planing craft

FACULTY OF MECHANICAL ENGINEERING

UNIVERSITI TEKNOLOGI MALAYSIA

2009/2010

SMK4562 [SMALL CRAFT TECHNOLOGY] (GROUP ASSIGNMENT)

Title: PLANING CRAFT

Name: ESAMUDDIN HAMIDON

FIRDAUS BIN MAHAMAD

MUHAMMAD FAIZAL BIN AS’SHAARI

Section: 01

Course: 4SMK

Lecturer: DR MOHAMAD PAUZI ABDUL GHANI

TABLE OF CONTENTS

CHAPTER TITLE PAGE

1.0 INTRODUCTION 1

1.1 Background 1

1.2 Definition 2

2.0 FUNDAMENTAL OF PLANING BOAT 3

2.1 Introduction 3

2.2 Savitsky Method 3

3.0 HYDRODYNAMICS CHARACTERISTIC OF PLANNING HULL 9

3.1 Water Rise And Splash-Up 9

3.2 Induced Resistance And Lift By Hassan Ghasemi And Mahmoud Ghiasi 11

4.0 TYPE OF PLANNING HULL 18

4.1 Introduction 18

4.2 Deep Vee Bottom 19

4.3 Type of Deep Vee Bottom 20

4.3.1 Convex Section Shape 20

4.3.2 Concave Section Shape 20

4.3.3 Straight Section Shape 21

4.3.4 Inverted Bell Section Shape 21

4.3.5 Spray Stripes Shape 22

5.0 STERN WEDGES AND SPRAY RAIL 23

5.1 Planning Hull 23

5.2 Spray Tail 25

5.3 Deigning a Spray Tail 28

5.4 Wedge 30

6.0 CASE STUDY OF PLANING HULL MODEL TESTS FOR 31

CFD VALIDATION

6.1 Introduction 31

6.2 Model & Tow Arrangement 32

6.2.1 Planing Boat Model 32

6.2.2 Tow Arrangement 33

6.3 Test Program 35

6.4 Result 37

6.4.1 Resistance 37

6.4.2 Running Trim 38

6.4.3 Sinkage Results 40

6.4.5 Hull Pressure 41

6.4.6 Wave Profiles 42

6.4.7 Boundary Layer Velocity Profiles 43

6.5 Summary 48

7.0 REFERENCES 49

SMK4562 [SMALL CRAFT TECHNOLOGY]

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CHAPTER 1

INTRODUCTION

1.1 Background

The expectation for high-speed craft performance has become far greater over the year as

people compete to have a better and faster craft. People demanding faster craft for reasons, some

use the craft as pleasure, some for traveling, but mostly the high-speed craft is used in military

and coast guard purpose. The limitation in speed for typical vessel leads engineer to design a

different and advanced vessel, as a result several type of vessel has been introduce over the

decade to overcome the speed boundary in more efficient and economical way. This can be

achieve by any 3 methods below

a. By placing displacement volume below free surface.

b. By placing displacement volume above free surface.

c. By having slander water plane area.

Planing craft is one of the vessel types that able to achieve high speed on the surface of the water

which follows the (b) method.


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1.2 Definitions

Baird (1998) defines a high-speed vessel as a craft with maximum operating speed higher

than 30 knots, but this definition is too convenient and incomplete to define a high-speed vessel

because lot of considerations and factors must be included in a way to picture and classified the

high-speed vessel. Odd M. Faltinsen (2005) state in his book that hydrodynamicists define a

high-speed craft by using the Froude number, �� = � ��⁄ where � is the ship speed, � is the

overall submerge length �� of the ship, and � is the acceleration of gravity. A craft is

considered has a high-speed craft when the Froude number is larger than 4.0 for vessel that

supported by the submerge hull, such as mono-hulls and catamaran. The value of Froude number

is different for different type of vessels.

Where else the definition of planing vessel is, according to savitsky (1992), a vessel is

considered as a planning vessel when the length Froude number, Fn > 1.2. However, according to

ODD M. Faltinsen (2005) sometimes for Fn > 1.0 also can be considered as a lower limit for

planning vessel.

According to Dr M. Pauzi Abdul Ghani(200X), he define severals mode of small craft including planning vessel in several term as shown in the table 1 below:

Mode �� = �� √� ��∇ = �

��∇� �� = �� ℎ�ℎ� !��ℎ

Displacement 0.40 1.35 < 0.75 < 0.50 " �� = 1

Semi-Planng 0.56 1.90 0.75 - 2.25 0.50 – 1.50 " �� = 2

Fully-Planing 0.80 2.70 > 2.25 > 1.50 " �� = 4

Table 1 : Definitions of small craft for Displacement, Semi-Planing and Planing mode


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CHAPTER 2

FUNDAMENTAL OF PLANING BOAT

2.1 Introduction

There is no uniform definition of planning and in practice; there are many borderline cases in which it is difficult to decide on the basics of any definition. For prismatic surface where the buttocks must be straight and the variation on the beam and deedrise in the planning area must not be great; when a surface of this type moves with a positive angle of attack and the flow separates cleanly from its chine and transom, it is planning.

2.2 Savitsky Method

Another criterion, Savitsky considers a boat to be planning when CV/√λ >1.0. This is good criterion but is not practical for field observation.

For steady state planning all the forces and moments acting on the boat must be in equilibrium. The simplest case is that of a flat plate planning at trim angle, τ.


