpolation (Fig. 27). The maximum number of pressure terms is 10.
78
0 0.075 0.175 0.275 0.375 0.475
-1
-0.5
0
0.5
1
y
P
*
n = 1 - oversampled
n = 1 - actual
n = 10 - oversampled
n = 10 - actual
Fig. 27 Oversampled Pressure Basis Function
It is obvious in this plot that the sampling captures the character of the low frequency
pressure, but does not capture the high frequency oscillations. The amplitude of the high
frequency oscillations remains constant, but the amplitude of the sampled signal seems to
increase moving away from the centerline.
It seems likely that similar behavior will be observed in the wake basis function that
results from this pressure. The model does not guarantee that the oscillations carry over
to the wake, but physically it makes sense. The plot only shows one slice of the wake,
but the behavior is similar throughout the entire wake (Fig. 28). Once again the low
frequency component is captured well. A blip at y = 0.175 is not captured, but it is most
likely noise and should be ignored. As with the pressure basis function the high
frequency component, on the other hand, is not captured. The amplitude actually
decreases slightly, but the sampled function has a peak amplitude in the middle of the
hull then it decreases again.
79
0 0.075 0.175 0.275 0.375 0.475
-0.5
0
0.5
1
y
z
n = 1 - oversampled
n = 1 - actual
n = 10 - oversampled
n = 10 - actual
Fig. 28 Oversampled Wake Basis Function
m = 1, x = -1.1
The data loss can also be examined by looking at the characteristics of the total wake
basis function compared to an oversampled basis function (Fig. 29). The oversampled
function shows the oscillations are reasonably consistent in both directions; the peaks and
valleys create a regular grid. The sampled basis function, however, seems to have a
series of nearly plane waves with peaks at a slight angle from the centerline. The two
behaviors are quite different. It is also important to note that the data loss issue discussed
above is in the transverse direction. A similar behavior is observed in the longitudinal
direction.
80
Actual
Oversampled
Fig. 29 Oversampled Wake Basis Function Surface
m = 1, n = 10
This problem becomes particularly pressing when the hull shape is known and the
pressure coefficients are unknown. Because of the data loss the higher frequency basis
functions become closer to linearly dependant and the matrix inversion harder to
accurately compute. In practice, a high frequency term in a forward pressure strip is
erroneously given a large amplitude. This creates large high frequency oscillations
further aft along the hull which are fixed by assigning an even larger amplitude to the
high frequency terms in these pressure strips. The inversion results in an estimated hull
that when oversampled is nothing but an exponentially growing high frequency
oscillation. The wake this hull creates is obviously not accurate.
In conclusion, it is possible to take advantage of the symmetry of boat hulls to reduce
the number of calculations needed. With the constant pressure method, this requires
slightly modifying the basis function. For the sinusoidal pressure method all that is
needed is to discard the pressure terms that are asymmetric. These procedures effectively
reduce the computation time for each method. The fact that the sinusoidal pressure
method is based on a Fourier series also raises issues related to the number of points used
81
to capture the system behavior. In general, these issues along with all the inadequacies
discussed in chapter 3 make the sinusoidal pressure method unacceptable for computing
the shape of a boat wake. Throughout the remainder of the work the constant pressure
method is used exclusively.
4.4. ISSUES WITH INVERSION
As was mentioned in chapter 2, the wave height over a grid of points is determined by
summing a series of basis functions multiplied by pressure coefficients at each point
(2.170). In other words, the wave heights are a linear combination of basis functions:
g??= AE . (4.3)
The ideal method to determine a boat?s wake is to define the surface of the hull and
compute the basis functions over the wetted area. The number of points defining the hull
should be the same as the number of unknown pressure coefficients (
hull
E is square).
The pressure coefficients are then determined using
()
1
hull hull
Ag??
?
= E . (4.4)
A new set of basis functions is then computed over the wake area and the pressure
coefficients determined with the hull are used to solve for the height of the wake:
.
wake
wake
A
g
?
?
=
E
(4.5)
The problem with this method is that it involves inverting the basis function matrix.
Because of the uncertainties that go into creating this matrix the inversion is not a stable
numerical operation.
82
While the basis function matrix is nonsingular (theoretically invertible) it is very
poorly conditioned and thus sensitive to noise. The condition number of a matrix is the
ratio of the largest and smallest singular values of the matrix. For the ideal case (an
orthogonal matrix) the condition number is one. As the condition number grows the
matrix inversion becomes less stable and the accuracy of the inverted matrix decreases.
Chapter 3 presents two cases to validate the solution methods, a sinusoidal pressure and a
simplified hull pressure. In these cases the condition numbers of the hull basis functions
are 7.5?10
9
and 1.2?10
11
respectively. In these tests the pressure (as opposed to the hull
shape) was defined; however it would not be safe to invert the hull basis function
matrices.
