Mini electric surface drive

Discussion in 'Surface Drives' started by Timmcg, Jul 11, 2022.

  1. Timmcg
    Joined: Jul 2022
    Posts: 6
    Likes: 1, Points: 3
    Location: New Zealand

    Timmcg Junior Member

    Hi. I'm currently looking at building a mini electric surface drive, and I have some questions.
    I'll start with some background.
    About a year ago I put together an 8' ply hydroplane following the minimix plans. At first it was powered by a Mercury 9.9 2 stroke using a 9.25x10 prop. It ran really good, but eventually I got sick of premix and all the other things that come with little petrol outboards.
    I then converted the boat to electric. It is powered by a 7.5kw (15kw peak) brushless motor driving the same prop via a shaft drive. It also runs pretty good on this setup, but not quite as good as the outboard, likely due too a poorly matched prop.
    I bought the electric motor to try match to wot shaft speed of the outboard (3000 rpm), but failed to take into account the fact that the electric motors actual speed under load is quite a bit different to its theoretical speed.
    Anyway... I am now building another 8' hydroplane, this time a hasty hydro. This one will also be electric powered, with a slightly larger (10kw continuous) motor. I have not ordered the motor yet as I want to get the rest of the drivetrain sorted first.
    I have been thinking of building a surface drive because it would be easier to build than a conventional shaft drive in my situation and will free up lots of space inside the boat.
    Has anyone made a small surface drive before? What prop do I use? Im guessing it needs to be larger than a subsurface prop since only half of it is in the water.
    All the larger surface drives I see use cleaver type props with a lot pitch, but the smallest I can find is 13x23, which I'm guessing will be far to big for my motor.
    Do I need something custom made or is there something suitable out there?

    My goal is to go as fast as possible without exceeding the 10kw rating of my motor
     
  2. DogCavalry
    Joined: Sep 2019
    Posts: 2,414
    Likes: 1,217, Points: 113
    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, what is your expected speed, given the known power and weight? Based on your experience with your previous boat, what RPM do you expect to get from your motor... Roughly. Slip should be in the range of 5% going at hydroplane speeds. With that info in hand you can back out a value for prop pitch. Don't fuss too much about prop diameter- an electric motor won't stall with an oversized prop, which is the concern with a regular ICE. Read Hickman's original work on surface piercing props- my thread about it is close to the top. He's using something like a 24" prop on a 15hp engine. Worked fine.

    DC
     
  3. DogCavalry
    Joined: Sep 2019
    Posts: 2,414
    Likes: 1,217, Points: 113
    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    Just retead it. Pair of 22" props on a 17hp engine, turning 800 to 900 rpm.
     

  4. Timmcg
    Joined: Jul 2022
    Posts: 6
    Likes: 1, Points: 3
    Location: New Zealand

    Timmcg Junior Member

    I’m not really sure what my target speed is, current boat gets to 24mph on electric with 6kw of power. With the petrol outboard it would do 29mph.
    Maybe 35mph?
    Shaft speed doesn’t really matter as I can order the motor to suit, and motors are $300 where props are $500+
    I’ll have a read through that thread thanks
     
    DogCavalry likes this.
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