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In the case of a planning boat with a fixed LCG, the trim angle will adjust itself to place the centre of pressure under centre of gravity. Therefore we have to determine the trim angle, which produces equilibrium. There are two method of direct calculation, i.e.

Clement’s Method

This method is suitable only for high speeds where the buoyant contribution to lift is neglected

Savitsky’s Method

This method takes into account the buoyant forces and is therefore applicable to vary low speeds.

Savitsky (1964) has given formulas for lift and drag force on planning hulls. These formulas are based on a large number of resistance tests with prismatic, or wedge-type surfaces, in which the trim angle, dead rise angle, wetted length and length-beam ratio, were varied systematically.

L = Lift

W = L = Weight

F = Normal Force

R = Resultant Force

J = Drag

E = Friction

ϴ = Trim Angle (Angle of Attack)


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The following approach is followed.

Given

V speed

b maximum beam over the chines or spray strips

& displacement volume at rest

Fn& = ��'()/+ , volumetric or displacement Froude number

CV = ��', , Froude number based on b

CLb= -'().-�.,. , the equivalent flat plate lift coefficient

The following Savitsky’s formula can then be applied to determine the thrim angle for equilibrium:

� /, = 0�.� ( 0.0120√" + (0.0055"67)/��7 Where

τ is trim angle, deg

λ is mean wetted length-beam ratio, Lm/b

V is speed, m/sec

b is beam of planning area, m

g is acceleration of gravity


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The first term of equation represents the dynamics component of list while the second term is the buoyancy component of lift. At Cv > 1.0, there is little buoyant lift, so that, all other condition being equal, lift varies as the speed squared.

The lift coefficient for a finite deadrise;

CLβ = CLb – 0.0065βCLb0.6, where β is the deadrise at the mid- chine positionin degree.

Savitsky’s (1964) also give a formula for the location distance, p, i.e. the centre of pressure forward of the tramsom. However, in many cases it may assumed that the resultant nor,al force on the planning bottom, N, passes through the CG, i.e. p = LCG as shown in figure below.

Resultant Normal Force on Planning Bottom


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With appropriate value of Cv and p/b= LCG/b, the corresponding λ and CLb/ τ1.1 are then read off

the nomograph in figure below:

Nomograph for equilibrium condition when all force act through CG (Koelbel)

This nomograph by Koelbel is valid when the propeller thrust, the resistance force and the resultant of the planning force all act through the CG. Hence, the mean wetted length Lm and trim angle, τ can be determined.

Savitsky also gives a formula to correct the mean wetted length ratio, λk if desired,

λk = λ -0.03 + ½ [0.57 + ( β/1000)][(tanβ/2tanλ)-β/167]

where the value of β should be taken at the mid-chine length position.

The value of λk should now be compared with the value of LWL /b. If λk ≥ LWL/b then the bow is not clear of the water and craft is not fully planning.


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When λk ≤ LWL/b the bow is essentially clear of water and the resistance can be predicted from the following equation:

RT = W tan τ + ½ ρ V2λ b2 CFO /(cos τ sec β)

Where

CFO is the friction coefficient according to ITTC 1957 as a function of the Reynolds number,

Rnb = V1λb/ υ.

�� = � 91 − 0.01200�.�√" cos τ ?�/7

Here V1 is the average bottom velocity which is less than the forward planning velocity owing to the fact that the planning bottom pressure is large than the free stream pressure.


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CHAPTER 3

HYDRODYNAMIC CHARACTERISTIC

OF PLANING HULLS

3.1 Water rise and splash-up

The dynamic forces develop on relatively flat straight bottom (flat) of planning craft help lift it up on the water surface. This able to laminate the wave making resistance component and the total resistance is the sum of the surface resistance of the wetted surface area and pressure drag term. In theory, the wetted surface are varies inversely as the square of the vessel’s speed, so that the high speed resistance is roughly independent of speed.

As shown in figure below , for simplest case of prismatic planning surface,

� The pressure drag is a factor of @ sin 0 where 0 is trim angle and R is the total normal force, if the vessel component speed is CD so the energy input for drag component is equal to CD @ sin 0. Half of the energy converted into dynamic lift force R and the other haft turns to kinetic energy of water spray and both divided by the ‘dividing streamline’.

� For planning hull, water always rises in front before breaking forward or sideways and rearward. Water rise the greatest when the deadrise angle β =0 and will rapidly diminished as the β increased.


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� P.R Payne (1994) suggest to calculate the water rise in front of any prismatic planning surface is best approximated by:

E√,F = G sin7 0 and G = 2ℯI7.6JK

Where; d = vertical water rise at the water/keel intersection

B = beam

L = submerged length of the keel

0 = trim angle

β = Dead rise angle in radians


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3.2 Induced resistance and lift by Hassan Ghasemi and Mahmoud Ghiasi

Once model dimensions and running conditions have been defined, the potential flow solver based on the BEM is employed to obtain the induced dynamic pressure, dynamic lift, and resistance. Frictional resistance is computed by the two-dimensional thin boundary layer, and the practical method is utilized to evaluate the spray resistance. 3.2.1 Theoretical and Calculations

Consider a Cartesian coordinate system fixed in the space O-XYZ as figure above the planing hull travels with constant forward speed, Vs, on calm water surface and unrestricted flow. The fluid motion generated by the planing ship can be treated as equivalent to the disturbance created by a pressure distribution acting on the bottom of the ship. Several assumptions are been made:

1. Inviscid 2. Incompressible 3. Irrotational 4. No surface tension


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These lead to a boundary value problem for the velocity potential. The flow around the pressure distribution which harmonic in the fluid domain can be define using the Laplace equation which is the total velocity potential, Φ : Φ = ϕ − VNNOP. XNNO, (1) Where: ϕ is the perturbation velocity potential XNNO is the position vector. The total potential and perturbation potential are both governed by Laplace’s equation ∇7Φ = 0 ∇7ϕ = 0 (2) The potential ϕ is computed by the BEM, which is based on Green’s identity. In general, the boundary surface includes the body surface (SB) and the free surface (SF). Thus, the perturbation potential ϕ is given by the following integral expression with points Q on body surface and free surface SB+SF and P in the fluid domain D.