To demonstrate the instability the basis functions from the simplified hull pressure
will be used with two hull shapes and the resulting pressures compared. The first hull
shape is the one computed using constant pressure patches (Fig. 30).
Fig. 30 Original Test Surface
When the basis function matrix is inverted the pressure (Fig. 31) is identical (to machine
precision) to the actual pressure used to generate the surface (Fig. 18).
83
Fig. 31 Pressure Resulting From Original Test Surface
The surface is then rotated by 0.00001? about the y ? axis (adjusting the trim by a
miniscule amount). This small change in the hull should produce virtually no change in
pressure; however, the pressure solved for (Fig. 32) is unrealistic. The pressure rise at the
front of the surface displacement is still visible. However, further aft the pressure
oscillates uncontrollably. As the surface is rotated more (or other perturbations are
applied) the amplitude of the oscillation becomes much larger.
84
Fig. 32 Pressure Resulting From Rotated Test Surface
To better understand the reason for these oscillations the basis function matrix must
be examined. The hull shape equation (4.3) can be written in block form as a series of
smaller matrices due to its formation from a multidimensional tensor equation, i.e.
1,1 1 ,1,1,1 1 ,2,1,1 1 , ,1,1 1 ,1
2,1 1 ,1,2,1 1 ,2,2,1 1 , ,2,1 1 ,2
,1 1,1,,1 1,2,,1 1,,,1 1,
hull
JNJNJ NMJ
JNJNJ NMJN
IJ NIJ NIJ NMIJ NM
gA
EE E A
g
EE E A
??
?
?
?
?
????? ???
????? ???
????? ???
=
??? ??
??? ??
??? ??
=
??? ??
??? ??
??? ??
E
L
L
MMMO
L
?
?
N
?
?
?
?
M
(4.6)
where
,ij
? , , and are as in (2.170)and ?? ? represents the whole range of
values (i.e. a vector/matrix). The basis matrix can be simplified by noting that the
contribution of a pressure patch only affects the flow aft of the patch:
,,,nmi j
E
,nm
A
,,,
0 ; 1
nmi j
Eim= ??. (4.7)
85
This leads to:
86
?
?
?
?
?
?
. (4.8)
1,1,1
1,1 1,1
1 ,1,2,1 1 ,2,2,1
2,1 1,2
,1 1,
1 ,1, ,1 1 ,2, ,1 1 , , ,1
00
00
0
NJ
J N
NJ N J
J N
IJ NM
NIJ NIJ NMIJ
E
A
EE
A
g
A
EE E
?
?
?
?
??
? ?
?? ??
? ?
? ?
?? ?? ? ?
??
?? ?
??
?? ?
?? ?
=
?? ?
?? ?
?? ?
L
L
MMOM
L
The first row of hull heights (
1,1 J
?
?
) can be determined independent of any pressures
except for the first row ( ). Conversely, the first row pressure vector can be found
by inverting the first row basis function matrix ( ) to match the first row hull
height vector. This pressure creates a disturbance down stream. The second row of
pressures must cancel this disturbance as well as match the second row height to the hull.
Due to noise in the basis function calculations, the down stream height disturbances
contain small oscillations. To cancel the oscillations, the pressure in the next inversion
contains the same oscillation with a slightly larger amplitude. As the solution propagates
aft the oscillation in the pressure continues to grow causing the instability.
1,N
A
? 1
J1,1,1N
E
??
0.1 0.2 0.3 0.4
-15
-10
-5
0
5
10
y
P
x = 1.5
x = 1.7
x = 1.9
Fig. 33 Increasing Pressure Oscillation
This instability and sensitivity to small hull shape changes require a change in
solution procedure. Instead of defining the hull, determining the pressure that produces
that hull, and finally determining the wake the new procedure is to define a pressure
shape with unknown parameters; vary those parameters over a reasonable range; and then
determine both the hull and wake shape the pressure produces. One potential problem
with this method is that the hull shapes produced may not be realistic. The hull shape
needs to be monitored to guarantee that it is practical. This may also be a benefit,
however; since the hull is not being varied directly, some shapes may be discovered that
are possible to manufacture but would not otherwise be investigated.
87
88
5. HULL OPTIMIZATION
The goal of this research is to study methodology for designing hulls for different
types of professional and recreational tow boats, specifically for towing a skier or a
wakeboarder. Algorithms for predicting the wake (and hull shape) given a pressure
disturbance have been developed. The next step is to describe what is ?good? for a wake
and what parameters are defined in each event. The wake shape equations depend on the
speed the boat is traveling; the speed is in turn dependent on what and who is being
towed. Additionally, it does not make sense to examine the shape of the entire wake; the
rider will only be in a portion of the wake defined by the tow rope length which depends
on the event and the rider. Different events require different parameters and different
riders have different preferences.
