Prop Power Consumption

Discussion in 'Props' started by Divan, Sep 6, 2019.

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  1. Divan
    Joined: Sep 2019
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    Location: South Africa

    Divan Junior Member

    Good day,

    I am trying to work out how much power a 9" prop with 9" pitch at 1200 rmp will consume.

    I am unable to find any formulas or calculators to work this out any help will be appreciated.
     
  2. jehardiman
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    jehardiman Senior Member

    How many blades and what is the blade area ratio?
     
  3. Divan
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    Divan Junior Member

    It is a 3 blade prop and the BAR 0.53
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    It will vary with boat speed as well.
     
  5. Divan
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    Divan Junior Member

    It is a very large and heavy boat so top speed at 1200 rmp will probably be around 5 to 6 km/h
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Perhaps some more information would help. Where are you getting this figure of 1200 rpm from ? In any case, you are talking about an engine of less than 10hp
     
    Last edited: Sep 6, 2019
  7. jehardiman
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    jehardiman Senior Member

    Just for a benchmark, the Honda 7.5 4-cycle OB on my Catalina 22 has a 9x9 prop and drives the boat at about 5 knots at half throttle (? rpm ... WO is ~2000 rpm i.e. ~5k shaft/2.5 final). I could work the B3-35 Bp-delta chart, but as Mr Efficiency says it's not much hp.
     
  8. Divan
    Joined: Sep 2019
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    Divan Junior Member

    The 1200 rpm is the maximum rated speed of my electric motor, I am trying to figure out if I will have enough power onboard to run the motor at full speed with out installing it on the boat since it will take a lot of effort and time replacing it with my current motor.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the boat ? Length ?
     
  10. Divan
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    Divan Junior Member

    It is an catamaran and 6 meters in length
     
  11. gonzo
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    gonzo Senior Member

    The maximum rated power of the motor is the amount of deliverable power you need onboard. It is pretty straightforward.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Is there the ability to fit a larger diameter propeller ? Sounds a good idea if possible
     
  13. Divan
    Joined: Sep 2019
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    Divan Junior Member

    It is possible but would prefer to keep the prop as small as possible due to areas with shallow water. I am aware in my situation that an bigger prop will be a lot more efficient and I might be able to go with a bit bigger prop but for now it will have to do.
     
  14. Divan
    Joined: Sep 2019
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    Divan Junior Member

    True but I am unsure if the motor will be strong enough thus I want to calculate the amount of power needed to turn the prop at the rated speed of the motor and if it doesn't have enough power then to be able to see what size motor I need. It should be an fairly strait forward formula. Lest assume you have a prop that is 9" in diameter with 3 blades and a pitch of 9" and an BAR of 0.54 how much energy is required to rotate the prop at 1200rpm in a body of stationary water? This is what I want to calculate.
     

  15. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    A 9" 1.0 P/D B series 3-50 requires 11.328 ft-lbs of torque at a J=0 and n=20 (20 rps=1200 rpm). Since hp = Torque * rpm/5252, you would need a motor of at least 2.59 rated shp @ 1200 rpm.
     
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