7174460 Basics of Ship Resistance

8/6/2019 7174460 Basics of Ship Resistance

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Chap 7 Resistance and Powering of Ship

Objectives

Prediction of Ships Power

- Ships driving system and concept of power

- Resistance of ship and its components

frictional resistance

wave-making resistance

others

- Froude expansion

- Effective horse power calculation

Propeller Theory

- Propeller components and definitions

- Propeller theory- Cavitation


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Ship Drive Train and Power

Ship Drive Train System

Engine ReductionGear

Bearing Seals

ScrewStrut

BHP SHP DHP

THP

EHP


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Brake Horse Power (BHP)

- Power output at the shaft coming out of the engine before

the reduction gears

Shaft Horse Power (SHP)

- Power output after the reduction gears

- SHP=BHP - losses in reduction gear

Horse Power in Drive Train



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Delivered Horse Power (DHP)- Power delivered to the propeller

- DHP=SHP losses in shafting, shaft bearings and seals

Thrust Horse Power (THP)

- Power created by the screw/propeller

- THP=DHP Propeller losses

Relative Magnitudes

BHP>SHP>DHP>THP>EHP

E/G R/G

BHP SHP

ShaftBearing Prop.

DHP THP EHP

Hull



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Effective Horse Power (EHP)

EHP : The power required to move the ship hull at a givenspeed in the absence of propeller action

(EHP is not related with Power Train System)

EHP can be determined from the towing tank experiments at

the various speeds of the model ship.

EHP of the model shipis converted into EHP of the full scale

ship by Froudes Law.

VTowing Tank Towing carriage

Measured EHP


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0

200

400

600

800

1000

Eff

ectiveHorsep

ower,EHP

(H

0 2 4 6 8 10 12 14 16

Ship Speed, Vs (Knots)

POWER CURVEYARD PATROL CRAFT

Typical EHP Curve of YP


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Efficiencies

Hull Efficiency

THP

EHPH =

-Hull efficiency changes due to hull-propeller interactions.- Well-designed ship :

- Poorly-designed ship :

1H

1H

Well-designed

Poorly-designed

-Flow is not smooth.- THP is reduced.

- High THP is needed

to get designed speed.


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Screw


Efficiencies (contd)

Propeller Efficiency

DHPTHPpropeller=

Propulsive Coefficients (PC)

SHP

EHPp =

propellerdesignedfor well6.0p

SHP DHP

THP

EHP


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Total Hull Resistance

Total Hull Resistance (RT)

The force that the ship experiences opposite to the motion of

the ship as it moves.

EHP Calculation

=

P

ST

P

s H

ft lb

s

ft

V(lb)R)EHP(H

550 shipofspeedV

resistancehulltotal

S ==TR

( )

P

ST

Hatts

Wattss

J

s

ftlb

s

ftlbVR

550/1W1

:

=

===

Power


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Total Hull Resistance (cont)

Coefficient of Total Hull Resistance- Non-dimensional value of total resistance

5.02

SV

RC

s

TT

=

hullsubmergedtheonareasurfacewetted

shipofSpeed

densityFluidresistancehullTotal

watercalminresistancehulltotaloftCoefficien

====

=

S

V

R

C

S

T

T

dimension-nonlb

2

2

4

2

ftsft

ftslb


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Coefficient of Total Hull Resistance (contd)

-Total Resistance of full scale ship can be determined using

ST VSC ,, and

TST CSVlbR =2

5.0)(

speedshipscaleFull

formofCurvesfromobtained

tablepropertywaterfromavailable

testmodelthebydetermined

:

:

:

:

S

T

V

S

C


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Relation of Total Resistance Coefficient and Speed

0

5000

10000

15000

20000

TotalR

esistance,

0 2 4 6 8 10 12 14 16

Ship Speed, Vs (knots)

TOTAL RESISTANCE CURVEYARD PATROL CRAFT

speedhighat5ot

speedlowat2from

2

=

n

V

VCR

n

S

STT

speedhighat6to

speedlowat3from

2

=

n

V

VVCVREHP

n

S

SSTST


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Components of Total Resistance

Total Resistance

AWVT RRRR ++=ResistanceViscous:R

V

ResistanceMakingWave:RW ResistanceAir:RA

Viscous Resistance

- Resistance due to the viscous stresses that the fluid exerts

on the hull.