5.1. EVENT DEFINITION
Skiing has existed for the longest time and its events are the most formalized. In
particular two governing bodies oversee most of the professional skiing tournaments, the
International Water Ski Federation (IWSF) and the American Water Ski Association
(AWSA). These two bodies have very similar rules concerning boat speed and rope
length for events. Two events will be considered for skiing: jumping and slaloming.
Jumping is defined as the skier using a ramp to jump and get the maximum distance
before landing. The skier can choose a speed between 45 and 57 kph (28 and 35 mph)
and the tow rope length is 23 m (75 ft) [AWSA].
Fig. 34 Ski Jumping Example and Course Layout
Slaloming is defined as the skier going around a series of buoys to either side of the
boat?s path. The speed is set at 58 kph (36 mph); the rope length varies between 11.25
and 18.25 m (37 and 60 ft) [AWSA]. The rope gets shorter with each run making the
event more difficult.
89
Fig. 35 Slaloming Example and Course Layout
The conditions for recreational (amateur) waterskiing vary much more based on the
skier?s preference. Typically the speed is less than for professionals and the tow rope
length is towards the longer end of the professional range. One factor ties each of these
activities together: the activity is not based on the wake. The boat must be towing the
skier, but fewer disturbances from the wake are better.
Wakeboarding is a newer sport; two bodies also govern its professional events, the
World Wakeboard Association (WWA) and the World Wakeboard Council (WWC).
Tournaments sponsored by these two bodies have different events that mainly consist of
various freeform jumps, flips, etc. Both agree, however, that boat speed and rope length
are at the discretion of the rider [WWC]; in general the riders prefer boat speeds much
slower than skiing, 12 to 20 mph (19 to 32 kph), and tow rope length in the middle of the
slalom lengths [Favret, Solomon]. This is also true for recreational wakeboarding. One
factor that is consistent for most wakeboarding activities is that the wake itself provides
the launching platform for many of the tricks. Therefore, the wake should be larger and
provide a ramp for the wakeboard.
90
Fig. 36 Wakeboarding Examples
In summary, besides the actual hull shape there are two parameters that are varied for
the different activities: the boat speed and the tow rope length, or distance behind the boat
the wake will be examined. Two basic types of wake are also desired depending on the
activity. For skiing a flat wake as small as possible is desired. For wakeboarding a larger
wake with ramp characteristics is ideal.
To characterize these wake shapes two parameters will be examined: the maximum
height of the wake and the slope of the main disturbance. These parameters are
calculated along the arc the skier makes behind the boat and are defined as
() ()
1
tan .
max min
max min
max min
height z z
zz
slope
Rz Rz??
?
= ?
?? ?
=
? ?
?
? ?
(5.1)
91
z
?
z
min
z
max
( )
z
max
?
( )
z
min
?
R
?
y
x
Fig. 37 Wake Parameters
For each speed and tow length under consideration the pressure distribution that creates
the hull will be varied to attempt to either minimize or maximize (depending on the
activity) these parameters. The hull shape that created the wake will then be examined
for reasonability and shape.
5.2. TYPICAL HULL SHAPE
Because the hull shape is being determined from the surface deformation made by the
pressure instead of being defined directly, the shape must be examined to guarantee that
it is reasonable. The gross shape of most planing hulls has not changed drastically in
92
many years, although minor improvements have been made. The shape is characterized
by a ?V?-bottom that is sharper near the bow with decreasing deadrise angle towards the
stern. This basic shape is shown below [Comstock].
L
C
L
B
0
?
1?
1
2
3
4 5 6
7
8
9
10
Fig. 38 Basic Planing Hull Shape
Two critical parameters characterize this shape. The chine is the sharp break in the
transverse slope of the hull (the edge of the ?V?-bottom). The slope of the ?V? is known
as the deadrise; as was previously mentioned it varies along the length of the hull.
A third parameter needed to characterize the hull?s shape in the water is the trim
angle. This angle is how parallel the keel is to the free surface. This is not a parameter of
the hull itself; the trim angle is maintained to balance the pitch moments acting on the
hull.
Modern planing hulls are more complicated than this example. For instance most
have strakes, longitudinal ridges running along the bottom of the hull; however, the
chine, deadrise, and trim angle characterize the overall free surface deformation created
by the hull well enough for the purposes of this research.
93
94
5.3. TEST CASES
As mentioned in section 5.1, the conditions that represent each event include speed
and tow rope length. Six cases are studied to find optimal hull shapes. To represent ski
jumping the rope length is defined as 23 m and the speed can vary between 45 and 57
kph; these limits are tested. In slaloming the speed is set at 58 kph and the rope length
can vary between 11.25 and 18.25 m; 11.25 and 14.25 m are evaluated. Wakeboarding is
not regulated but two typical cases, 14.75 m rope length at 19 kph and 32 kph, are tested.
These parameters are summarized in Table 1.
