Ship tecnic Sharif university Lecture 6

8/4/2019 Ship tecnic Sharif university Lecture 6

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Chapter 6

Ship Resistance


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As a ship moves through the water, it experiences forces that work

against its forward movement. The sum of all these forces is the

- This is designated as RT- It is from this value that theEffective Horsepower, EHP, is calculated


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Resistance Values and Coefficients

Resistance values, denoted by R, are dimensionalvaluesRT = Total hull resistance is the sum of all resistance

RT = RAA + RW + RV

RAA = Resistance caused by calm air on the superstructure

RW = Resistance due to waves caused by the ship

- A function of beam to length ratio, displacement, hull shape &Froude number (ship length & speed)

RV = Viscous resistance (frictional resistance of water)

- A function of viscosity of water, speed, and wetted surface

area of ship


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Total Resistance and Relative Magnitude of Components

- At low speeds Rv dominates

- At higher speeds Rw is dominates

- Hump (Hollow)- location is function of ship length and speed

Viscous

Air Resistance

Wave-making

Speed (kts)

Res i s

t anc

e( lb)

Hum

p

Hollow

The amount of each resistance component will vary depending on speed:


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Speed-Power Trends EHP = (Resistance) x (Speed)


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Similar to the resistance components are the

- Resistance Coefficients, C, are dimensionless values of resistance- Allow the comparison of dissimilarly shaped vessels

- Used extensively in modeling


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Coefficients

CT = Coefficient oftotal hull resistance

CT = CV + CW

- CV = Coefficient ofviscous resistance over the wetted area of

the ship as it moves through the water

- CF = Tangential component (skin resistance)- KCF = Normal component (viscous pressure drag)

- CW = Coefficient ofwave-making resistance


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Coefficient of Viscous Resistance, CVLets look at the resistance due to the water, CV, first

- Consists of tangential and normal components

FF KC+C=+= normaltangentialV CCC

- Tangentialresistance, CF, is parallel to ships hull and causes a net forceSkin Friction opposing the motion by the water

-Normal resistance, KCF, is perpendicular to the ships hull. K is unique

to the hull form

flow shipbow sterntang

ential

normal


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Coefficient of Viscous Resistance, CV

Laminar Flow

Laminar flow - Fluid flows in layers that do not mix transversely but

slide over one another

Tangential Component, CF

Also called the hull frictional resistance, CF can be characterized by the fluid flowaround the hull:

Turbulent Flow

Turbulent flow -The flow is chaotic and mix transversely- Denoted by the Boundary Layer

- The boundary layer forms at the Transition point where flow changes from

laminar to turbulent


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Normal Component, KCF - 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- Is affected by hull shape

Fuller shape ship has larger normal component than slender

ship

Full ship

Slender ship

large eddy

small eddy



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- The viscous resistance component CV can be related to another

common dimensionless coefficient, the Reynolds Number

Rn = L V

Reynolds Number


Laminar Flow Turbulent Flow

Rn < 5 x 105 Rn > 1 x 106


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How to Reduce the Viscous Resistance Coefficient

- For tangential component, increasing the length decreases the

skin resistance

- For normal component, a more slender ship decreases the pressure

drag on the hull

Very long, narrow, slender hull is favorable ( A slender hull form will createa smaller pressure difference between bow and stern)

Increase L while keeping the submerged volume constant


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Froude Number, Fn

The Froude Number is another dimensionless value derived from model testing

Fn = V

\/gL

Also used, but not dimensionless, is the Speed-to-Length Ratio:

Speed-to-Length Ratio = V

\/L

...Velocity is typically expressed in Knots (1 knot = 1.688ft/s)


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Typical Wave Patterns are made up ofTRANSVERSE and

DIVERGENT waves

Transverse wave

Stern divergent wave Bow divergent waveBow divergent wave

Coefficient of Wave Resistance, CW

Wave

Length

L


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Wave-Making Resistance

Transverse Wave System

- Travel at approximately the same speed as the ship

- At slow speeds, several crests exist along the ship length because the wave

lengths are smaller than the ship length

- As the ship increases speed, the length of the transverse wave increases

- As the 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 Length

Wave

Length

SlowSpeed

High

Speed

Vs < Hull Speed

Vs Hull Speed

When the transverse wave length equals the ships length the vessel has

reached its HULL SPEED(Wave making resistance drastically increases

above hull speed)



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Divergent Wave System

- Divergent waves consist ofBow and SternWaves

- Interaction of the bow and stern waves create the Hollow or Hump on the

resistance curve


- Hump: The bow and stern waves are in phase, the crests are added up

creating a larger divergent wave system

- Hollow: The bow and stern waves are out of phase, the crests match

the troughs 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 is far too difficult and inaccurate from

any theoretical or empirical equation

- Model test in the towing tank and Froude expansion are needed

to calculate the Cw of the real ship



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It takes energy to produce waves, and as speed increases, the energy

required is a square function of velocity!

