Homebuilt Personal Watercraft

Discussion in 'Boat Design' started by preventec47, Jul 31, 2016.

  1. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    Fifty or more years ago before the advent of jet based personal watercraft
    I seem to recall all kinds of plans in magazines such as Popular Mechanics for
    tiny run-abouts and tunnel hull inboards and outboards that you would build
    from plans. I have not followed real well the evolution of PWC personal watercraft
    but the ones I have seen lately have a lot more hp than all the boats my family
    had growing up that we would water ski behind and zoomed all around the lake in.
    That and the fact that I remember as a ten or twelve year old I even
    skied behind my grandpa's fishing boat that had only 35 hp and it was just
    me an him on the lake. So I am therefore led to believe that the jet
    drives are MUCH less efficient and probably became popular because they
    did not want kids swimming around small boats with props exposed to
    sever limbs etc. Today our little industrial engines from Briggs and Honda
    have made great strides in that the largest V-Twins are up in the low to mid
    30 hp ranges now and I think a small little wooden boat with an inboard
    lawn mower engine could be designed ( redesigned ) to again zoom around
    a lake at least 35 or 40 mph or more. The other great benefit would be
    the fuel savings for a whole day at the lake of using 35 hp instead of 150hp
    as exists on a lot of the PWC.
    I have actually found a few pics on the old POP MEchanics mags etc. and will try to attach a few.

    Also I went to a few of the big boat plans websites and found a few 11 and 12 foot
    plans for homebuilt inboard boats. One boat was even smaller and would get up
    onto plane with one person while using only a ten HP outboard motor.
    18mph I think they said it could go. not bad for just ten HP.
     

    Attached Files:

  2. PAR
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    PAR Yacht Designer/Builder

    Making a garden tractor engine work in a boat is more trouble than you might realize, assuming you'd prefer it didn't blow up on you, underway. I've done this a number of times (used small gas engines), but some marinization needs to be preformed and frankly, these engines aren't the best suited for this type of work.

    It's usually just cheaper (much) to find a used outboard, fix whatever ills it might have and go have fun, than to convert a lawn mower engine. You see the engine is just part of the problem. You'll need a transmission and if it's a simple straight shaft, you'll need a rudder too. Rudder steered boats steer like crap, BTW. So, you have to ask yourself, what advantages will I get if I go the homemade drive route, compaired to an outboard or jet.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    The blowing up part came to mind readily. PWC are highly evolved these days, and those old mini-boats with outboards would have chopped up a few swimmers back in the day.
     
  4. fcfc
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    fcfc Senior Member

  5. PAR
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    PAR Yacht Designer/Builder

    The major issues are fuel vapor collection inside enclosed spaces, heat build up (big issue), how to handle the exhaust and of course eliminating spark production, within the enclosed engine space. Then you'll need to figure out how to shift FNR on a transmission, how to setup a throttle, remote wiring for the starter, charging circuit, maybe some gauges to monitor the engine. This doesn't address the steering concerns, just the basics for the drive system. Yeah, it's a bit more complicated than first glance might suggest.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    If that lot doesn't convince him it is a dud idea, nothing will !
     
  7. DCockey
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    DCockey Senior Member

  8. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Good points about the requirements to put an alternate engine on a boat.
    Makes a nice check list for those who want to try.

    But how about letting the guy try. Something simple and useful might come of it.

    Every useful idea started someplace.
     
  9. WestVanHan
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    WestVanHan Not a Senior Member

    Before you go off building a mini speed boat and spending money,I humbly suggest you take one out in water that is not smooth.
     
  10. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    What ! Not a single comment about the photos I attached. Are all
    you guys heavily invested in PWC companies or what? With the price of
    PWC up in the 20K and 30K thousands of dollars.... how can there not be
    more interest in smallish homebuilt boats ? I also liked those teeny tiny tunnel
    hull mini racing boats but perhaps they are using small outboard motors.

    Thanks thanks thanks for the Crab Skiff links.... Thats what I am talking
    about but I cant find any technical info about the boats other than you can
    buy the plans for the wooden boat.

    I did also see a reference to 20 hp Vanguard as the motor type on those
    Crab Skiffs so I would think an upgrade to 35 hp might be about as much as we
    could stand.
     
  11. Kailani
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    Kailani Senior Member

  12. Kailani
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    Kailani Senior Member

    Plenty available used.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I can't see much point in putting a 20 or 30 hp engine into a tiny little thing that has little practical use, you'd be bored with it after doing a couple of figure-eights. And as had been pointed out, will bash you around in any chop. Might as well put it on a 12 or 13 foot "proper" boat.
     
  14. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    Where? I am not seeing them.

    I perused through the Glen-L website and they have a wonderful selection
    except they really dont have any small boats for 20hp inboards.

    Full Disclosure here.... I'd love to work with an outboard motor as suggested
    but I have been given a 25hp Kohler VTwin and the little Crab Skiffs seemed
    to scoot along nicely on their 20hp inboards. And what about those little
    hydroplane racers ? Are they all using outboards?

    Maybe it is just difficult( or expensive) to piece together the hardware
    necessary to put a shaft drive through the hull with bearings and waterproofing etc.
    for the real inboard function. I have no idea where to start and I do
    not want to reinvent from scratch. There must be some smallish inboards being built and
    enjoyed somewhere that they have worked out all the pieces and parts necessary.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    "crab skiffs" ? Are these boats used professionally ? Seems to me that if they can't afford a "real" marine engine, the game is a crock !
     
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