Table 1 Test Parameters
Speed (kph) Tow Rope Length (m)
Jump 1 57 23
Jump 2 45 23
Slalom 1 58 11.25
Slalom 2 58 14.25
Wakeboard 1 19 14.75
Wakeboard 2 32 14.75
Three other parameters must be set for each test: the displacement, length, and beam
of the boat. When planing the volume of water displaced does not balance the weight of
the boat due to the lift created by planing. Instead of measuring displacement the lift
generated by the pressure is adjusted to match the weight of the boat. The length and
beam are more difficult. As more lift is created the boat rises out of the water.
Therefore, the wetted area will not necessarily stay the same as parameters change. In
order to minimize variations and concentrate on how hull shape (as opposed to size)
affects the wake for this study the length and beam remain constant for each test.
From section 5.2, planing hulls normally have a chine where the shape drastically
changes; above the chine the hull is nearly vertical. When planing the waterline is very
near the chine (see Fig. 39). Therefore, the wetted beam is assumed to be 90% of the
overall beam. Similarly, approximately two thirds of the length of the hull is in the water
when planing (see Fig. 39). For the tests a typical tow boat with parameters in Table 2 is
used.
Fig. 39 Examples of Planing Boats
95
Table 2 Typical Tow Boat Parameters
Overall Value Wetted Value
Mass (kg) 1445 1445
Length (m) 6.45 4.3
Beam (m) 2.31 2
The pressure distribution is varied for each test. The pressure distribution used in
section 3.2 represents a simplified pressure produced by a planing hull. In particular it
has characteristics that are present in planing craft such as a singularity at the leading
edge and zero value at the other edges [Scullen]. These characteristics can be maintained
and the pressure allowed to vary by modifying the pressure equation to
11
c d
L y
P
x B
??? ?
?? ??
=
??
??? ?????
????
? ???
. (5.2)
In this equation c effectively controls how long the peak of the pressure is and d controls
how wide the peak is. For the skiing tests (jumping and slaloming) the goal is to
minimize the wake height and slope; for the wakeboarding tests the goal is opposite, to
maximize the wake height and slope.
In each test using the conditions listed above (Table 1 and Table 2) the pressure is
varied according to equation (5.2) and then modified so that lift matches weight. The
wake generated by this pressure is computed and examined in an arc defined by the tow
rope. The maximum wake height and wake slope along this arc are determined using
equation (5.1). These two parameters are then compared over the range of pressure
parameters so that for each test the slope and height can either be maximized or
minimized as appropriate.
96
97
5.4. JUMP 1 TEST RESULTS
The first test is the higher speed ski jump test. The surfaces shown below represent
the maximum wave height and slope as a function of the two pressure parameters (c and
d). The goal of this test is to minimize both the maximum height and the slope. The
maximum height is minimized when either c or d is at the minimum value tested; the
absolute minimum occurs when both c and d are at the minimum tested value. The
minimum value of c tested is 0.01. Values below this do not appreciably modify the
pressure from this value and so are not tested. Similarly for d testing any lower does not
change the results.
The minimum slope occurs where d is approximately 4 and c is greater than 5. It
stays near this minimum along the line c = 3.5. To compromise between these two
minimizations values of c = 2.3 and d = 2 are chosen for the optimal wake.
c
d
2 4 6 8 10
5
10
15
20
25
30
0.4
0.45
0.5
c
d
2 4 6 8 10
5
10
15
20
25
30
28
29
30
31
32
Fig. 40 Jump 1 Minimization
5.4.1. Resulting Pressure and Hull Shape
These pressure parameters result in the pressure shown below (Fig. 41). This
pressure produces the wake also shown below. In general this hull shape looks
reasonable. The trim angle is 5.7 deg; at the stern of the boat the deadrise is 6 deg.
Unlike the typical hull presented in section 5.2 the deadrise decreases toward the front of
the boat. This is due to two reasons. First, the wetted area is defined as a rectangle
instead of the triangle that a more typical hull would have. Without knowing the hull
shape beforehand it is not possible to know the shape of the wetted area triangle so a
98
99
rectangle must be assumed. Second, near the bow the pressure function used (5.2) has
value for the width of the boat instead of only near the centerline (again a rectangular
wetted area instead of a triangle). A different pressure function might produce a more
typical hull shape. One interesting feature of the hull shape is the positive height on the
outside rear corner. At the transom approximately 0.8 m from the centerline the hull
begins to rise quickly. This is characteristic of the chine line. The chine widens as it
approaches the transom; this is not typical, but has the same explanation as the atypical
deadrise. Also, outside the chine the height becomes positive. It is common for the
water to spray up on the sides of the boat (see Fig. 39). The positive height is physically
reasonable in this light.