4RST(U) = V WT(X) YZY� − YT(X)Y� Z[P\]P^ _�

(3) Where:

E is the solid angle

E = ½ If point P is placed on the boundary (body surface), E = 0 If point P is placed inside and outside of the body

G is Green’s function G = 1/r+1/r’, where : r is the distance between the field point P and the source point Q.

r0 is the distance between the field point P and the image of source point relative to the mean free surface.


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3.2.2 Boundary conditions I. On the body surface:

YΦY` = −�a.NNNNO �NO + �b/

(4)

Here: vBL is the velocity by Takinaci et al. (2003)

�b/ = Y(�cd∗)Yf

(5) s = line along the surface of the craft, Ue = flow velocity at the edge of the boundary layer d = displacement thickness.

II. On the free surface: (∇T − �a)NNNNNO, ∇ξ = ϕh on z = ξ (x, Y) (6)

Where: ξ is the wave elevation l = 1� m− �a.NNNNO ∇T + 12 ∇T. ∇Tn

On z = ξ (x, Y) (7)

III. At infinity: limq→∞s∇Tt = 0

(8) IV. Boundary condition on the free surface:

The linearized equations are, −�a. lu = Tv , w� x = 0 (9) l = − 1� �aTy, w� z = 0

(10) By (9) and (10) Tuu − {DTv = 0, w� z = 0 (11)


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3.2.3 Pressure on hull The pressure on the craft hull surface is calculated by the derivative of perturbation potential �| NNNNO = ∇T, So, U = UE + U} = 0.5 ~ (2��NNNO . �|NNNO − �|NNNO . �|NNNO) + ~�ℎv (12) UE is the dynamic pressure produce by the velocity U} is the hydrostatic pressure

The hydrodynamic force (lift and induced resistance) can be obtain by

�y = @� = V Ua �u_f , �� = V Ua ��_f = 0, �� = @� = � = V Ua �v_f , (13) Where �NO(�u , ��, ��)is outward unit normal vector on the craft 3.2.4 Boundary layer theory and frictional resistance The resistance can be obtain by

R^ = V τDP

Ddxds ds = 0.5ρV C�U�7dx��(�)

D = 0.5ρ � C�U�7(∆x��

�� ) Where, C� = Local friction coefficient τD = Shear stress NP�� = number of strip in longitudinal direction


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3.2.5 Upwash geometry and spray resistance

Figure x

Figure x above show the wetted bottom area which divided into 2, the aft part of the

stagnation line is the pressure area. It is surrounded by the wetted keel length, LK and wetted chine length, Lc. The relation ship between those lines is as below: According to Savitsky(1964) L� − L� = B

π

tan βtan τ

(15) Where else, Bowles and Danny (2005) state that L� − L� = B

π. tan βtan τ . 1

¢1 1� + tan β tan ¢β 2� ££� 7� + 1

(16)


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The hydrodynamic pressure of the spray is proportional to the geometrical configuration of the hull (such as the deadrise, the trim, and chine wet/dry regions) and operational conditions (such as the craft speed and the resulting free surface waves). Spray surface may practically be expressed by the following equation:

¤a¥q¦� = {�(�§ − �¨)�cos ©

(17)

Where K1 depends on the craft speed.

{� = ª(��∇) = « 0.2 �ª ��∇ < 30.4 �ª 3 ≤ ��∇ < 50.7 �ª ��∇ ≥ 5 ±

(18) Where ��∇ is volumetric Froude number,

��∇ = ��∇� �⁄

The pressure of spray is given by, U� = {7.

(19) Where, p = obtain from equation (11)

{7 = ª(©, 0, ²(³)) = « 2 �ª ²(³) < 0.5�1.5 �ª 0.5� ≤ ²(³) < 0.9�1.2 �ª 0.9� ≤ ²(³) < � ±

(20) Finally, the spray resistance and lift generated can be estimate using equations below, @a¥q¦� = U�¤a¥q¦� sin 0

(21) �a¥q¦� = U�¤a¥q¦� cos 0 (22)


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3.2.6 Total resistance and lift The total resistance of planning is given by, µ¶ = µ· + µ¸ + µ¹º»¼½ (23) From here we can get the effective power which by ¾¿ = µ¶À¹ (24) The hydrodynamic lift and buoyancy is expressed as following

�E = V UE��_�aÁ

�a = V Ua��_�aÁ

(25) Other useful coefficients are,

Non-dimensional pressure coefficient �¥ = U12 ~�a7

Non-dimensional hydrodynamic coefficient �/Â = �E12 ~�a7�Ã

Non-dimensional hydrostatic coefficient �/Ä = ��12 ~�a7�Ã

Non-dimensional resistance coefficient �� = Å12 ~�a7��Ã


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CHAPTER 4

TYPE OF PLANNING HULL

4.1 Introduction

The following sections will give a brief description on various type of planning hulls available namely:

a) Deep vee bottom b) Inverted vee bottom c) Round bottom

From the above three configuration, they could further be developed into their own classes. Since limited information is available on the inverted vee and round bottom type, so in this section the focus will on the deep vee hull geometry. The various type of deep-vee hull section are describe in the following paragraph.


