Propulsion and Resistance

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Chap 7 Resistance and Powering of Ship

Objectives

• Prediction of Ship’s Power - Ship’s driving system and concept of power - Resistance of ship and its components · frictional resistance · wave-making resistance · others - Froude expansion - Effective horse power calculation•

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Introduction• When the hull form has been decided upon, it is necessary to determine the amount

of engine power that will enable the ship to meet its operational requirements.

• Knowing the power required to propel a ship enables the naval architect to select a propulsion plant, determine the amount of fuel storage required, and refine the ship’s center of gravity estimate

• naval architects have endeavored to increase the speed of ships.

• wind was the force used to propel ships through the water and ships could only go as fast as the wind would propel them.

• wind was the force used to propel ships through the water and ships could only go as fast as the wind would propel them.

• Testing of full-scale ships and models determined that the power required to propel a ship through the water was directly related to the amount of resistance a hull experiences when moving through the water.

• the modern screw propeller was developed, replacing the paddle wheel as the prime mode of ship propulsion. The screw propeller, with many modifications to its

original design, remains the principle method of ship propulsion to this day.

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Ship Drive Shaft and Power

Ship Drive Shaft 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

Ship Drive shaft and Power

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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/GBHP SHP Shaft

Bearing Prop.DHP THP EHP

Hull

Ship Drive Train and Power

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Effective Horse Power (EHP)

• EHP : The power required to move the ship hull at a given

speed 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 ship is converted into EHP of the full scale

ship by Froude’s Law.

VTowing Tank Towing carriage

Measured EHP

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0

200

400

600

800

1000

Effe

ctive

Ho

rse

po

we

r, E

HP

(H

P)

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 =η

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

1Hη1Hη

Well-designed

Poorly-designed

- Flow is not smooth.- THP is reduced.- High THP is neededto get designed speed.

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Screw


Efficiencies (cont’d)

• Propeller Efficiency

DHP

THPpropeller =η

• Propulsive Coefficients (PC)

SHP

EHPp =η

propeller designed for well 6.0≈pη

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 Hft lb

sft

V(lb) R)EHP(H

550 ship of speedV

resistance hull total

S ==TR

( )

P

ST

Hatts

Wattss

J

s

ftlb

s

ftlbVR

550/1W 1

:

=

==⋅=

⋅⇒⋅ Power

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Total Hull Resistance (cont)

• Coefficient of Total Hull Resistance

- Non-dimensional value of total resistance

5.0 2 SV

RC

s

TT ρ

=

hull submerged theon area surface wetted

ship of Speed

density Fluid

resistance hull Total

watercalm in resistance hull totaloft Coefficien

==

===

S

V

R

C

S

T

T

ρ

dimension-nonlb

2

2

4

2⇐

⋅⇒

ftsft

ftslb

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• Coefficient of Total Hull Resistance (cont’d)

-Total Resistance of full scale ship can be determined using

ST VSC , , andρ

TST CSVlbR ⋅= 25.0)( ρ

speed ship scale Full

form of Curves from obtained

table property water from available

test model the by determined

:

:

:

:

S

T

V

S

C

ρ

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• Relation of Total Resistance Coefficient and Speed

0

5000

10000

15000

20000

To

tal R

esis

tan

ce,

Rt (lb

)

0 2 4 6 8 10 12 14 16

Ship Speed, Vs (knots)

TOTAL RESISTANCE CURVEYARD PATROL CRAFT

speed highat 5 o t

speedlow at 2 from

2

=∝

⋅≈

n

V

VCRn

S

STT

speed highat 6 to

speedlow at 3 from

2

=∝

⋅⋅≈≈

n

V

VVCVREHPn

S

SSTST

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Components of Total Resistance

• Total Resistance

AWVT RRRR ++=Resistance Viscous: RV

Resistance Making Wave: RWResistance Air: 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, ship’s 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

ista

nce

(lb

)

- Low speed : Viscous R - Higher speed : Wave-making R- Hump (Hollow) : location is function of ship length and speed.

Hump

Hollow

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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 ship’s hull and causes a 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

tangential

norm

al

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Wave-Making Resistance

Typical Wave Pattern

Bow divergent waveBow divergent wave

Transverse wave

L

Wave Length

Stern divergent wave

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Wave-Making Resistance

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

WaveLength

SlowSpeed

HighSpeed

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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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 is far difficult and inaccurate from any theoretical or empirical equation. (Because mathematical modeling of the flow around ship is 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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Bulbous Bow


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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

- 2∼ 24% 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.

?

DimensionSpeedForce

prototype Model

prototype shipmodel ship

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Basic Theory behind Ship Modeling

• Geometric Similarity - Geometric similarity exists between model and prototype 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 :

:

Factor Scale

3

33

2

22

)(ft

)(ft

)(ftS

)(ftS

(ft)L

(ft)L

λ

M

S

M

S

M

S

∇∇=

=

=

=

λ

λ

λ Length ModelM

shi scale fullS

:

p:

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• Dynamic Similarity - Dynamic Similarity exists between model and prototype if 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

==

==

====

,

,

)( ),(

,

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• Corresponding Speeds

M

M

S

SnMnS

gL

V

gL

VFF == ,

- Example : Ship length = 200 ft, Model length : 10 ft Ship speed = 20 kts, Model speed towed ?

ktsktsV

LLV

L

LVV

S

MSS

S

MSM

47.4 20

120

1

/

1

===

==

λ

(ft)L

(ft/s)V

(ft)L

(ft/s)V

M

M

S

S =

1kt.=1.688 ft/s

Propulsion and Resistance

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