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
2
3
4
5
6
x 10
4
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-0.4
-0.2
0
0.2
Fig. 41 Jump 1 Pressure and Hull Shape
5.4.2. Resulting Wake
The wake this pressure produces is shown below in Fig. 42. The black line represents
the arc a skier would follow. The height the skier encounters is also plotted vs. the
distance along the arc. The square indicates the highest point on the arc and the circle the
lowest. A typical wake behind a boat is reasonably flat directly behind the boat with a
crest spreading in a ?V? outside this flat area. Outside the crest is a valley which levels
off to the undisturbed free surface height. The shape shown below has several of these
characteristics. In particular the size and location of the crest and valley are the
100
101
parameters used to characterize the wake. The area inside the wake crest (inside the ?V?
behind the boat) is not accurate. This area has longitudinal oscillations that are not
present in physical wakes. These oscillations narrow further behind the boat until the
surface inside the wake becomes essentially noise (around x = 25 m in this test). These
oscillations can also be seen in the centerline validation tests (section 3.3). However, in
this area of the wake the extra disturbance from the boat?s propeller affects the fluid flow;
the disturbance from the propeller is not included in this simulation. Therefore, the area
inside the ?V? behind the boat can safely be ignored when analyzing the wake shape.
Ignoring the area directly behind the boat, the height profile along the skier?s arc
appears reasonable. The valley outside the wake is not typically that much more extreme
than the surface height inside the wake crest, but this is most likely due to a combination
of the atypical hull and the no propeller assumption. The height of the wake is 39.8 cm
and the slope is 28.4 deg.
x (m)
y
(m
)
5 10 15 20 25
1
2
3
4
-0.4
-0.2
0
0.2
1 2 3 4 5
-0.3
-0.2
-0.1
0
0.1
0.2
Distance Along Arc (m)
z
(m
)
Fig. 42 Jump 1 Wake
5.5. JUMP 2 TEST RESULTS
The next test case is the slower ski jump. As before, the surfaces to minimize (wake
height and slope) are shown below. The height shows a strong trend: as c is decreased
the height decreases. The slope is not as clear. There is a rough minimum along the line
c = 2.1 and the line d = 2 is also a minimum for c greater than 4. The slope is also near
minimum at c = 0.01 and d = 14.6; this is the condition chosen as the optimum
compromise.
102
c
d
2 4 6 8 10
5
10
15
20
25
30
0.56
0.58
0.6
0.62
0.64
0.66
c
d
2 4 6 8 10
5
10
15
20
25
30
25
25.5
26
26.5
27
Fig. 43 Jump 2 Minimization
5.5.1. Resulting Pressure and Hull Shape
These parameters result in a pressure that is nearly linear while still maintaining the
required boundary conditions. The hull shape is also interesting. The trim is normal at
5.2 deg, and as before the hull gets wider forward. The deadrise, however, is not
apparent. In fact, the rear outside edge is lower than the centerline. This shape is known
as a catamaran and while not typically used in towboats is common in other power boat
designs.
103
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1000
2000
3000
4000
5000
6000
7000
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-0.4
-0.2
0
0.2
Fig. 44 Jump 2 Pressure and Hull Shape
Although this hull shape is not common for towboats, some manufacturers are
beginning to produce boats with similar features. The boat shown below is the
MasterCraft X-Star. It is not a true catamaran because the ?V? hull is still present;
however, outriggers are present similar to a tri-hull. The X-Star is a wakeboarding tow
boat; it is not meant to imply that the hull shape presented above is necessarily the best
ski tow boat. It does, however, show that similar designs are feasible and being used in
industry.
104
Fig. 45 MasterCraft X-Star
5.5.2. Resulting Wake
As before, the wake looks reasonable except for the area immediately behind the
boat. The maximum wake height is 56.2 cm and the slope is 25.9 deg. In general, this
wake is not as preferable as the previous test case. As boat speed increases typically the
wake becomes smaller and flatter because less of the hull is disturbing the free surface to
achieve the same lift. The comparison of the two speeds qualitatively matches the
physical expectations. The magnitude of the height for both this and the previous test
seems high. More evaluation of this technique is warranted to guarantee its accuracy.
105
x (m)
y
(m
)
5 10 15 20 25
1
2
3
4
-0.5
0
0.5
1 2 3 4 5
-0.2
-0.1
0
0.1
0.2
Distance Along Arc (m)
z
(m
)
Fig. 46 Jump 2 Wake
5.6. SLALOM 1 TEST RESULTS
The next two tests are the slalom runs. The first slalom test is with the shorter rope.
The trends in both the height and slope are strong. The height decreases as either c or d
decrease; the absolute minimum occurs at c = 0.01 and d = 2. The slope has similar
trends except the minimum in c occurs at 2; for c < 2 the slope begins to rise again. The
gradient in the d ? direction is stronger, however (d has more affect on slope than c).
Because c = 0.01 and d = 2 minimizes the height and is near the minimum slope this
point is chosen as the optimal compromise.