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4.2 Deep Vee Bottom

"Deep-V" boats have an undesirable tendency to pitch severely in rough seas in resonance with the frequency of the wave action. When moving with the waves, these hulls fall off one wave and plow into the next where their sharply inclined bow surfaces act like a rudder around which they rotate or broach. Yet their wide amidship bottom surfaces still pound against oncoming waves. Further, at high planing speeds, these hulls need to have their center of gravity substantially rearward of amidship in order to keep the bow up and reduce wetted area which reduces frictional drag, but such a rearward center of gravity causes excess bow rise at low speed.

The lifting characteristics of the continuous hull result in a non-level ride, and the boat exhibits lateral instability at rest. In high speed turns such a boat banks severely, and a large turning radius is required for low speed turns because the boat pivots on its bow. Trim tabs or similar devices are often necessary to provide the necessary lift at the stern area, depending on the orientation of the power unit. Further, if the angle of the V is not deep, these hulls tend to skid excessively in a turn.

Boats with deep-V hulls produce a large wake with a heavy spray, displace a great deal of water at all speeds, have a relatively high aerodynamic and hydrodynamic resistance, and generally have poor fuel economy.

One reason for the poor efficiency of deep-V hull boats is their tendency to ride at an angle to the water with the bow up high and the stern low. Thus, the hull presents a large frontal surface area to encounter wind and water resistance. In addition, visibility is reduced as a result of the high bow. Some boat designers have attempted to overcome this characteristic by adding trim tabs and/or lifting strakes to the hull; however, these additions cause an increase in drag and add to the cost and maintenance of the boat while reducing fuel economy.

4.3 Type of Deep Vee Bottom

4.3.1 Convex Section Shape

This is inherently a wet section (wetted surface of the hull is quite high) but can be overcome by spray rail and bulwark, which have anti spray characteristic, hence wetness will be The section pounds less than other of equal deadrise because it is less likely to contact the water on large area at the same time.

4.3.2 Concave Section shape

This shape in inherently considered as a dry section, but it is found to be very hard shape. Owing to the fact that the hollow area underneath almost always ‘packets’ the water and produces impacts, it is hence to be known to have a very high riding shape.

Depth of concave/convex section are extremely important, too much depth reducedamping coefficients and may adversely other parameters.


Type of Deep Vee Bottom

Convex Section Shape

This is inherently a wet section (wetted surface of the hull is quite high) but can be overcome by spray rail and bulwark, which have anti spray characteristic, hence wetness will be The section pounds less than other of equal deadrise because it is less likely to contact the water on large area at the same time.

Concave Section shape

This shape in inherently considered as a dry section, but it is found to be very hard shape. Owing to the fact that the hollow area underneath almost always ‘packets’ the water and produces impacts, it is hence to be known to have a very high riding shape.

Depth of concave/convex section are extremely important, too much depth reducedamping coefficients and may adversely other parameters.

[SMALL CRAFT TECHNOLOGY]

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This is inherently a wet section (wetted surface of the hull is quite high) but can be overcome by spray rail and bulwark, which have anti spray characteristic, hence wetness will be avoided. The section pounds less than other of equal deadrise because it is less likely to contact the water

This shape in inherently considered as a dry section, but it is found to be very hard riding shape. Owing to the fact that the hollow area underneath almost always ‘packets’ the water and produces impacts, it is hence to be known to have a very high riding shape.

Depth of concave/convex section are extremely important, too much depth reduces some

4.3.3 Straight Section Shape

These shapes are found to be about as good as any section shapes by some model testers, when transverse section is considered, this opinion has some degree of merit bwhen the entire forebody surface produced by straight section is considered, it because evident that this section shapes have all the concave and convex section shape i.e they produce a wet pounding boat.

4.3.4 Inverted Bell Section shape

This section is designed as the constant force sections. The advantage is that the rounded keel does not pound, but the disadvantage is that the shape produces a strong tendency to directional instability. This shape needs an external centerline keel and low spflow as to avoid the directional instability.


Straight Section Shape

These shapes are found to be about as good as any section shapes by some model testers, when transverse section is considered, this opinion has some degree of merit bwhen the entire forebody surface produced by straight section is considered, it because evident that this section shapes have all the concave and convex section shape i.e they produce a wet

Inverted Bell Section shape

section is designed as the constant force sections. The advantage is that the rounded keel does not pound, but the disadvantage is that the shape produces a strong tendency to directional instability. This shape needs an external centerline keel and low spray strips to break up the cross flow as to avoid the directional instability.


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These shapes are found to be about as good as any section shapes by some model testers, when transverse section is considered, this opinion has some degree of merit by its own. But when the entire forebody surface produced by straight section is considered, it because evident that this section shapes have all the concave and convex section shape i.e they produce a wet

section is designed as the constant force sections. The advantage is that the rounded keel does not pound, but the disadvantage is that the shape produces a strong tendency to directional

ray strips to break up the cross

4.3.5 Spray Stripes Shape

With these spray stripes at the bottom, they may lead to the flow separated especially for high deadrise boats and hence perform antispray characteristicscraft planning on a wide bottom in preplanning regime (low speeds) and on narrow bottom at high speed.