( due to friction of the water against the surface of the ship)

- Viscosity, ships velocity, wetted surface area of ship

generally affect the viscous resistance.


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Components of Total Resistance

Wave-Making Resistance

- Resistance caused by waves generated by the motion of the ship- Wave-making resistance is affected by beam to length ratio,

displacement, shape of hull, Froude number (ship length &

speed)

Air Resistance- Resistance caused by the flow of air over the ship with no

wind present

- Air resistance is affected by projected area, shape of the ship

above the water line, wind velocity and direction

- Typically 4 ~ 8 % of the total resistance


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Components of Total Hull Resistance

Total Resistance and Relative Magnitude of Components

Viscous

Air Resistance

Wave-making

Speed (kts)

Res is t a

nce( lb)

-Low speed : Viscous R- Higher speed : Wave-making R

- Hump (Hollow) : location isfunction of ship length and speed.

Hump

Hollow


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Why is a Golf Ball Dimpled?

Lets look at a Baseball (because thats what I have

numbers for) At the velocities of 50 to 130 mph dominant in baseball the air

passes over a smooth ball in a highly resistant flow.

Turbulent flow does not occur until nearly 200 mph for a smoothball

A rough ball (say one with raised stitches like a baseball) inducesturbulent flow

A baseball batted 400 feet would only travel 300 feet if it wassmooth.

A non-dimpled golf ball would really hamper Tiger Woods longgame


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Coefficient of Viscous Resistance

Viscous Flow around a ship

Real ship : Turbulent flow exists near the bow.

Model ship :Studs or sand strips are attached at the bow

to create the turbulent flow.


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Coefficient of Viscous Resistance (cont)

Coefficients of Viscous Resistance

- Non-dimensional quantity of viscous resistance- It consists of tangential and normal components.

FF KCC +=+= normaltangentialV CCC

Tangential Component :

- Tangential stress is parallel to ships hull and causesa net force opposing the motion ; Skin Friction

- It is assumed can be obtained from the experimental

data of flat plate.

FC

flow shipbow stern

FC

tange

ntial

norm

al


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Sn

n

F

FV

LVR

RC

CC

=

=

=

)2(log

075.0

2

10

ofComponentTangential

Semi-empirical

equation

watesaltfor

watfreshfor

/sft101.2791

/sft101.2260

/s)(ftViscosityKinematic

)Speed(ft/sShip

(ft)L

NumberReynolds

25-

25-

2

pp

=

=

=

=

==

S

n

V

L

R


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Tangential Component (contd)

- Relation between viscous flow and Reynolds number

Laminar flow :In laminar flow, the fluid flows in layers

in an orderly fashion. The layers do not mix transversely

but slide over one another.

Turbulent flow :In turbulent flow, the flow is chaotic and

mixed transversely.

Laminar Flow Turbulent Flow

Flow overflat plate

5

105 aboutRn


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Normal Component

- Normal component causes a pressure distribution along the

underwater hull form of ship

- A high pressure is formed in the forward direction opposing

the motion and a lower pressure is formed aft.-Normal component generates the eddy behind the hull.

- It is affected by hull shape.

Fuller shape ship has larger normal component than slender

ship.

Full shipSlender ship

large eddy


small eddy


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Normal Component (contd)

- It is calculated by the product ofSkin Friction with Form Factor.

