Prop Efficiency Formulas

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

  1. hwsiii
    Joined: Nov 2008
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    Location: Pascagoula, MS

    hwsiii Junior Member

    Prop Efficiency Formulas Please Help

    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

    Last edited: Mar 2, 2010
  2. conceptia
    Joined: Nov 2009
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    conceptia Naval Architect

    get the wageninger B-series propeller charts.. this will help you...
  3. jehardiman
    Joined: Aug 2004
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    jehardiman Senior 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"?
  4. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior 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.

  5. bmsyachts
    Joined: Jun 2005
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    bmsyachts naval 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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