Spray Stripes Shape

With these spray stripes at the bottom, they may lead to the flow separated especially for high deadrise boats and hence perform antispray characteristics. This allow the possibility of the craft planning on a wide bottom in preplanning regime (low speeds) and on narrow bottom at


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With these spray stripes at the bottom, they may lead to the flow separated especially for . This allow the possibility of the

craft planning on a wide bottom in preplanning regime (low speeds) and on narrow bottom at


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CHAPTER 5

STERN WEDGES AND SPRAY RAIL

5.1 Planing hull

To be able to develop or improve such theory, it is important to learn and study about the

hull nature first. One of the planning hull criteria is to generate a hydrodynamic lift, which

contributes to the reduction of the wave-making component thus increase the efficiency.

Unfortunately, comes with this advantage, planing hull invites other problem such as augmented

slamming, porpoising, and dynamical heel. High-speed planing craft also generates spray with

impact pressure at the bow region. It is important in planing craft study to consider the spray

generated because the ratio of spray mass to the boat mass is big compared to other type of

vessel. At very high speed, the effect of spray, the quantity and not to mention the pressure

produced becomes more obvious and should not be taken lightly. The direction of the spray will

induce the position of the pressure to its maximum level.

Hassan Ghasemi and Mahmoud Ghiasi (2007) in their study about a combined method

for the hydrodynamic characteristics of planing crafts, they have studied the effect of spray

resistance on four different models by using the boundary element method to analyze the


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hydrodynamics of the model. In the end of their study, they come up with conclusions as

follows:

• Comparison of the pressure distribution for the three models shows that a concave

planing model may give further lift and induce resistance relative to the other

models.

• The practical method is very effective for estimating the spray resistance.

• It is calculated that the hydrodynamic lift to weight ratio (L/W) is about 65% and

85% at Fn of 3.35 and 5.0, respectively.

• Greater emphasis on numerical computation of the jump pressure due to the spray

is recommended as an alternative to the present practical method

Where else, Muller-Graf claims that well shaped and properly arranged spray rail, if

combine with a transom wedge, are the most effective devices to reduce the hull resistance of

given semi-displacement round bilge hull. It is also state that the advanced spray rail system

(ASRS), developed in the Berlin Model Basin, combine with a wedge, leads to remarkable

power saving, which are larger than those obtained by each component solely. Additionally, the

seakeeping qualities of round bilge hulls are improved by this special spray rail system and the

apparent loss of metacentric height of this hull type at high speeds can be reduces considerably


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5.2 Spray rail

By knowing the flow nature on the hull, the designer is able to improve the boat

performance by doing some modification or adjustment on the hull. This modification is usually

in the form of rail or strake. The use of spray rail has proven that the maneuvering of the craft

has increased. Spray rail is a horizontal profile on the hull, the function is to spray out water and

reduce the added mass thus increase the lift force and hydrofoil effect. When more of the craft

hull is above the water surface, the resistance and drag force is less and thus increase the craft

speed. ITTC (1984) suggests that semi-displacement round-bilge hulls should be tested with

spray rails, in order to avoid substantial increase of the wetted surface area which could reach 50

to 60 % of the wetted surface area at rest. The effect of stern wedges and controllable flaps (trim

tabs) could decrease 5% in resistance for 0.4 < Fn < 0.45 and up to 11% for 0.5 < Fn < 0.9.

Andi Haris Muhammad (2008) has done research on the effect of spray-strake on

maneuvering performance of a planing hull patrol vessel and conclude that the use of Spray-

strake improve the aspect of maneuvering depending on the position, shape and angle of the

strake. Muller-Graf (1991) claims that well shaped and properly arranged spray rails, if

combined with transom wedge, are the most effective devices to reduce the hull resistance of

given semi-displacement round bilge hulls. It is also stated that the advance spray rail system

(ASRS), developed in the Berlin Model Basin, combined with a wedge, leads to remarkable

power saving, which are larger than those obtained by each component solely. Additionally, the

sea-keeping qualities of round bilge hulls are improved by this special spray rail system and the

apparent loss of metacentric height of this hull type at high speed can be reduced considerably.

Even though the rail on the hull is the same through out the entire hull but it serves for

many purposes. Rail located at the side of the bow is to create spray separates the water from the

hull or else the water will stick to the hull and become additional mass. This type of rail is known

as spray-rail or spray-strake. Rail located at the bottom of the hull is designed to keep the craft in

its course while in a high speed. The use of the rail is also able to improve the lateral stability

and reduce the rolling tendency of the craft.


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Yushu Washio (1995) in his patent document, titled high-speed lateral-stability hull

construction with patent number 5,425,525 has designed various style and position of rail on

hull. The design is to provide a hull of a single-hulled ship having a transom and either a chine or

round bilge, which suppresses wake and exhibits excellent lateral stability even at high speed.

The design also has the capability to produce a restoring force to counter large-amplitude rolling.

Figure 7: A former hull type (on left) and the later hull type (on right) [Yushu Washio, 1995]

In figure xx, the former hull is designed to employ a chine to form a squarish bilge while

the later hull design is to employ a slender hull. In addition, for the later hull design, to prevent

waves from washing up along a surface of the hull or spray from arising at the bow region during

high speed navigation, it is common to install a small reaction flap or spray strip as shown in

figure 8.