23

)()(

)()()()(ft19K

K

=

==

=

ftLftB

ftTftBftL

C

CKC

F

Fv

FactorForm

Coeff.FrictionSkin

ofComponentNormal



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23

)(

)(

)()()(

)(ft19K

=

ftL

ftB

ftTftBftL

FF CKC+=+= normaltangentialV

CCC

2

10 )2(log

075.0

=

n

FR

C

Summary of Viscous Resistance Coefficient

watesaltfor

watfreshfor

/sft101.2791

/sft101.2260

/s)(ftViscosityKinematic

)Speed(ft/sShip

(ft)L

NumberReynolds

25-

25-

2

pp

=

=

=

=

==

=

S

n

Sn

V

L

R

LVR K= Form Factor


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Reducing the Viscous Resistance Coeff.

- Method :

Increase L while keeping the submerged volume constant

1) Form Factor K Normal component KCF Slender hull is favorable. ( Slender hull form will create

a smaller pressure difference between bow and stern.)

2) Reynolds No. Rn CF KCF

Summary of Viscous Resistance Coefficient


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Typical Wave Pattern

Bow divergent waveBow divergent wave

Transverse wave

L

Wave Length

Stern divergent wave


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Transverse wave System

It travels at approximately the same speed as the ship.

At slow speed, several crests exist along the ship length

because the wave lengths are smaller than the ship length.

As the ship speeds up, the length of the transverse wave

increases.

When the transverse wave length approaches the ship length,

the wave making resistance increases very rapidly.This is the main reason for the dramatic increase in

Total Resistance as speed increases.


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Wave-Making Resistance (cont)

Transverse wave System

Wave Length

Wave

Length

Slow

Speed

High

Speed

Vs < Hull Speed

Vs Hull Speed

Hull Speed: speed at which the transverse wave length equals

the ship length.

(Wavemaking resistance drastically increases above hull speed)


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Divergent Wave System

It consists ofBow and Stern Waves.

Interaction of the bow and stern waves create theHollow or

Humpon the resistance curve. Hump : When the bow and stern waves are in phase,

the crests are added up so that larger divergent wave systems

are generated. Hollow : When the bow and stern waves are out of phase,

the crests matches the trough so that smaller divergent wave

systems are generated.



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Calculation of Wave-Making Resistance Coeff.

Wave-making resistance is affected by

- beam to length ratio

- displacement

- hull shape- Froude number

The calculation of the coefficient isfar difficult and inaccurate

from any theoretical or empirical equation.

(Because mathematical modeling of the flow around shipis very complex since there exists fluid-air boundary,

wave-body interaction)

Therefore model test in the towing tank and Froude expansion

are needed to calculate the Cw of the real ship.



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Reducing Wave Making Resistance

1)Increasing ship length to reduce the transverse wave

- Hull speed will increase.

- Therefore increment of wave-making resistance of longer

ship will be small until the ship reaches to the hull speed.-EX :

FFG7 : ship length 408 ft Which ship requires more

hull speed 27 KTS horse power at 35 KTS?

CVN65 : ship length 1040 fthull speed 43 KTS



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Reducing Wave Making Resistance (contd)

2)Attaching Bulbous Bow to reduce the bow divergent wave

- Bulbous bow generates the second bow waves .

- Then the waves interact with the bow wave resulting inideally no waves, practically smaller bow divergent waves.

-EX :

DDG 51 : 7 % reduction in fuel consumption at cruise speed

3% reduction at max speed.

design &retrofit cost : less than $30 million

life cycle fuel cost saving for all the ship : $250 mil.

Tankers & Containers : adopting the Bulbous bow



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Bulbous Bow



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Coefficient of Total Resistance

AllowancenCorrelatio

1

:C

CCK)(C

CCCC

A

AWF

AWVT

+++=++=

Coefficient of total hull resistance

Correlation Allowance

It accounts for hull resistance due to surface roughness,

paint roughness, corrosion, and fouling of the hull surface.

It is only used when a full-scale ship prediction of EHP is madefrom model test results.

For model,

For ship, empirical formulas can be used.

.0 smoothissurfacemodelSince=AC


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Other Type of Resistances

Appendage Resistance

- Frictional resistance caused by the underwater appendages

such as rudder, propeller shaft, bilge keels and struts

- 224% of the total resistance in naval ship.Steering Resistance

-Resistance caused by the rudder motion.

