Jet drive, tunnel, or tunnel outboard for low HP low draft

Discussion in 'Propulsion' started by Liopleurodon, Mar 16, 2017.

  1. Liopleurodon
    Joined: Jan 2017
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    Liopleurodon Junior Member

    Greetings! I'm a long time lurker, and have been paddling or driving a boat in some way since I was 3.

    I’m considering a small boat project with a custom inboard powertrain in the range of 5-10HP, let’s say 7HP for now. The requirements are that it seat 4 people, have remote steering, be capable of running against some current, and have minimum draft. (Not necessarily all at the same time.) What makes this different from an outboard jet is HP limitation and a desire to spend a lot of time at low speed. Instead of flying over shallows at significant throttle, this would gently rub over logs at speeds similar to a canoe.

    I’m aware of the issues with jet drives at low speed. The persistent question to me is whether the alternatives are any better. I’m not expecting high performance, but I picture a PWC drive (sans motor) as a modular and readily available tunnel drive. I picture a home-made prop tunnel with a small mud prop and wonder if it’s an improvement or not.

    A more unusual idea involves putting my custom powertrain into a large outboard module that has its own outboard tunnel and bears a slight resemblance to a longtail. This provides the thrust vectoring of an outboard and avoids cutting up the boat’s hull. It would seem a little bit like an inline jet outboard instead of the common 90deg version.

    With this all said, what could I realistically expect from a PWC jet at low HP, compared to a tall prop tunnel designed for low HP? Recognizing that the tunnel would not be factory. And should my custom drivetrain have a reverse, could I consider reversing the impeller instead of redirecting the thrust at the rear?
     
  2. Liopleurodon
    Joined: Jan 2017
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    Liopleurodon Junior Member

    Ok, let's try this tweak: Picture a surface drive, but build a tunnel around the prop and use a more traditional propeller. And maybe raise it so that the bottom of the prop is at hull level or barely below. Any expectations on how this arrangement would work well or badly?
     
  3. Irie
    Joined: Jul 2016
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    Irie Junior Member

    How minimum a draft are you looking for? I've no experience with jet drives, or boat propulsion in general, so I can't answer your questions. I did build a simple surface drive for my small boat. I think the shallowest its been is 6-8". Ive got a thread on it here

    No idea if its what your looking for but something to look at until someone more qualified chimes in
     
  4. Liopleurodon
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    Liopleurodon Junior Member

    Thank you, that info is very helpful. A good way to visualize my latest suggestion is to picture your surface drive,then extend theta plate all the way back to the transom, and extend the side fins downward and forward to form the tunnel. Your closeups of the shaft and coupling are helpful too.

    Post #44 on your thread is a reasonable visualization as well. Move the "existing tunnel" and "forward tunnel extension" rearward so that the forward tunnel extension starts at the transom, and remove the other parts, placing the rudder right behind the prop.
     
  5. Liopleurodon
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    Liopleurodon Junior Member

    To answer the question of how minimum a draft, I'm looking for a 14ft boat to run at displacement speed without any part of the propeller below the hull. If that's not doable, I would want a strong skeg or pair of skegs to ride up over any obstacle, like a log. This boat would meet many submerged logs. Picture coming up on a log, getting a little stuck, and instead of revving up the engine, or running at speed to clear it, you have the driver and passengers move around a little bit, like you might in a canoe.
     
  6. Irie
    Joined: Jul 2016
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    Location: Usa

    Irie Junior Member

  7. Liopleurodon
    Joined: Jan 2017
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    Location: Pennsylvania

    Liopleurodon Junior Member


  8. Liopleurodon
    Joined: Jan 2017
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    Location: Pennsylvania

    Liopleurodon Junior Member

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