Figure 8: Implementation of spray strip [Yus


Figure 8: Implementation of spray strip [Yushu Washio, 1995]


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hu Washio, 1995]


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5.3 Designing a spray rail

Although there is various design of spray strip in hull design, there are still no formal

steps and guide line in designing a spray strip. The entire existing spray strip nowadays is

designed based on the designer experience. Though there are many research and study about the

effect of spray strip that can be used as a guideline such as presented by Muller-Graf (1991) in

which he did a test on the effect of various geometry of spray strip for semi-displacement hulls.

The geometry description of the spray rail can be seen in figure 9.

The spray rail had a triangular cross-section with constant bottom width bSR=0.0055LWL.

Transversal slope of spray rail, (β), where located at the bottom of the rail and the horizontal line

with angle 00 < β < 450. Break angle of spray rail, ζ > 900. Height of rail above DWL (hSR) can be

calculated in figure 10..

Figure 9: Example of spray rail geometry. (Muller-Graf, 1991)


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Figure 10: Guideline to determine the rail high. (Muller-Graf, 1991)


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5.4 Wedge

Figure XX: Definitions of Wedge

Wedge is an appendix on vessel located at aft and bottom as shown in figure above.

Wedge angle Æ is measured relatively to the buttock line slope. The wedge’s aft edge can be obtain by the equation propose by Predrag Bojovie and Prasanta K. Sahoo :

© = tanI� ÇÈ�ÉI�)yÉIy)ÈÊ

xË = x� + (ÌD − Ì�). tan(© − Æ)


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CHAPTER 6

CASE STUDY OF PLANING HULL MODEL TESTS

FOR CFD VALIDATION

6.1 Introduction

This case study was taken form the experiments were performed in the Clearwater Towing Tank at the National Research Council of Canada's Institute for Marine Dynamics and consisted of a series of resistance tests with a planing craft. Model scale experiments were done to collect data to validate developments in computational fluid dynamics methods. Tests were done over a range of speeds and in 6 different ballast configurations (displacement and longitudinal center of gravity). Measurements were made of tow force, running trim and sinkage, hull pressures, wetted surface area, and wave profiles. Additional tests were done to measure the boundary layer thickness at two locations along the hull using a laser Doppler velocimeter. These were done for four speeds in a single ballast configuration. The boundary layer at each position and at each speed was delineated using about 20 runs.


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6.2 Model & Tow Arrangement

6.2.1 Planing Boat Model

The hull shape used in the experiments discussed in this paper was a 1:8 scale model of a full scale vessel currently in operation. It was constructed out of carbon fiber reinforced plastic strengthened with transverse and longitudinal stiffeners, a watertight bulkhead near the stern, and a shear deck with coaming. A plastic splash guard cover was fitted during tests. The hull surface, shown in Figure 1, was marked with station numbers on the bottom and port side. Knife edges extending 1mm from the hull surface, were fitted along the chines to promote flow separation. The hull was not prismatic but did have a simple shape as shown in Figure 2. This cross section was constant from the transom for about 2/3 the length of the hull (covering the wetted length of the model for all ballast conditions when planing). A small flat bottom area at the centerline turns to a low deadrise of 5.9°. This deadrise then turns sharply to 40.8° near the chine (see Figure 2).

Figure 1. Model Hull (LOA = 1.475m)


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Figure 2. Model Hull Cross Section

6.2.2 Tow Arrangement

The model was fitted to the tow carriage using a gimbal and yaw restraint. Tow force was transmitted from the heave post through a linear bearing to an ‘S’-shaped load cell (max. load = 50 lb.) and then through a universal joint to the model (see Figure 3). The universal joint allowed the model to pitch and roll freely and the heave post was free to move vertically in the tow post arrangement. The model was prohibited from rotating about the heave post by a yaw restraint which was counterbalanced so that it did not affect the ballast. The tow arrangement is shown in Figure 4.


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Figure 3. Gimbal. Figure 4. Tow Arrangement.


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6.3 Test Program

The test program consisted of two phases. The first phase focused on testing the effects of different ballast conditions over a range of speeds. Measurements were made of tow force, running trim, sinkage, hull pressures, wetted surface areas, and wave profiles. The second phase was performed solely at the design ballast condition, and was used to measure boundary layer velocity profiles below the hull surface using a laser Doppler velocimeter (LDV).

As planing craft performance is sensitive to ballast condition, tests were performed over a range of displacements and locations of the longitudinal center of gravity (LCG). These conditions are given in Table 1, which also shows the static trim angles of the model. The first column lists the three displacements (design displacement ±15%) and the first row lists the three LCG positions (design LCG ±7%). LCG position was referenced from the transom base.

A plan view of the model hull bottom is given in Figure 5 showing the relative locations of the LDV windows, pressure transducers (labeled P1 through P9), tow point, and LCGs.

Displacement LCG =0.49m LCG =0.53m LCG = 0.57m

25.2 kg - 1.0o -

29.6 kg 2.0o 1.1o 0.4o

33.9 kg - 1.3o -

Table 1. Static trim angle for ballast conditions,


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Figure 5. Instrument Positions in Model


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6.4 Results

6.4.1 Resistance

The resistance curves for the model were typical for a planing vessel and had the characteristic ‘hump’ speed at the onset of planing. Figure 6 shows the resistance results for the various ballast conditions. Only the design condition was tested over the full speed range. The curves closest to the design condition show the effect of a 7%change of LCG (both fore and aft) on resistance, while the two more distant curves show the effect of a 15% change in displacement.

Figure 6. Model Scale Resistance.


