Three point hydroplane questions

Discussion in 'Boat Design' started by Timmcg, Jul 30, 2022.

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

    Timmcg Junior Member

    I am building a small ply 3 point hydroplane. Roughly following the design of the 'Hasty Hydro' with plenty of modifications.
    By far the largest modification is that it will be powered by a 25kw (33hp) electric motor and be shaft driven. Most of my questions are around prop position and angle. For reference target shaft speed is around 3000 rpm and prop is a 10x19 cleaver that I already have on hand.

    From what I understand, a 3 point hydroplane rides on the 3 points that are the trailing edge of the 2 sponsons and the prop (and the cushion of air that is forced under the hull)
    Keeping this in mind, the location of the prop must be pretty important so I want to get it right before I start drilling holes.
    Most of the small hydros are designed around outboards, and that outboard is usually kicked down at around 12°. I'm guessing that 12° would be an ideal starting point for a shaft drive as well.
    Should my prop be below the bottom of the hull and forward of the transom or mounted behind the transom and higher up? How much of a cleaver prop do you want in the water when planing? And does the rear of the boat actually come out of the water when planing or just skim along?

    My last question, since I have a large prop and a small, light boat, I'm guessing prop walk will be quite significant. Would it make sense to offset the rudder in other direction so the extra drag from the rudder counteracts the prop walk? I'm not too worried about turning, the goal is going as fast as possible in a straight line.

    Thanks
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  2. kapnD
    Joined: Jan 2003
    Posts: 1,085
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    Location: hawaii, usa

    kapnD Senior Member

    Building a boat to suit a propeller is a huge mistake.
    Have you done any calculations on power consumption?
    A 33 hp electric motor is going to need some serious battery bank to get around the pond, and 3 point hydros are not notorious for their load carrying abilities.
     
  3. Timmcg
    Joined: Jul 2022
    Posts: 6
    Likes: 1, Points: 3
    Location: New Zealand

    Timmcg Junior Member

    I'm not building it to suit that prop, that was what I am planning on starting with. I have a few others lying around and I'm happy to buy more if required.
    Yes I am aware will be power hungry. I am currently in the process of building a custom lipo battery pack. It is 74V 55Ah and weighs 19kg. It will run for about 10 mins with motor at full power, and can be recharged in 15 mins assuming I can find a big enough power supply. More than enough runtime for my needs.
    Rest of the driveline is Motor (4kg), Speed control (1kg), Shaft, prop tube, rudder etc ~10kg
    This puts me at a weight less than a 20hp outboard with the advantage that the weight can be spread around and is not all hanging off the back.
     
  4. clmanges
    Joined: Jul 2008
    Posts: 541
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    Location: Ohio

    clmanges Senior Member

    I think you have problems. If that 25KW motor is running at battery voltage, it will be pulling ~340 amps. Is it going to be liquid-cooled? Is the battery going to be liquid-cooled?

    Also, where did you find a 25KW motor that only weighs about ten pounds?
     
  5. Timmcg
    Joined: Jul 2022
    Posts: 6
    Likes: 1, Points: 3
    Location: New Zealand

    Timmcg Junior Member

    The motor is liquid cooled. It is a motor for an industrial heavy lift drone, with some of my own modifications.
    350 amps is nothing for the battery I'm building, It is theoretically capable of ~6000A. It uses the same cells as the batteries I use for drone racing. Those batteries are sub 200g and very small. They regularly hit peaks of 150 amps and get discharged completely in a couple of minutes, they get warm but nothing crazy.
    I have a lot of experience with electric drivetrains, having built my own EV race car and many heavy lift drones.
    What I am not an expert in is hydroplanes, hence the reason for this thread.
     

  6. clmanges
    Joined: Jul 2008
    Posts: 541
    Likes: 115, Points: 43, Legacy Rep: 32
    Location: Ohio

    clmanges Senior Member

    Wow. Seems my knowledge base is behind the current state of the art.
     
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