Lwave = 2V2

g

The limiting speed, or hull speed, can be found as:

V = 1.34\/Ls

Note: Remember at the hull speed, Lwave

and Lsare approximately equal!



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Reducing Wave Making Resistance

1) Increasing ship length to increase the wave length

- Hull speed will increase- The hull speed will be greater for the longer ship (the wave-making

resistance of longer ship will be small until the ship reaches to the hull speed)


2) Attaching Bulbous Bow to reduce the bow divergent wave- Bulbous bow generates the second bow waves

- The waves interact with the bow wave resulting in smaller bow divergent waves


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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 a problem

in

sail boats)

Added Resistance

- Resistance due to sea waves which will cause the ship motions (pitching,

rolling, heaving, yawing)


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Increased Resistance in Shallow Water

Resistance caused by shallow water effect

- Water flow is restricted under the vessel,so water velocity under the hull increases

- The faster moving water decreases pressure causing the ship to squat

- Increases wetted surface

- Increases surface friction

- Waves tend to be larger compared to waves in deep water at the same speed

- Traveling through a canal can produce the same effect

Other Type of Resistances


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( )212

, Re,Fr TR VL V

C f fSV

Lg

= = =

When a model and its prototype are geometrically similar and

their two dimensionless coefficients (Re, Fr) are the same, theirresistance coefficients (CT) should be the same.

Dimensional analysis reduces the number of the related

parameters involved in model tests. However, it can take the

problem no further than the above conclusion.


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Model Tests of Ship Resistance

Model tests are widely used in the design and study of large

engineering constructions, such as harbor, breakwater, bridge

constructions, and ship buildings.

A ship model is geometrically similar to its prototype. Thesize of the model is usually much smaller than that of the ship.

Ship model tests are employed to predict the resistance, the

interaction between the hull and the propeller, seakeepingproperties of a ship, etc. Therefore, model tests are very

important in ship design and ship research. Here we focus on

model resistance tests.


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A typical resistance curve in a model test

V

gL


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A Towing Carriage and A Ship Model


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Towing tank

Resistance tests in calm waterResistance tests in calm water


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30

Resistance Test in Towing Tank


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33

Resistance Test in Towing Tank


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35

Seakeeping test in Laboratory


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Propulsion


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Sub Cavitating Propeller Fully Cavitated Propeller

Surface Piercing Propeller

(S.P.P.) Waterjet

Air Propeller


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Engine Reduction

Gear Bearing Seals

Propulsor

Strut

Shaft

37


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HUBROOT

BLADE TIP

TIP CIRCLE

ROTATION

LEADING

EDGETRAILINGEDGE

PRESSURE

FACE

SUCTION

BACK

Screw Propeller

PROPELLER

DISC

37

57


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Hub

pitch

dia

meter

The distance that the blade travels in one revolution, P

- measured in feet

Propeller Pitch57

P ll


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Propeller


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Propeller Geometry


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Propeller Coefficients


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Typical Chart


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B-Series Charts

43


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43


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56


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56

Blade Tip Cavitation


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p

Sheet Cavitation

Flow velocities at the

tip are fastest so thatpressure drop occurs

at the tip first.

Large and stable region of

cavitation covering the

suction face of propeller.


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Consequencesof

Cavitation

1) Low propeller efficiency (Thrust reduction)

2) Propeller erosion (mechanical erosion as bubbles

collapse, up to 180 ton/in pressure)

3) Vibration due to uneven loading

4) Cavitation noise due to impulsion by the bubble

collapse

S P P


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S. P. P.

.

.


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S.P.P.


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S.P.P.

Waterjet


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Waterjet


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.


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Cavitation Tunnel


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Cavitation Tunnel

Applications: Assessment of Propeller and Duct Performance

Flow Visualization and Determination of Drag Characteristics forVarious Appendages

Cavitation Studies


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Propeller Test


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60


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Geometrical similarity indicates the main characteristics of a

model & its prototype are in the same ratio.

or , for a model and its prototype

having the same Fr & Re, then we requir e

1, & ,

if both are run in water at the similar density &

temperature, .

