Prop for Electric Motor

Discussion in 'Props' started by Ron Mintle, Feb 13, 2021.

  1. Ron Mintle
    Joined: Feb 2016
    Posts: 2
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Fallbrook, California

    Ron Mintle New Member

    I have a 1975 O’Day 27 that has had the original Atomic 4 engine replaced with a SolidNav inboard electric motor. I have replaced the batteries with new ones, but am having handling problems with it.

    I know nothing about this motor such as HP, torque, RPM etc. but would like to learn more about it.

    At the lowest throttle setting, the speed of the boat is too high. I need to keep putting it in neutral and only putting it in gear for a quick moment when trying to dock.

    In open water, the boat makes about 6-7 knots at full throttle, and accelerates well.

    But what is really a problem is that when the boat is moving very slowly, like when leaving the dock, deflecting the tiller to turn, especially when in reverse, the boat does not change direction.

    I am having to use a small outboard motor for all maneuvers involving slow speed.

    And a friend said that he thought the prop was cavitating, but I can't confirm this is the case.

    I don’t know what the prop on it is, but I would like to know how to determine the best prop to use and it that is not what I have, then I would like to change it, especially if it will address these issues.

    Thanks for any help you may be able to give.

    Ron Mintle
    Fallbrook, CA
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    The maneuvering problem you are having is not a problem with the prop, it is a function of the motor and controller. It is important to remember, that the "throttle" (except for very expensive specialty control systems) does not control rpm, it controls motor torque and likewise power. The motor and prop will always speed up until the prop is able to absorb the motor torque. Unlike an ICE where the engine torque and prop torque track, limiting rpm; in electric motors, the torque is constant (or even greater at low rpm) throughout the rpm until some stall point is reached, meaning that the motor curve and prop curve cross at nearly right angles, giving a single operating point at the maximum rpm the motor can achieve.
  3. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The handling problems you describe are not unique to electric drives. You may not be able to steer in reverse when drifting forward, but at least the entire dock doesn't disappear in a cloud of blue smoke when docking downwind, and you might even be able to hear a person on the bow if they scream in your direction.

    When reversing, you should gain some helm control once you can overcome the prop walk. That could take a good long while though - like 30 seconds or more. And then the tiller will try to throw you overboard when it does start to work.

    Sadly, applying power in two second bursts every 15 seconds is a completely normal way to dock a large percentage of small auxiliary sailboats. Goose the throttle good when you're in it.

    One of the claims of electrics was that they were going to fix all of that nonsense. I don't know why you don't have infinite control over RPM, though. What is the operating principle of your controller and what type of motor is it? A PWM controller should be able to run right down to 1 rpm. Check the throttle mechanical stops. The throttle position rheostat or encoder may not be getting dialed all the way down.
    Last edited: Feb 13, 2021

  4. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Perhaps, perhaps not. What a PWM can do for a propulsion motor depends on motor type, number of poles, feedback circuitry/sensors, etc. Trying to control both ahead and astern rpm and torque to get a specific power at a specific rpm is a bit more complicated.
    Ron, without knowing more about the motor and controller there in not much more to say. Even large 400hp Secondary Propulsion Units on warships don't have full rpm control because of weight, space, and heat issues. Phil is correct that backing a vessel has it's own issues, and you will need to get some way on before the rudder will be effective.
    BertKu likes this.
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