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6.4.2 Running Trim

Trim angle is an important factor in planing craft performance as it changes the geometry of the hull relative to the water. The running trim angles for this model followed similar trends as the resistance curves, clearly identifying the ‘hump’ speed at which planing begins. Shown in Figure 7 are the absolute running trims for the various ballast conditions.

Figure 7. Running Trim.


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It can be seen from the plots that the different ballast conditions were not tested to the same maximum speeds. For instance, the aft LCG ballast condition was only tested to 6.0 m/s and the forward LCG condition was tested to8.0 m/s. This occurred because the model was prone to dynamic instability, or propoising, at high speeds. The aft

LCG position made the model susceptible to this instability at speeds above 6.0 m/s and therefore it was not tested beyond that limit.

Another way of presenting the running trim results is to plot the change in trim angle developed at speed from the static trim angle at rest (given in Table 1). This plot, Figure 8, shows that when in the planing regime, the threshold above which porpoising occurred was when the change in trim angle dropped below approximately 2.1°. More details of the porpoising characteristics of this model can be found in Thornhill et al. (2000).

Figure 8. Change in Trim.


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6.4.3 Sinkage Results

Sinkage refers to the change in the vertical position of the model at speed and was measured using an LVDT (linear voltage differential transducer) mounted on top of the heave post (see Figure 4). Shown below in Figure 9 is the sinkage profile for the design ballast condition. Also given in the figure is the trim profile for this condition.

These are presented together because sinkage is related to trim angle (the model did not necessarily rotate about the tow point where sinkage was measured). At low speeds, the model began to trim by the stern and sank downwards in the water. As it climbed its bow wave, trim peaked and then began to decrease while the model continued to rise upwards. At high speeds, trim angle continued to decrease while the vertical position leveled off to approximately 3.5cm above its original position.

Figure 9. Sinkage and Trim Results.


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6.4.5 Hull Pressures

Hull pressures on the model were measured using 9 pressure taps mounted flush to the hull bottom at various locations. Several of these pressure taps malfunctioned during tests while others encountered relatively high levels of noise. The final results could not therefore be relied upon for specific quantitative information of the pressure distribution on the hull. They can, however, be used to show the range of pressure on the hull and identify certain trends that developed with increasing model speed. The most notable of these are shown in Figure 10.

Figure 10. Hull Pressure at Two Locations.

The figure gives the results from two pressure transducers located fore and aft at the same longitudinal plane in the model (P1 and P6 shown in Figure 5). The forward transducer records increasing pressure with increasing speed, while the aft transducer shows the opposite trend, with negative pressure values at high speeds. These negative pressure values correspond to increased flow velocities near the hull as discussed in a later section.


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6.4.6 Wave Profiles

The surface wave profiles produced by the model at speed were captured by a transverse array of capacitance probes located midway along the tow tank. The 23 probes were spaced 7 inches apart, the first being 7 inches from the side of the model as it passed by. Sampled at 100 hz, the time traces from the probes show the wave elevations at the various longitudinal cuts. A proximity switch was used to correlate the position of the model with the probe data: when the switch was triggered, the model’s bow was in line with the probe array. The probe array is shown in Figure 11 attached to a beam fixed to the tank wall. An example of the data collected from the probes is shown in Figure 12.

Figure 11. Wave Probe Array in Tank.

Figure 12. Wave Probe Data


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6.4.7 Boundary Layer Velocity Profiles

The second phase of the experimental program was dedicated to determining velocity profiles in the boundary layer at two locations for four different model speeds in the design ballast condition. The measurements were made using a laser Doppler velocimeter (LDV) fitted in the model. This instrument has several advantages over other more common techniques for velocity measurements such as pitot tubes and hot-film anemometry. The primary advantage of the LDV is its non-intrusiveness; only the laser beams enter the water, so they do not influence the thin layer of fluid where measurements are being taken.

The LDV uses intersecting laser beams to make velocity measurements. Strictly speaking, the LDV measures the velocity of particles in the flow and not the flow itself. A particle, when traveling through the volume of intersection of the beams, reflects light as it passes through an interference pattern of light and dark bands caused by the lasers of matching wavelength. Processors in the LDV determine the frequency of this pulsating reflected light picked up by sensors in the probe. As the distance between the interference bands is known, the processor can then calculate the velocity of the particle. Numerous particle measurements are averaged to determine the mean flow velocity.

Particles are added as “seed” to the flow and are generally in the size range of 0.5 – 5.0 microns. The measurement volume of the LDV depends on both the beam diameter and the angle of intersection. For these experiments the volume was an ellipsoid 0.64 mm in height (perpendicular to the hull) and 76 µm in diameter.

Seeding is an important part of LDV testing as it controls both the data rate (the number of particles passing through the intersection volume per second) and validation (the percentage of particles that could be processed into velocity measurements). For these experiments, seed was added for each test by aiming a small stream of a concentrated water/seed mix in the path of the model. Several types of seed were used, including silver-coated glass micro-balloons and pre-sifted all-purpose flour. Data rates for the experiments ranged from 30 Hz to 3 kHz with validation between 60-95%. Typical values for most tests were data rates around 500 Hz with 75% validation.

The set-up for the experiments had the LDV probe mounted inside the model on a set of micrometer tables used to locate the probe for each measurement. The probe faced downward and projected the lasers through a small acrylic window in the hull. The beams intersected at a point just below the window where a measurement was taken (see Figure 13). The micrometer tables were used to precisely position the probe at different positions within the boundary layer. A single run of the carriage was used to measure the velocity of each point in the boundary layer at each model speed. Successive runs were needed to resolve the velocity profile for a given model speed.


