Hull Forming Material

Discussion in 'Boat Design' started by Fanie, Oct 25, 2007.

  1. Fanie
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    Fanie Fanie


    There is a product called 'PVC Foam' available in sheets. If this is used for hull forming instead of the ply wood, would there be a camparason in strength, value... ?
  2. Trevlyns
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    Trevlyns Senior Citizen/Member

    Hi again Fanie!

    Let me try get the ball rolling on this one. PVC foam would generally be used over a male mould or "plug" of the yachts hull. On top of this you would lay up the glass and resin "skin" and eventually remove the plug and the PVC, leaving you with an empty fibreglass hull. Certain closed cell foams can be used instead of the PVC and are left in and the hull is then glassed inside as well, forming a foam sandwich. This is obviously stronger than a single skin.

    In plywood boats, the ply is formed over frames and stringers (or stitched together with the keel and bulkheads in the stitch and glue method). You then have a boat with ply skins and they are generally sheathed with a thinner layer of fibreglass - mainly for protection against damage as the ply is strong enough to do the job on its own.

    There are far more expert people on this site and I'm sure you'll get a fuller answer soon.

    Lekker bly! :p
  3. Fanie
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    Fanie Fanie

    Hi Trevm and thanks again. Someone just sent me a 2000 page PDF aparently everything A-Z and beyond on fibreglass stuff (sigh) so I'll spend the next couple of months to go through it.

    Your answer is correct of course, the PVC foam will delaminate from the glass if left in place whereas the wood will bond properly forming a much stronger surface, while the foam is great for isolation and floatation it is still not as strong as the ply.

    I'm wondering if there is a replacement for the ply that is not wood but with the same or better strength / quality / value. The possibility may exist since new materials etc are introduced frequently.
  4. Petros
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    Petros Senior Member

    I know some but not a lot about fiberglass (I try and avoid it now after building one fiberglass boat way back in high school), but I think there are several types of foam core materials, some compatible with fiberglass resins and some not. The type that is not is less costly and used to make a mold plug, you build up the hull shape on a wood frame and paint and coat it with a compound that protects it from the fiberglass. You then lay up a mold and pull it off the plug. Then you form the actual hull inside the mold you made.

    The type of foam that is intended for bonding in a sandwich lay-up is different (and more expensive), I do not remember which is which right now but I am sure it is in your document (I have fiberglass reference books I could dig out as well). The resin will bond to it and the fiberglass/foam/sandwich lay-up is very strong and lighter than wood (and easier to form). The only place you need higher density wood or plastic inserts in the layup is where the core is at risk of compression as in through bolting locations. The fiberglass/wood/fiberglass layup is still stronger but it weighs more, and the extra strength of the wood is not as much an advantages since the primary loading are taken by the fiberglass, not the core material. So with the foam you have to to detailing it differently for hard point mounting location and put more layers of glass on surfaces to walk on (or you damage the fiberglass and underlying foam).
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    How about 'bond through' holes in the foam to allow the fiberglass

    to bond from the outer to the inner shells to negate any tendency of the fiberglass to separate from the foam.

    I'm thinking of some 'Z' clips out of fiberglass(or something else that bonds well) with the two end legs bonding to the inner/outer hulls and the middle leg spanning the thickness of the foam.

    These would be inserted through a slit made in the foam to allow them to be placed by the builder.
  6. Fanie
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    Fanie Fanie

    I've read an article on the different materials which indicated that wood ply does make a stronger and thinner hull. You can use a closed cell foam, but to achieve the same strength you have to use a thicker foam. From experience you always get some water in the wood, so I would rather use the thicker foam and sacrefice the space. I would also rather use some extra glass on the hull than to have a soft hull that wil twist or bend. If you create structural beams in the foam you add a lot of strength and stiffness, but it's a lot of work to do.

    Another very nice feature when using the foam is it can easily be shaped and formed.

    The foam I intend to use is a closed cell rigit foam, and the glass fibre bonds to it directly. An article on camparason between materials suggested that you use a lot of resin to fill the pores on the surfaces which adds to cost and weight, I however don't think you have to fill the pores to get proper bonding. You can also use flox to fill the cavities if they're that large - flox weighs like really nothing. I'm sure the weight saved by not using ply, non rot proprties as well as the added boyancy from the foam should add to it's credit.
  7. tuks
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    tuks Junior Member

    Fanie, the guys you are looking for are AMT composites. They carry a full range of core materials including baltek and ariex products. I think they may keep nidacore or divinicell too.

    Its a really cool shop to visit, they have all the cool hitech stuff, carbon, kevlar, Prepegs, Aluminium honeycomb and a bunch of different epoxies.

    Be carefull, a cored hull is much stiffer than an uncored hull for a given weight, think carefully before you design the laminate yourself, because the sheer strength of the foam in very important. There are many nasty stories of boats delaminating due to core failure from slamming loads. If properly designed a laminate will be light and strong and should not delaminate.
    Last edited: Oct 27, 2007
  8. Fanie
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    Fanie Fanie

    Hi Tuks, thanks I know of AMT, just didn't know they carry such an extensive range of products. The allu honeycomb is a nice hint, I'll need it for my laser cutter.

    Don't worry, I'll research everything untill I'm sure I'm doing the best thing or at the least the right thing.

    I was wondering, if one has a boat design idea, who could you ask to have a look at it ?

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    FAST FRED Senior Member

    I was wondering, if one has a boat design idea, who could you ask to have a look at it ?

    If you can stick your idea in a scanner , post it here!

    There will be dozens of OPINIONS , from a variety of backgrounds..

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