making a mold, 8ft speedboat(55 kph) 20 hp, homemade

Discussion in 'Boatbuilding' started by hardcopy, Mar 19, 2014.

  1. hardcopy
    Joined: Mar 2014
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    hardcopy New Member

    myself and my brother have built a fun little speed boat. its a tunnel hull design and its more fun than I had imagined. what I would like to do is recreate pretty much the exact same boat but make it lighter and change the top shell design for better weight management. I guess what I'm looking for is advice on how to recreate this boat but make it lighter and have a diferent shell on top. I apologize for this being a repeat thread but I have looked trough the archives and haven't found anything that fully helps understand the process.

    the boat itself is a plywood/fiberglass boat fully framed inside and has a homemade jackplate holding a 20hp Honda (2012) outboard. have had it going about 55 kph and turns on a dime. it actually holds its own in fair chop which was quite suprising
     

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  2. nimblemotors
    Joined: Jun 2009
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    nimblemotors Senior Member

    My impression from these small boats is that smooth water no problem, but in a chop, sitting down will pound your *** to death, unlike a jetski where you can stand and let your legs work as shock absorbers. Do you find that to be an issue?

    Making a mold is well documented in many places, key to it mostly is having the shape so it doesn't have 'undercuts' (which is probably the wrong word).
    Really might be best to just shape a new one altogether.
     
  3. hardcopy
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    hardcopy New Member

    I'm still not very versed in all nautical terms but the porpusing is aided by some flaps with actuators on each tunnel of the hull and it only porpuses if you take a sharp turn at 45 kph or more, anything under and it handles whatever you put it through. as far as it making you uncomfortable, I've ridden this and others until out of fuel before ever considering any discomfort. in fact the worst complaint about the build is that you hate taking it back to refuel which is an easy remedy if I can find a way to reduce weight and redistribute it in a more effective way. I would like to take a mold of the bottom and modify the deck part if that is possible

    I have had the boat in about 1 foot chop and it was a little uncomfortable but at half foot or less I is quite a fun ride at high speed and only my mother reduced the speed to less than full throttle in that chop. I'd say most small boats experience the same downfall but I'm still quite entranced into this model if I can redistribute some of the weight effectively

    what are undercuts? not sure on the term
     
  4. rwatson
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    rwatson Senior Member

    The term "a fair chop" is usually a lot more than the 1 ft you are talking about, perhaps two or three times that amount.

    Considering you designed the boat yourself, you are fortunate to have an animal that doesnt have really dangerous habits at hi-speed.

    However, to take it to professional performance, you are going to need to get a professional design - porpoising, weight distribution, wave penetration etc are all professional problems, and not one you can cure in an informal forum discussion.

    The cheaper option would be to buy an existing design, where the mysteries of fibreglass moulding should be explained.

    If you have a fair bit of spare cash, consulting with an experienced designer would be the best option.
     
  5. hardcopy
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    hardcopy New Member

    thank you for the replies. it was our first boat and it was well worth the time. maybe biting a bit more off than I can chew to improve on the model but I will give it a shot. the worst that can happen is it performing worse than the original. after researching as much as I could find I think I am going to recreate a similar model but add in the slight changes and see what comes from it.
     
  6. hardcopy
    Joined: Mar 2014
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    hardcopy New Member

    I have modified plans for a new hull and deck that should address some of the weight redistribution. I have decided to try a resin infusion over the new mold. being that its only 8 feet long, one of the local shipbuilders who gave me loads of information the first time became interested and has done the infusion himself quite often. the only thing we are going to do differently is infuse the complete boat plug(foam) and remove the plug piece by piece afterward from the cock pit. as it wont have any frames or bulkheads so we are currently trying to get an ideal layup for it. the only wood will be in the transom and the tunnel bottom.

    this is more of an experiment but I remain quite optimistic and am glad to have some professional help. I'm quite exited and it should be a lot of fun.
     

  7. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Was it weight distribution or buoyancy issues that create the porpoising, and how did you know ?
     
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