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Figure 13. LDV Mount.

Raw data from a typical test is given in Figure 14. It shows the acceleration, constant speed, and deceleration portions of the run. The figure also shows that the raw velocity data fell onto equally spaced discrete values (seen as bands of points). This feature is an artifact of the LDV’s internal processors that determine the particle velocities.

The width between these bands can be changed, but doing so also alters the range of velocities which can be measured. A smaller bandwidth results in a smaller velocity range. These experiments used a bandwidth of approximately 0.1 m/s.


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Figure 14. Typical LDV Data.

Boundary layer velocity profiles for two positions on the hull for each of four model speeds (4 m/s, 5 m/s, 6 m/s and 6.5 m/s) were measured. Results for the model speed of 4 m/s are given below in Figure 15.

Figure 15. Boundary Layer Velocities (Vm = 4 m/s).


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The results from these measurements clearly show the boundary layer velocity form, thickness, and the free stream velocity for both of the two locations at each speed tested (for a total of 8 profiles). In the figure, the forward position shows a boundary layer thickness of about 4 mm with a free stream velocity equal to the model velocity.

The aft position shows that the boundary layer had grown thicker and that the flow achieved a greater free stream velocity, exceeding that of the model speed. This is consistent with the negative pressures measured in the aft region of the hull. Profiles at the other model speeds tested were qualitatively similar as those shown in Figure 15. The percentage increase in free stream velocity from the forward to the aft position decreased as the model speed increased (trim angle also decreased). The boundary layer thickness also decreased with increasing model speed.

This drop in pressure and increase in speed in the after region of the hull can be partially explained by taking into consideration the potential head due to depth of immersion, a factor that is omitted in simple classical planing theory which predicts only positive pressures over the length of a planing surface. However, the pressure drops and speed increases for the corresponding trim and sinkage conditions were somewhat larger than expected from this cause alone. It is planned to investigate this behaviour further using CFD simulations.

The positions of the forward and aft measurement positions relative to the leading edge of the wetted hull area for a given model speed are shown below in Figure 16.

Figure 16. Vessel Attitude (4 m/s).


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One difficulty with the technique used to determine the boundary layer velocity profile was the determination of the reference or zero position of the hull surface. The procedure for finding this zero position consisted of systematically moving the measurement point closer to the lens until the photo-detectors gave an overload error.

This meant that the measurement volume was inside the lens, and that the beams were reflecting directly back to the detectors. It was, however, possible that measurements could be taken with a small portion of the measurement volume inside of the lens, without overloading the photo-detectors. The size of this overlap could not be determined.

The orientation of the probe meant that the largest dimension of the measurement volume (0.64mm) was perpendicular to the hull. It was assumed that measurements could not be made if more than half of the measurement volume was inside the lens. This gives an uncertainty in the hull zero position for the LDV measurements of approximately 0.32mm. The shape of the profiles is not affected by this bias, which would shift the entire curve up or down.

Another result from the analysis of the raw LDV data came from the standard deviations of the samples used to calculate the mean flow velocities. Shown Figure 17, the standard deviations followed a similar trend as the velocities. High standard deviations were measured close to the hull, while in the free stream they leveled off. The higher values close to the hull can be attributed to two primary factors: turbulence and velocity gradient. Wall bounded turbulence in the boundary layer can cause fluctuations in velocity that would result in increased standard deviation. The large velocity gradient close to the hull would also result in increased standard deviation since a broader range of velocities spanning from the bottom to the top of the measurement volume would have been captured.


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Figure 17. Standard Deviations from LDV Data.

6.5 SUMMARY

Tests were performed on a 1/8 scale model of a planing vessel to generate a set of performance data to be used in future validation of numerical simulations. Sample results were presented for the measurements of resistance, running trim, sinkage, hull pressures, wave profiles, and boundary layer velocity profiles. Resistance and running trim results showed characteristics common to planing craft. Hull pressures were found to increase in the forward part of the hull but decrease and become negative in the aft. Boundary layer thicknesses were found to increase in the direction of flow and to decrease with increasing model speeds as expected. Velocities measured just outside the boundary layer were found to be greater than free stream in the aft part of the hull, showing an acceleration from the forward position.


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REFERENCES

BERTRAM, V. (2000), Practical ship hydrodynamics, Butterworth+Heinemann, Oxford

BLOUNT, D.L.; CLEMENT, E.P. (1963), Resistance tests of a systematic series of planing hull forms, SNAME 71, pp.491-579

CODEGA, L.; LEWIS, J. (1987), A case study of dynamic instability in a planing hull, Marine Technology 24/2, pp.143-163

SCHNEEKLUTH, H.; BERTRAM, V. (1998), Ship design for efficiency and economy, Butterworth+Heinemann, Oxford

Thornhill E., Veitch B., Bose N., “Dynamic Instability of a High Speed Planing Boat Model”. Marine Technology, July 2000.

Savitsky D., “Hydrodynamic Design of Planing Hulls”, Marine Technology, vol. 1, no. 1, pp. 71 – 95, October 1964.

Du Cane P., High Speed Small Craft 3rd Ed. Temple Press Books, London. 1964.

Payne P.R., Design of High Speed Boats Volume 1: Planing. Fishergate Inc. Annapolis. 1988.

Dr Mohamad Pauzi Abdul Ghani (2010), Small Craft Technology, UTM.

planing craft

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