Plywood/foam equivalency?

Discussion in 'Materials' started by SpiritWolf15x, Jan 11, 2018.

  1. SpiritWolf15x
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    SpiritWolf15x Senior Member

    Hello everyone, I'm wondering if anyone knows what type and thickness of foam I can substitute in place of plywood.

    Ie: plans call for 1/4" marine ply, what thickness of foam would work strength wise while keeping weight the same or less.

    Thanks,

    Rob.
     
  2. Rurudyne
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    Rurudyne Senior Member

    Are you asking about the difference between foam vs plywood cores for fiberglass construction? Or just, as it seems, using foam where regular plywood panels are called for?
     
  3. rwatson
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    rwatson Senior Member

    Good question Ru.

    Mind you, you would never use foam alone, its always in conjunction with glass, as foam doesn't have the tensile or sheer strength of plywood.

    For small craft ( canoes, kayaks ) you might be pleasantly surprised. I would bet that a foam core of 5mm, with glass on two sides, would probably go very close. Especially if the panels are compound curved. Of course, that would depend a lot on the craft. You might have to go 10mm for a power craft.

    To give you a feel for the strength and stiffness, a 3mm foam with a layer of 6oz cloth either side is damn hard to snap with two hands, and a super lightweight small kayak might work ok with that schedule.

    The big trick is to make sure that the fabric on both sides can handle the stresses of the particular craft.
     
  4. fallguy
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    fallguy Senior Member

    Plywood/Glass strength Comparison http://multihullblog.com/2010/11/plywoodglass-strength-comparison/

    On raw strength, per Hughes above link, you need 17oz biax each side with 12mm foam core. But it might be major overkill as the 6mm ply is probably spec’d for convenience as that is a common size.

    Needs more information. Vessel size and propulsion at least.

    And then there is a flaw in my answer or your question because you don’t provide the lamination and so the weight compare is impossible and the boat might not make her waterline. The foam might be lighter; okay? You should do an estimate to see and make sure your waterline doesn’t move an inch up. If it does; you’ll need to realize the impacts in performance and fitout and finish. 12mm foam is super light. If you saw a number; make sure and read the units; typical given in density; not per square meter, but cubic!

    If the designer spec’d foam and ply; disregard the ply spec.
     
  5. rwatson
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    rwatson Senior Member

    ...... or you could just add some ballast ?
     
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  6. TANSL
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    TANSL Senior Member

    I would not like to spoil the party, maybe I have understood something wrong, but I think there is no plywood-foam equivalence. If the boat has been initially calculated so that the plywood supports the efforts, the calculation of the panels by the procedure of a sandwich with a core of foam is totally different and the constructive solutions may not be valid for both systems.
    I only recommend a thorough study of the problem.
     
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  7. rwatson
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    rwatson Senior Member

    Yeah, of course.
    But since the OP made such an abstract description with no detail, and no functional specs, and nothing that would give anyone the slightest clue about what the F they are talking about, we all get a chance to argue and extrapolate the impossibly nebulous concepts to the nth degree until the OP is so confused they close their account and go and make cement submarines.
    That's what this forum is all about.
     
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  8. TANSL
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    TANSL Senior Member

    @rwatson, I hope you have not been angry with me. I know that you and everyone else, including myself, in this case, just want to help.
     
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  9. PAR
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    PAR Yacht Designer/Builder

    True, but there can be a plywood/foam equivalence to a degree, given some subtle variances. For example, two layers of 8 ounce biax around 1/2" foam, would be very similar to 1/4" plywood, in most regards, but without more from the OP as to what he wants, a moot point.
     
  10. TANSL
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    TANSL Senior Member

    What I think, simplifying the problem, is that when you build a hull with plywood the calculations are made so that the plywood can resist the loads that each panel must support. The layers of fiber are placed over it to obtain the watertightness of the panel and give it a good surface appearance. Although the plastic has some resistance, it is not considered as a resistant element. On the other hand, in a sandwich foam construction the foam does not support the stresses (only shear forces) and the plastic must be calculated in a way that resists the loads on the panel. That is why I say that there is no equivalence and that the constructive system must be different, not valid for both types of construction.
     
  11. PAR
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    PAR Yacht Designer/Builder

    I understand, but this is an equation that many have wanted. It's not easy, if you need strength, stiffness and lightness, but some useful comparisons can be made, whereas you can make up a foam cored panel, that will act reasonably well as a panel replacement on a plywood build. Of course, attachments need to be worked out and it is a cored structure, not a stressed skin, like most taped seam plywood boats are, but in small craft, a reasonable substitute can be achieved. This is because the plywood spec'd in smaller boats revolve around the typical sizes available, which is usually slightly larger, than what you need structurally once the math is done; therefore a foam cored substitute can be incorporated, with the same understanding you'll end up with a thicker core and a little less fabric, still resulting in similar panel properties. This assumes you're not designing to a finite degree, for weight to strength tolerances, but to simply make a cored sandwich structure, that can be competitive with plywood attributes, except in cost and weight, which is what I think the OP is looking for.
     
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  12. TANSL
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    TANSL Senior Member

    In a first instance, although I am not sure that it is totally adequate, if you know the mechanical properties of plywood, you can assume the maximum load that board, with that thickness, can support and replace them with a PRF thickness that supports the same load. A little math, even if it is little, must be done. There are very simple formulas that serve to determine the mechanical properties of the PRF.
     
  13. jorgepease
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    jorgepease Senior Member

    impact resistance should be in your considerations as well.
     
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  14. rwatson
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    rwatson Senior Member

    Thats OK Tansl. I know English isnt your first language.

    Translated, my words were agreeing with you, by saying that from so little information, so much is assumed, but in a joking way. No criticism to your points at all.
     

  15. SpiritWolf15x
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    SpiritWolf15x Senior Member

    Why haven't I gotten any new post notifications for this?

    The plans I have are for a Buccaneer 24 trimaran. They are drawn and designed using plywood but as I have had bad experiences with wood in the past and now see it solely as a means of heating my cabin, I am wondering how to replace every step that calls for wood with foam. Essentially I'm trying to build a foam/ fiberglass/carbon Buccaneer 24.
     
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