106
c
d
2 4 6 8 10
5
10
15
20
25
30
0.4
0.45
0.5
0.55
0.6
c
d
2 4 6 8 10
5
10
15
20
25
30
28
29
30
31
32
Fig. 47 Slalom 1 Minimization
5.6.1. Resulting Pressure and Hull Shape
The pressure this condition produces is fairly linear longitudinally and curved
transversely. The hull, however, does not exhibit the catamaran shape of the previous
linear hull; it is a more traditional ?V? shape. The trim angle is 4.9 deg and the deadrise
at the transom is 2.6 deg.
107
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
2000
4000
6000
8000
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-0.3
-0.2
-0.1
0
0.1
0.2
Fig. 48 Slalom 1 Pressure and Hull Shape
5.6.2. Resulting Wake
The wake this pressure produces is slightly different from the previous two test cases.
The lowest height of the wake occurs inside the wake crest as opposed to the valley
outside the crest. In fact, no valley outside the wake crest is present. This difference is
due to the fact that the skier is so close to the boat; further from the boat the wake crest
for this peak dies and the wake crest the previous tests used is formed. This close to the
boat the skier is ahead of the formation of the main wake. The height the skier sees is
38.4 cm and the slope is 28.2 deg.
108
x (m)
y
(m
)
5 10 15 20 25
1
2
3
4
-0.4
-0.2
0
0.2
1 2 3 4 5
-0.4
-0.3
-0.2
-0.1
0
0.1
Distance Along Arc (m)
z
(m
)
Fig. 49 Slalom 1 Wake
5.7. SLALOM 2 TEST RESULTS
The longer rope slalom test results are interesting because for many pressure
distributions the wake is not valid; in particular the main wake crest is below the
undisturbed free surface which is not realistic. These obviously incorrect wakes are not
considered in the optimization. Of the wakes that are reasonable the heights and slopes
follow the trends seen previously in the minimization. As c and d decrease the height and
slope also decrease; d has a bigger effect on slope than c does. The optimal combination
for this test is c = 2.3 and d = 17.4.
109
c
d
2 4 6 8 10
5
10
15
20
25
30
0.5
0.52
0.54
0.56
0.58
c
d
2 4 6 8 10
5
10
15
20
25
30
39
39.5
40
40.5
Fig. 50 Slalom 2 Minimization
5.7.1. Resulting Pressure and Hull Shape
These pressure coefficients result in a distribution that is wide, but concentrated at the
front of the wetted area. The hull has the catamaran shape seen in a previous test. The
trim angle is 3.8 deg; the deadrise angle is meaningless in a catamaran hull.
110
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
2
3
4
x 10
4
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-0.4
-0.2
0
0.2
0.4
Fig. 51 Slalom 2 Pressure and Hull Shape
5.7.2. Resulting Wake
The longer rope in this test means the skier is back in the main wake so the valley
outside the wake crest is apparent. The wake height is 48.5 cm and the slope is 39.2 deg.
As with the ski jumping tests, the wake heights are excessive compared to physical
wakes.
111
x (m)
y
(m
)
5 10 15 20 25
1
2
3
4
-0.5
0
0.5
1 2 3 4 5
-0.4
-0.3
-0.2
-0.1
0
0.1
Distance Along Arc (m)
z
(m
)
Fig. 52 Slalom 2 Wake
5.8. WAKEBOARD 1 TEST RESULTS
The last two tests are for wakeboarding. The main difference between these tests and
the skiing tests is the lower speeds. Additionally, instead of minimizing the wake height
and slope the goal of the tests is to maximize them. The first wakeboard test is the slower
of the two. The height and slope show similar trends as the other tests except c has a
much larger effect on both the slope and the height than d; in fact unlike before as d
decreases the height and slope may increase. The maximum height and slope both occur
at c = 10 and d = 30.
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c
d
2 4 6 8 10
5
10
15
20
25
30
1.2
1.4
1.6
1.8
c
d
2 4 6 8 10
5
10
15
20
25
30
24
26
28
30
32
Fig. 53 Wakeboard 1 Maximization
5.8.1. Resulting Pressure and Hull Shape
This pressure is virtually constant over the width of the boat and is concentrated
exclusively at the leading edge of the wetted area. The hull shape this produces is
problematic. Before the transom of the boat the hull is rising implying the hull is bowl
shaped, which is not a practical planing hull design. Considering only the centerline
forward of the minimum hull height the trim angle is 35.2 degrees, very steep for a
planing hull. As boat speed increases the boat transitions from operating in a
displacement mode (lift comes from displaced water) to a planing mode. During this
transition the boat trim is very large. The hull in this test with these pressure parameters
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is going slow enough that it has not made the transition to full planing. The wake shape
equations are only valid for full planing. This is the most likely cause for the unrealistic
hull shape.