- Small in warships but troublesome in sail boats

Added Resistance-Resistance due to sea waves which will cause the ship

motions (pitching, rolling, heaving, yawing).


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Other Resistances

Increased Resistance in Shallow Water-Resistance caused by shallow water effect

- Flow velocities under the hull increases in shallow water.

: Increment of frictional resistance due to the velocities: Pressure drop, suction, increment of wetted surface area

Increases frictional resistance- The waves created in shallow water take more energy from

the ship than they do in deep water for the same speed.

Increases wave making resistance


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Basic Theory Behind Ship Modeling

Modeling a ship

- It is not possible to measure the resistance of the full-scale ship- The ship needs to be scaled down to test in the tank but

the scaled ship (model) must behave in exactly same way

as the real ship.

-How do we scale the prototype ship ?- Geometric and Dynamic similarity must be achieved.

?

Dimension

Speed

Force

prototype Model

prototype shipmodel ship


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Basic Theory behind Ship Modeling

Geometric Similarity

- Geometric similarity exists between model andprototype if the ratios of all characteristic dimensions

in model and prototype are equal.

- The ratio of the ship length to the model length is typically

used to define the scale factor.

Volume:

Area:

:

FactorScale

3

33

2

22

)(ft

)(ft

)(ftS

)(ftS

(ft)L

(ft)L

M

S

M

S

M

S

=

=

=

=

Length

ModelM

shiscalefullS

:

p:

B i Th b hi d Shi M d li


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Dynamic Similarity

- Dynamic Similarity exists between model and prototypeif the ratios of all forces in model and prototype are the

same.

- Total Resistance : Frictional Resistance+ Wave Making+Others

S

MSM

M

S

S

MSM

M

M

S

S

M

MM

S

SS

nMnSnMnS

nWnV

L

LVV

L

L

v

vVV

gL

V

gL

V

v

VL

v

VL

FFRR

FfCRfC

==

==

====

,

,

)(),(

,

B i Th b hi d Shi M d li


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Dynamic Similarity (contd)

- Both Geometric and Dynamic similarity cannot be achieved

at same time in the model test because making bothRn and

Fn the same for the model and ship is not physically possible.

)(1

100

10)(10

kts

ft

ftkts

LLVV

S

MSM

=

=

=

)(100

)(assume10

100)(10

kts

vvft

ftkts

L

L

v

vVV

SM

M

S

S

MSM

=

==

=

Example

Ship Length=100ft, Ship Speed=10kts, Model Length=10ft

Model speedto satisfy both geometric and dynamic similitude?


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Dynamic Similarity (contd)

- Choice ?

Make Fn the same for the model.

Have Rn different

Incomplete dynamic similarity- However partial dynamic similarity can be achieved by

towing the model at the corresponding speed

- Due to the partial dynamic similarity, the following

relations in forces are established.

WSWM CC =

VSVM CC


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Corresponding Speeds

M

M

S

SnMnS

gLV

gLVFF == ,

-Example :

Ship length = 200 ft, Model length : 10 ft

Ship speed = 20 kts, Model speed towed ?

ktsktsV

LLV

L

LVV

S

MS

S

S

MSM

47.420

120

1

/

1

===

==

(ft)L

(ft/s)V

(ft)L

(ft/s)V

M

M

S

S =

1kt.=1.688 ft/s


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Modeling Summary

AWFAWVT CCKCCCCC +++=++= )1(AMWMMFMTM CCKCC +++= )1(

)5.0*(

550

)(

)1(

2sSSTSTS

STS

ASWSSFSTS

VSCRVR

hpEHP

CCKCC

=

=

+++=

AMFMTMWM CKCCC += )1(Froude

Expansion

Measured in tank

smooth)isModel(0

)givenorCalculated.factorscaletodue(

d)(calculate,

)//V,( S

==

===

AM

MS

FSFM

MMSnMnSWMWS

C

KK

CC

gLVgLFFCC

1)

2)

3)

7174460 Basics of Ship Resistance

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Transcript of 7174460 Basics of Ship Resistance