Looking for more thrust.

Discussion in 'Props' started by duluthboats, Aug 12, 2022.

  1. duluthboats
    Joined: Mar 2002
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    Location: Minneapolis,MN, USA

    duluthboats Senior Dreamer

    I have 9.9 Elco outboard. It came with a 7.5 dia x 7 pitch prop. This porp is not the best for a displacement hull. The diameter is restricted to 7.75. I can get a 7.25 dia x 5 pitch,that will fit, but not sure if the reduction in dia will keep me where I am at now.
    Would like any help finding a better prop. The shaft is .438 dia with 9 splines.
    Before you ask, Elco has been ZERO help, this is very disappointing.
     
  2. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    I'm assuming electric outboards are propped like gas, are you turning max rated rpm with the current pitch?
     
  3. duluthboats
    Joined: Mar 2002
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    Location: Minneapolis,MN, USA

    duluthboats Senior Dreamer

    The boat is at hull speed at 2/3 throttle. If I go to max rpm I'm just wasting energy. There is very slow response from a dead stop. I would expect some lag but not as much as I have now. I am guessing a prop with more thrust (smaller pitch) would give me more control at slow speeds.
     
  4. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Just a guess but I'm guessing your hitting max thrust low in the rpm range. In small gas kicker motors (pushing big heavy boats slowly) they stall or prop slip causes aeration if they are over pitched for speed. Guessing the electric motor can put the torque and stalling isn't an issue.

    Lower pitch prop will definitely help with the upper where it turns into a bubble maker, it will also put less load at the lower end.

    The heavy boat/hull speed outboard has been a quandary that's puzzled most sport fisherman for the last 40 years. Every gas manufacturer has a version with a gear leg with a larger ratio allowing for 10 inch props instead of the normal 7s on small outboards.

    Correct pitch to get a more linear bite will help outboard longevity, but it may only incrementally change performance.
     
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  5. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    In order to get more thrust out of a wheel of given diameter you must increase the velocity of flow through the wheel at the give speed of advance; i.e. the apparent advance of the wheel (rps*Pitch) must be greater than the advance (boat speed) of the wheel. This difference is the slip and the higher the slip, the greater the thrust. You can do this by upping the pitch and holding the rpm or lowering the pitch and increasing the rpm.
    Unlike an ICE or a brushed DC motor, the 9.9 Elco is a brushless PMAC; a permanent magnet synchronous AC motor. This means that the "throttle" is actually controlling speed (i.e. rpm), not torque (to a given point). Since you apparently have enough torque to cavitate at full throttle (i.e. maximum frequency of the AC field generator); indication is that you need to up pitch and/or add area to the blades to increase thrust.
     
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