skin thickness for small boat

Discussion in 'Boat Design' started by stephen smalzel, May 18, 2004.

  1. stephen smalzel
    Joined: May 2004
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    Location: colorado usa

    stephen smalzel New Member

    I am designing an ocean going row boat that most closely resembles a hawaiian outrigger canoe or ocean kayak, with overall dimmensions of 22 foot length and 25 inch beam (6.7 x .61 meters). I am cosidering using 3mm okume covered with fiberglass or a thin foam core for the skin, either material with four structural bulkheads. I have done lots of reading but have found no information on the actual weight of fiberglass/kevlar/carbon for this application. Has anyone built a small boat ( kayak or canoe especially ) stitch and glue or foam core and have any practical suggestions for me before I start doing experiments with the materials. More specifically: Is 6oz. fiberglass over 1/8" okume strong enough to trust for prolonged ocean travel? What weight and type of composite would you use on 1/8" or 1/4" airex, and would you use airex in this application? (I have read some disparraging comments about airex, but would prefer to use it for its ability to bend. ) Incorporating divinycell I would work with 1/4" sheets scarfed and cut into skinny planks, layed longitudinally over a male mold, sanded smooth and finally covered with composite. Again what composite and weight would you suggest for the skins? Using foam core I would vacuum bag.
    I was in hawaii this winter and tried to cull what information I could from canoe builders there, but they seemed reluctant to give away their trade secrets and what I did gather was that they were using much lighter weight carbon and kevlar cloths over thin foams than I would trust for long travel. Any suggestions would be greatly appreciated! Cheers.
  2. tonyr
    Joined: Oct 2003
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    tonyr Junior Member

    Is this going to have an outrigger? If not, then I would have thought that you will be really challenged by such a small beam. How good is the stability of a boat of these dimensions, especially in ocean wave conditions?

  3. Guest

    Guest Guest

    If you are looking for a strong, lightweight hull, I understand that Twaron is a pretty good fiber to use. It is supposed to be significantly better than regular E and S-glass, but not quite up to carbon fibers tensile strength, however, it is supposed to be more ductile and I would assume cheaper and eaiser to use than carbon fiber. Some of the new Hylas yachts are using it in there hull construction.

    I would think the combonation of Twaron, an Airex core, and epoxy resin would produce a superior hull.

    What about longitudinal stringers?
  4. stephen smalzel
    Joined: May 2004
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    Location: colorado usa

    stephen smalzel New Member

    Tonyr & guest-
    Thanks for your replies. The boat is definately going to have one or two out riggers, and that is somewhat the purpose of the whole affair- to see if a narrow, minimal wetted surface area hull using outriggers for balance can be rowed practically in the open ocean. I foresee two potential problems. Firstly, that the whole thing will tend to trip over the out rigger(s) in a beam sea. This was experienced by Hans Lindeman crossing the Atlantic by canoe. I'd be willing to take a few cold dips if the boat functioned well otherwise. Secondly, I imagine that wave action on the outriggers would make the whole rig rocky and that might make sculling impractical (as apposed to paddling). I am hoping that by keeping the ama seperation easily adfjustible one could keep rocking to a minimum given the ambient wave conditions.
    In regards to twaron, that is a brand name for aramid fibers, like kevlar, which I imagine when sandwiching airex would be nearly ideal. I have never worked with aramid fibers, though, and understand that they are much more difficult to work with and repair than carbon or glass. I have very limited experience with composites all around which is why I am seeking all of your sage advice.
    I am leaning towards a fiberglass covered okoume hull and divinycell ceck. Certainly longitudinal stringers woul help stiffen the hull, but I imagine that the time and weight would be better placed in extra skin layers. That's the theory with monocoque design as I see it. With okoume rather than foam core construction I would probably add a couple of stringers to be safe.
    Thanks- stephen
  5. Karsten
    Joined: Jun 2004
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    Location: Sydney

    Karsten Senior Member

    The approximate density of glassfibre is 2.5 g/cm^3, carbon 1.8 g/cm^3 and epoxy 1.2 g/cm^3. For a rough weight calculation you can assume that your laminate is made from 50% fibres and 50% resin by volume.
    I would go for the carbon option. It's much stronger stiffer and safes some weight. You don't need that much so the extra costs wouldn't be too much. The problem with carbon is that you can't see air boubles under the cloth because it's black and not seethrough like glass. Therefore I would recommend vacuum bagging.
    It is usually better to increase the thickness of the foam core instead of adding an extra fibreglass layer. The problem will be that the fibre layer ends up to be very thin and therefore easy to punkture. Kevlar is supposed to be good for impact strength because it stretches much more than carbon before it breaks. You have to put some fibres on the inside as well otherwise you don't have a propper sandwith.

  6. Bill the Cat
    Joined: Oct 2002
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    Bill the Cat Junior Member

  7. Not A Guest
    Joined: Jan 2004
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    Location: Great Lakes

    Not A Guest Junior Member

    It seems that we and you are confusing 2 goals:

    1) proper scantliings and

    2) a proper design

    You appear to want to test a design. If so there is little or no reason to worry about the durability of scantlings. I think you need a support boat along side during testing.

    While Dave Gerr's book is a good reference, your boat is below the size that his scantlings appear to be valid for.
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