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PROPULSI

( 3 SKS)Jurusan Teknik Sistem Perkapalan

ITS Surabaya

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G eometry of the Screw Propeller

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Propeller Area

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1. Disc area: The area of the circle swept out by the tips of the blades of apropeller diameter

2. Projected outline: It is the view of the propeller blade that is actually seenwhen the propeller is viewed along the shaft centreline normal to y-z plane(when viewed forward).

3. Developed outline: It is a helically based view, but the pitch of each sectionhas been reduced to zero. The intersection of the blade with the axial cylinder isrotated along the blade reference line into a plane parallel to the propeller. Theamount of rotation is equal to the pitch angle at every radius.

4. Expanded blade outline: It is really not an outline in any true geometric senseat all. It is a plotting of the chord lengths at their correct radial stations about thedirectrix. Such that the outline is constructed by laying off at each 10radius r, the chord length along a straight line. The outline is formed by thelocus of the end points of the chord lines laid out in the above manner.In general, the developed area is greater than the projected area and slightly lessthan the expanded area.

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Skew

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B-Series

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R ange of applicability of B series propellers

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Cavitation

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6 .7 Cavitation

A typical pressure distribution in a blade element is shownbelow,

Pressure (+)

Suction (-)B ack V R

face

As the pressure on the back of a propeller falls lower andlower with the increase in a propellers n , the absolutepressure at the back of the propeller will eventuallybecome low enough for the water to vaporize and localcavities form. This phenomenon is known as cavitation.( , vapor pressure of water)v P

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Cavitation on a propeller will1. lower the thrust of the propeller, & thus decrease its

efficiency,2. cause vibration of hull & the propeller and generate

uncomfortable noise , &3. cause erosion of the propeller blade.

Criteria for prevention of cavitation

M ean thrust loading coefficient21

2c

R p

T V A

X V

!

22 2

- density of water, - Thrust,

- project blade area, 1.067 0.229 ,

- the relative velocity at 0.7 of a propeller

2 0.7

p p

D

P D

n

V

T

!

!

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F or a particular streamline such as A B ,

B ernoulli`s theorem gives the relation

C V P !200 21 V

At the point P

200

211 2

12

1 V P V P V V !

? A2120012

1 V V P P P !! VH At the point S : V 1 = 0

2001 2

1 V P P P VH !!Stagnation Pressure

202

1 V q V!

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? A P P P

V V P P P

H

VH

!

!!

01

21

2001 2

1

IF P 1 < Pv C avitation

P P P v H u 0

v P P P u 0H

or

q

P P

q

P v

u0H

W !q

P P v0 Cavitation Number

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Cavitation number

The cavitation is most likely to occur at the tips of blades

where the relative velocity is the largest and the hydro-static pressure is the lowest when blades rotate to thehighest position. It can also occur near the roots whereblades join the boss of a propeller because the attackangle is the largest.

2227.0 836.462.192.188

DnV h

A

R !W

2

7.0

7.0

21

R

vatm

RV

gh P P

V VW !

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Cavitation diagram (SNAME)