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
2
4
6
8
x 10
4
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-1.5
-1
-0.5
0
Fig. 54 Wakeboard 1 Pressure and Hull Shape
5.8.2. Resulting Non-Optimal Pressure and Hull Shape
To find a reasonable hull the parameters which minimize the wake height and slope
(c = 0.01, d = 2) are examined. The hull this produces still curves upward at the stern,
but not much.
114
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
2000
4000
6000
8000
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-1.5
-1
-0.5
0
Fig. 55 Wakeboard 1 Non-Optimal Pressure and Hull Shape
5.8.3. Resulting Non-Optimal Wake
The wake the non-optimal pressure coefficients produce is obviously not the best
wake, but it does look reasonable. The crest ?V? is very wide which is typical of slower
boats, particularly when not fully transitioned to planing mode. The lowest point of the
wake is inside the crest which is not unreasonable. The wake height is 110.6 cm and the
slope is 28.5 deg. As with all the other tests this height seems excessively large.
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x (m)
y
(m
)
5 10 15 20 25
1
2
3
4
-1.5
-1
-0.5
0
0.5
1 2 3 4 5
0
0.2
0.4
0.6
0.8
Distance Along Arc (m)
z
(m
)
Fig. 56 Wakeboard 1 Non-Optimal Wake
5.9. WAKEBOARD 2 TEST RESULTS
The final test is the faster wakeboard test. The height and slope profiles have similar
trends as the slower wakeboard test. The maximum of each occurs at c = 10 and d = 30.
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c
d
2 4 6 8 10
5
10
15
20
25
30
0.65
0.7
0.75
0.8
0.85
0.9
0.95
c
d
2 4 6 8 10
5
10
15
20
25
30
31.5
32
32.5
33
33.5
34
Fig. 57 Wakeboard 2 Maximization
5.9.1. Resulting Pressure and Hull Shape
This pressure is identical to the optimal wake in the previous wakeboard test, but this
hull is more reasonable. One interesting feature of the hull is a depression near the
forward outside corner of the wetted area. This is similar to a catamaran, but vanishes at
the stern of the boat. No tow boats are currently manufactured in this shape, but it is not
infeasible. The trim angle is 11 deg and there is no deadrise.
117
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
2
4
6
8
x 10
4
x (m)
y
(m
)
1 2 3 4
0.2
0.4
0.6
0.8
1
-1
-0.5
0
0.5
Fig. 58 Wakeboard 2 Resulting Pressure and Hull Shape
5.9.2. Resulting Wake
This test has a high enough speed that the wake looks more like the skiing wakes.
The valley outside the wake crest is the lowest point although the valley inside the crest is
not much higher. The wake height is 95.3 cm and the slope is 34.3 deg. It does not seem
reasonable for a boat to create a wake nearly one meter high.
118
x (m)
y
(m
)
5 10 15 20 25
1
2
3
4
-1
-0.5
0
0.5
1
1 2 3 4 5
-0.6
-0.4
-0.2
0
0.2
0.4
Distance Along Arc (m)
z
(m
)
Fig. 59 Wakeboard 2 Wake
5.10. RESULTS SUMMARY
In each test the wake the boat produced appears reasonable. A ?V? shaped crest is
visible with a valley inside it and outside either a valley or drop to the undisturbed free
surface. The wake is wide when the boat is slow and narrows as speed increases
matching reality. The area directly behind the boat is not realistic, but this could be due
to ignoring propeller effects. The wake shape also shows a strong correlation to the
pressure distribution, and the hull shapes while not always typical are usually reasonable.
There is however a problem with the wake height. In every case tested the height is
larger than can be expected in a real wake. The shape of the wetted area also needs to be
119
120
examined. In particular, each hull produced is flat at the leading edge of the wetted area
instead of showing the characteristic deadrise present in tow boats. Without the ability to
directly define the hull shape defining the shape of the wetted area becomes problematic.
121
6. CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH
6.1. CONCLUSIONS
In conclusion, this work attempts to predict the wake shape behind a given boat
moving at a given speed. To do this two methods for determining the free surface
deformation from a pressure distribution are presented. Both methods derive from the
same velocity potential function; the difference is the form of the pressure distribution
and the resulting integrals that must be evaluated numerically. Each method is compared
to previously published results to verify its accuracy. The sinusoidal pressure basis
function method does not yield results matching the original authors. Furthermore, this
method has potential instabilities beyond the numerical inaccuracies; in order to produce
a square linear system the pressure oscillates too quickly for the solution grid to capture.
The constant pressure basis function method yields results that closely match the
published results. The numeric algorithm for the constant pressure basis function also
runs much faster than the sinusoidal pressure basis function. Because of the sinusoidal
pressure basis function method?s shortfalls, the constant pressure basis function method is
used to predict the wake shape behind the boat. Even the constant pressure basis function
method is not stable enough to invert and determine a pressure distribution from a boat
hull shape. Instead a variety of pressures are examined for wake shape and hull shape.
