# Prop Efficiency Formulas

Discussion in 'Props' started by hwsiii, Feb 28, 2010.

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### hwsiiiJunior Member

I am trying to find if there is more than one formula for finding the efficiency of a prop, NOT the efficiency of that prop from the manufacturer as designed, but the efficiency of the prop on one particular planing boat with one particular motor.
Maybe a better way to state this question is, the propeller efficiency for any particular installation, without having to go through a super detailed analysis of fluid dynamics.

Let's say these are the parameters for this installation:

Motor RPM: 5,750
Gear Ratio: 2:33 to 1
Propeller RPM: 2,468 for both props
HP at prop at WOT: 74
Prop: SS 3 Blade 36.9 MPH 13" Diameter 17" Pitch AND a
Prop: SS 4 Blade 35.1 MPH 12 3/4" Diameter 16" Pitch
Boat Weight: 2,000 pounds
Boat Length: 18'

With a Speed Length Ratio of 7.8
The 3 blade has a crouch number of: 192
The 4 blade has a crouch number of: 182

H

Last edited: Mar 2, 2010
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### conceptiaNaval Architect

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### jehardimanSenior Member

FWIW, there are some methods, but they are not very percise.

17" pitch x 2468 rpm = Advance of 58.3 ft/sec
16" pitch x 2468 rpm = Advance of 54.8 ft sec

58.3 ft/sec - 36.9 mph = Slip of 4.2 ft/sec
54.8 ft/sec - 35.1 mph = Slip of 3.3 ft/sec

Thrust EHP= .5*1.990*((13/12)^2/4)*pi*4.2*54.12^2 = 20.5 hp
Thrust EHP= .5*1.990*((12.75/12)^2/4)*pi*3.3*51.48^2 = 14.2 hp

20.5 hp / 74 hp = 28%
14.2 hp / 74 hp = 19%

Both wheels are over bladed and under pitched for top speed with the 13" wheel doing better, nearer to typical for small speed boats. But how do you feel about each's hole shot "power"?

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### jehardimanSenior Member

I have notice an error in my formulation here, there should not be a 0.5 in the Thrust EHP calculation (Bad engineer, no cookie...).

In that case EHP should be 41 and 28.4 with efficiencies of 56% and 38% respectively. This means that the 13" wheel is not doing too bad, about what I'd expect for a good small boat match, but the 12.75" wheel is still over bladed and under pitched.

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### bmsyachtsnaval architect

Prop Efficiency

For the 3 blade case with speed of 36.9 mph or 32.1 knots, prop RPM 2468, diameter 13", the best prop pitch ratio is 1.5, thus pitch 19" and open water efficiency in the range of 73.5%. The slip is ablut 19% This according to a couple of sources.

The efficiency has the potential of increasing to 76% with a 13.6" prop with pitch of 19". The slip is about 16%.

You have to deduct the losses of all of the components between the prop and the engine shaft to determine the shp available at the prop. The above numbers were based upon the 74 hp being available at the prop.

Hope this gives you some numbers to ponder.

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