Newbie with maybe a stupid project?????

Discussion in 'Jet Drives' started by cosmic12, Oct 8, 2016.

  1. cosmic12
    Joined: Oct 2016
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    cosmic12 Junior Member

    Hi guys, new here but found a lot of very good info sofar.
    I have a few projects going and one of them I hope is going to be a jet drive. I have 2 hull choice's 1st being a early 60s Glass Par G3 that is for all purpose's shot and will be needing a complete resto. That's no big deal but I am thinking of putting a Sea Doo jet drive in it. If not familiar with the G3 its 13ft long and when new was 375lbs. I hope to use some new materials to lighten the hull a lot. Anyone done this sort of conversion ? I have the power plant already its a 96 787 twin they say is 85hp. The sea doo ran 50mph'ish I am wondering what it will do in a little larger hull that actually weighs less than the PWC.
    The other choice is a 1959 Crest Liner Jet Streak Alum hull that is less than 200lbs but I am kinda stuck on how it would seal and stay sealed. This hull is my 1st choice for style but I am sure to get a lot of flack from the purest.
    Oh and the reason for this madness if I am just tired of trying to keep old outboards running and really can't afford new ones and I live on a very calm river ( part of the Eire Canal) So Ideas and or thoughts?
     

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  2. big_dreamin
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    big_dreamin Junior Member

    I'm sure my response will be stupider because where I am i'm not even on the water. :^)

    I'd think a PWC jet drive might be designed for a different drag and speed profile, as in more drag, youre going slower, but the impeller may not be sized for that speed correctly. Efficiency may seriously suck, and alot of work done for not very good performance. A dedicated outboard jet drive for your size boat would likely be better.

    I'm aware the size and weight difference isn't massive vs a PWC its just what comes to mind - I could also be dead wrong but nobody else responded so far and I like crazy projects. I'd be surprised if even lightening the boat it's less than the PWC with the jet drive in there - i'd think the jet drive is the vast majority of the weight, those PWC hulls are as small as they typically can be to contain engine, jet, and gas...

    If the sole issue were keeping old outboards running I think i'd try modifying the outboard before trying to stuff a jetdrive in there though - even if it means an engine swap on a reliable lower unit to upgrade the reliability or make it more readily serviceable/replaceable. I'd be hacking around on an outboard before I hacked a boat apart to stuff a mismatched jet in there anyways.
     
  3. upchurchmr
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    upchurchmr Senior Member

    I like it.

    Either one.

    Don't know anything about Jetski efficiency.

    I'd much rather have a complete propulsion unit rather than some Frankenstein mix of outboard components.

    But I don't do power boats, so what do I know.

    But I have rebuilt lots of motors - everytime I got cute with changing something I had a waste of money.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    In both cases the jet can be installed, but you'll not see the performance envelope of a PWC, with it mounted in one of these old warped bottom hull forms. These older hulls are shaped very differently than the PWC hulls, so you can expect the old hull will jump up on plane very quickly, but will not be as maneuverable and top speed will be considerably less, because of the drag associated with the older warped bottom shapes.

    The jet isn't particularly efficient, especially at lower speeds. They need a lot of volume running through them to work well, so when they slow down, they tend to be sluggish at everything. I know of several conversions like this and all typically need about 20% - 30% more power in the jet, for comparable outboard performance. Simply put, if you want the old warped bottom to do about the same as it did with a 40 HP outboard on it, you'll need at least 50 HP jet, for a similar amount of performance.

    Though the Jet Streak is interesting, the G3 is the better choice, simply because you can more easily mate the drive tunnel, stringers, etc. to the hull. What most do is cut the whole jet assembly out, with the hull bottom included, so you can simply bond this assembly (appropriately aligned) to a cutout on the bottom of the old hull.

    The first thing you should do is calculate how much power you'll need for either of these hulls. You don't want to over power them, because all sorts of weird physics come to play and they can become quite dangerous. There are simply formulas that can get you the max outboard HP, for these puppies. Then select a jet with 30% more and you're good to go.
     
  5. ondarvr
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    ondarvr Senior Member

    The 30% reduction in HP is for an outboard pump, PWC pumps are more efficient, not that they're equal to a prop though. The 85hp rating should be at the pump, so it should have plenty of power, the hull could limit the performance, these old hulls weren't engineered, just sketched out on a napkin after too many beers at a cheap bar, with a little fine tuning after the fact.

    You can get different impellers for some pumps, so you may be able to fine tune it a little.
     
  6. cosmic12
    Joined: Oct 2016
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    cosmic12 Junior Member

    Thanks guys and keep the thoughts coming. On the warped hull, I am planning on blueprinting it and at least fairing out the last 3/4 ft of it so it will be as true as I can get it. Pretty sure it will be the G3 simply for the ease of install, glass to glass. On weight, well it took 4 guys to pick up the Sea Doo and only 2 for the G3 and it was full of rotten wood soaking wet. I am thinking pwr to weight will be better than the PWC.
    Now how it all works out with the CG difference I don't know. Time will tell. I have looked into the mercury sport jet set up and have not seen or heard much good about them. I hope a good SS impeller and a performance ride plate will help.
    I agree with the low speed handling and not looking forward to that part.
    Tell me more I am soaking up as much as I can get,
    Thank you.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    jet boats need a hull that is directionally stable, anything that has a pronounced forefoot, and little deadrise aft, is not really suitable, there is not a drive leg or rudder to act to limit gyrations that might occur if your bow drops into the back of a substantial boat wake. Hull shape is critical, there is no forefoot to speak of with the mass-market PWC's. Another factor is a jet drive has a higher line of thrust, a boat that does not like having an outboard trimmed in hard, will probably not be suitable for a jet.
     
  8. ondarvr
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    ondarvr Senior Member

  9. cosmic12
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    cosmic12 Junior Member

  10. ondarvr
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    ondarvr Senior Member

    What did you see when you clicked on the link?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    An index, not a particular subject.
     
  12. ondarvr
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    ondarvr Senior Member

    When I click on it it takes me to a thread on a guy building a 10' hull with a Yamaha PWC motor in it.
     
  13. cosmic12
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    cosmic12 Junior Member

    A fishing forum front page.:confused:
     
  14. ondarvr
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    ondarvr Senior Member


  15. cosmic12
    Joined: Oct 2016
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    cosmic12 Junior Member

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