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Tests are performed for a range of scenarios encountered in real life. In particular, ski
jumping and slaloming, and wakeboarding conditions are simulated. To find the optimal
hull shape for each of these activities the wake shapes produced by the various pressure
distributions are examined for characteristics important to the activities: wake height and
slope. The hulls produced by the tested pressures are feasible for the most part.
However, because the hull is not directly defined some characteristics are not typical.
Specifically, every hull produced is flat at the leading edge of the wetted area; this is not
typical of modern towboats. The flat hull shape is due to the fact that a rectangular
wetted area must be assumed. Additionally, several hulls had depressions along the
outside edge. This shape is common in power boats, but not in tow boats. The wakes
observed in the simulations are also qualitatively reasonable. A definite ?V? crest is
visible behind the boat. This ?V? widens as the boat moves slower matching physical
intuition. The wake height and slope also show a strong correlation to pressure
distribution. Oscillations appear inside the ?V? that do not represent reality, but these are
most likely due to ignoring the effects of the propeller. The major discrepancy between
the simulated wake and a physical wake is the difference in height. The simulated wakes
are much larger than is typical of tow boat wakes. This discrepancy warrants further
research.
One final important note is that the hull optimization performed in this research only
considers wake parameters. Boat hulls must be designed with many other considerations
such as handling, drag (engine size), and ride comfort in mind. For example, most of the
hulls studied in this research have nearly flat bottoms. Flat bottom boats are common for
small one or two person, low power boats for use in calm waters. Larger boats (such as
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tow boats) never have flat bottoms because of considerations completely outside wake
shape. Flat bottom boats are not as maneuverable as ?V? hulls; they slide over the water
instead of cutting through it. This sliding behavior also makes their ride in rough water
very violent; the boat goes up over each wave instead of cutting through them. Any
designer who wished to make use of the procedures developed in this research would
need to combine the wake considerations with other boat performance considerations.
Until a method to invert the surface deformation procedure (define a hull shape instead of
a pressure distribution) is developed it will be very difficult to use this method in
conjunction with other hull considerations.
6.2. SUGGESTIONS FOR FUTURE RESEARCH
There are several areas that could be investigated to improve the method developed in
this research. These areas generally fall into two categories: numerical algorithms used
and range of tests to optimize hull shape. The most complex algorithms in this research
involve the numerical integration of functions. These functions have features making
them difficult to integrate, singularities and rapid oscillations. Transformations are used
to remove the singularity and slow the oscillations, but these change the integration range
to negative and positive infinity introducing inaccuracies when the integration is
truncated. Better analytical transformations may be possible to improve the numerical
integration accuracy. The particular pressure basis functions used can also improve the
numerical algorithms. The sinusoidal pressure basis function method has the problem of
data loss from not capturing higher frequency oscillations. The method does satisfy the
Nyquist criterion for number of points per period necessary to uniquely define a sine
wave; however, this absolute minimum is not enough to accurately capture the sine wave.
124
This apparent discrepancy is due to the fact that the Nyquist criterion is for fitting the
data with a Fourier series instead of the piecewise linear interpolation used in the method
developed for this research. If the wake surface were described by a Fourier series
instead of being determined by inverting the basis function matrix it is possible the higher
frequency oscillations could be captured thereby stabilizing the sinusoidal pressure basis
function method. Another improvement that could be made with the pressure basis
functions is in the constant pressure basis function method. This method approximates
the pressure distribution by piecewise constant rectangular patches. This creates
discontinuities in the applied pressure disturbance. The free surface disturbance may be
smoother if these pressure discontinuities were not present. Specifically if the pressure
distribution could be approximated by either two dimensional piecewise linear or cubic
spline rectangles a smoother wake may result. For this method to succeed the velocity
potential function would need to be derived for this new pressure type. All of the
numerical improvements have the same goal in mind: to allow the free surface
deformation problem to be inverted. For this research to move from an analysis tool to a
design tool, a way to define the hull shape, and thereby determine the wake shape, must
be developed. In particular this means lowering the condition number of the surface
basis function matrix. The condition number may be lowered either by more accurate
numerical integration or by using better pressure basis functions.
The second category for recommended future research is in the range of tests to
optimize the hull. If a method to directly define the hull shape is developed many more
tests become available. One of the more interesting scenarios to investigate is the mixed
design tow boat, one that is designed to tow both skiers and wakeboarders. These
125
activities have different wake requirements, but are also performed at different speeds.
Could one hull be designed to produce an ideal wakeboard wake at low speeds and an
ideal ski wake at high speeds? If the pressure must be defined instead of the hull shape
then a wider range of pressure distributions should be examined. Specifically the
knowledge that a ?V? hull produces a triangular wetted area should be taken into account.
Regardless of whether the hull shape or pressure distribution is defined the results need to
be compared to physical experiments. This work highlighted results that do not seem
physically reasonable. These results need to be compared to a real wake.
126
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