Unsure on aluminium doubler attached to fiberglass hull

Discussion in 'Fiberglass and Composite Boat Building' started by BornByAccident, Aug 29, 2020.

  1. BornByAccident
    Joined: Aug 2020
    Posts: 2
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    Location: Sydney

    BornByAccident New Member

    Hey people,

    I'm new to this forum, only reason I've decided to join is that I'm embarking on a water jet conversion on what seems to be some sort of hydroplane boat, its 10ft and fiberglass. (See image below)

    To stiffen the hull in the places to support the 150cc engine (motorbike engine mounted east-west behind ththe seat) and the jet unit I've acquired from a jetski. I've decided to add two 1" thick partial longerons in the places coloured in black in the photo below.
    To do this I'm going to add a 90° angle and a 45° angle on each longeron.
    Now heres the problem regarding getting the hull to support this as I need to put a hole both in the transom and in the bottom of the boat. I'm thinking about riveting a doubler to the deck (both sides so the rivets dont pull though and crack the fiberglass) and the transom if needed.
    I'm going to use 45thou aircraft grade aluminium on the inside and possibly 20though underneath then do a wet layup over the top so the water doesnt grab onto the edge while traveling.

    Am I correct to be doing this or should I be using a different material


    Thankyou, any advice regarding this would he appreciated 20200804_120127.jpg 20200829_193825.jpg
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What makes you think this set-up will work, the small motorbike engine coupled to the jetski drive ? Could be a total mis-match.
     
  3. BornByAccident
    Joined: Aug 2020
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    Location: Sydney

    BornByAccident New Member

    With a direct drive straight to the impeller and a few gadgets here to help water venting though the water jet on slowing down it should work relatively speaking.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    I doubt it will work, jetski engines are much more powerful, and how do you propose to keep this motorbike engine cool ?
     
  5. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    BBA, welcome to the forum.

    A water jet impeller is no different from a conventional propeller when it comes to matching it to the power output from the engine.
    The jet unit that you acquired from the jet ski will have probably had at least a 700 cc engine driving it - it might even be a 'newer' one that had an 1100 cc engine.
    If you connect your 150 cc engine to this jet, it will be a bit like connecting a 10 hp engine to a propeller that is designed for a 75 hp engine.
    In other words, it won't work.
    The impeller has to be matched to the power available.

    Even if you can find a suitable impeller for a 150 cc engine, I think you will be very disappointed in the performance of your conversion - and as Mr E says, you still have a lot of work to do to cool the engine (you would have to effectively 'marinize' it).
    I think that the best bet for that little hydroplane would be a conventional outboard motor.
     
  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The center of gravity will be shifted forward a lot. The boat will be nose heavy and won't run right. The most dangerous change will be that it will oversteer. Also a 150 cc motorcycle engine will be terribly underpowered for a jetski drive. The smallest motors they used were 400cc .
     
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  7. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    BBA seems to have tuned out of this thread, but it is seemingly a case of "it seemed like a good idea at the time", you can save yourself a lot of bother by thoroughly investigating the practicalities of a novel idea, before attempting to execute it, as well as the hurdles mentioned earlier in this thread, it could also be a case of a hull form not suited to a jet drive, which with just the one picture, can't be safely adjudged.
     
    bajansailor likes this.
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