Since 1

s

m

s s s m s

m m m s m

s m

Lm

L

V L V Lm

V L V L m

m

=

= = =

=

;

?

( ) ( ) ( ) ( )

, it is ,

and Re Rem s m s

Fr Fr= =

almost impossible to satisfy both


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1. In order to overcome this fundamental difficulty to satisfy

the similarity laws, a major (first) assumption was made

by Froude that the frictional and the wave-making

resistances are independent, and the frictional-resistancecoeff. depends only on the Reynolds #. The wave-making

orresidual resistance coeff. depends only on the Froude # .

1 2212

1212

2212

Frictional Resistance:

Wave-making Resistance:

T F R

FF

RR

R VL VC C C f f SV gL

R VLC f

V S

R VC f

V S gL

= = + = + = =

= =


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2. It is also assumed that the frictional resistance coeff. of a ship

(or a model) is the same as that of a smooth flat plate with

the same length and wetted surface area as the ship (or themodel). Therefore, CF orRF of a ship (or a model) can be

computed given the length according to the half-analytically &

half-empirically friction formulas.

3. Based on these two assumptions, we may determine the

resistance of a ship at a constant velocity given the results of

model resistance test. The steps are detailed below.

212

a. At , the total resistance of a model, , can be measured.

Thus ,

where is the model's wetted surface area.

m Tm

TmTm

m m

m

V R

RC

S V

S

=

d


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ndb. According to the 2 assumption, , can be computed given

the length of model according to a friction coefficient formula.

c. Computing the model's resistance coefficient

FmC

residual

2

.

d. If , namely, , then

,

the ship's residual resistance coefficient is computed.

e. Same as in Step b, can be comput

Rm Tm Fm

s m s s

m ms m

Rm RS

FS

C C C

V V V Lm

V LgL gL

VC C f

gL

C

=

= = =

= =

( )

ed given the ship's length.

f. The total resistance coeff. of a ship is given by,

.

TS FS RS

FS Rm FS Tm Fm Tm Fm FS

C C C

C C C C C C C C

= +

= + = + =

g The total resistance of a naked ship (excluding appendages)


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212

g. The total resistance of a naked ship (excluding appendages)

can be obtained, , at . When

two geometrically similar ships are running at speeds which

conform to the F

S TS S s S mR C S V V mV= =

2

2

roude Law, , they are said to be running

at . It is noticed that, .

rs rm

s s

m m

F F

S Lm

S L

=

= =

corresponding speeds

In most cases, the total resistance of a ship can be determined

accurately based on the model test results using the above method.

However, the method is based on the 2 major assumptions (a. CF

& CR are independent, b. CFS of a ship is equal to that of a flat platewith the same length). Sometimes the errors due to the

approximations may be significant. We will study the frictional,

wave-making and eddy-making resistances in detail, for

understanding the computation using the method & its validity.


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5.5 Frictional Resistance

Laminar and Turbulent Flow (review of CVEN 311)

Laminar flow: the fluid appears to move by the sliding of

laminations of the infinitesimal thickness relative to adjacent

layers.

Turbulent flow: is characterized by fluctuations in velocityat all points of the flow field and these fluctuations with no

definite frequency.

Whether a flow is laminar or turbulent flow depends mainly

on its Reynolds #. For a plate flow,6

8

6 8

when Re < 10 the flow is laminar,

Re > 10 the flow is turbulent,

10 < Re < 10 the flow is transitional

Friction form las for a flat plate


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Friction formulas for a flat plate

The following formulas are commonly used.

( )

15

5 1.5

212

10

1) Blasius formula. (Laminar flow)

1.32 / Re, Re 4.5 10 . Re , , thus, .

2) Prandtl and von Karman formula (turbu lent flow)

log Re , 0.074( ) , thus,

FF F F

F F N FF

RVLC C R V

SV

A

C M C R RC

= < = =

= + =

( )

1.8

8

10

.

3) Schoenherr formula (1947 ATTC line , derived based on 2))

0.242log Re , for Re 4.5 10 .

4) 1957 ITTC line formula (known as ship-model correlation line

not a friction coef

F

F

V

CC

=

( )

7

2

ficient for a flat plate, turbulent flow)

0.075, for Re 10 .

lFC =

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Ship tecnic Sharif university Lecture 6