# Open Water Efficiency

Discussion in 'Props' started by stalin, May 14, 2010.

1. Joined: May 2010
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Location: asia

### stalinJunior Member

I have optimised the propeller as follows:

Propeller Diameter = 1.1
P/D = 0.75
Pitch = 0.8250
AE/A0 = 1.0328
RPM = 689.43
open water eff = 0.42
cav.number = 0.2857

Vessel particulars:

LWL = 21 M
dISP = 117T

Engine is 1x400 hp @1800 rpm, with gear ratio 2.59. Max diameter of the propeller is 1.1m, according the stern shape. Can anybody confirm that the open water efficiency is reasonable for this size of vessel.

2. Joined: Jun 2005
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Location: Richmond, Virginia

### bmsyachtsnaval architect

Hi Stalin,

What is the design speed of the boat at which the prop is to be selected?

3. Joined: Nov 2009
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Location: Houston

### conceptiaNaval Architect

4. Joined: May 2010
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### stalinJunior Member

Sorry for late reply. No.of Blades are 4 & design speed of the vessel is 10 knots.

5. Joined: Nov 2009
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Location: Houston

### conceptiaNaval Architect

hey stalin, the effieciency value is ok. But you can improve it if you want to. I feel the main problem is the space constraint for accomodating propeller, right? In that case, you can ahead wid dis. But also listen what Daiquiri, Leo and others has to say about it.

6. Joined: Feb 2008
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Location: Trieste

### bitStudent

Pitch too high?
D = 1100 mm
P = 711 mm
Ae / Ao = 0.7
z = 4
w = 0.1344
Eta0 = 0.466
T = 31.11 kN
Pe = 195.7 hp
Pb = 415.3 hp
No. Cav. = 0.271
Cavit. = 4.91%
My opinion ...

7. Joined: Jun 2005
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### bmsyachtsnaval architect

Your selection is most likely correct, but in terms of efficiency not close to "optimized".

The potential open water efficiency could be as high as 76% but with a much lower prop RPM and much larger diameter (in fact, huge!) Twin screws ease the situation but still very slow RPM and large diameter props to enjoy full potential prop efficiency.

So it's a balancing act with compromises in efficiency (fuel consumption) fighting prop size and space constraints and first cost.

8. Joined: Feb 2008
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Location: Trieste

### bitStudent

Ok.
Is D = 1100mm : stop.
Better not be
Bye

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