plywood and foam sandwich comparison

Discussion in 'Boatbuilding' started by tsanakou, Nov 10, 2011.

  1. tsanakou
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    tsanakou Junior Member

    Somewhere I have seen a comparison of plywood thickness and foam sandwich thickness. Don't remember where.
    Has somebody an idea?
    Tsan
     
  2. CatBuilder

    CatBuilder Previous Member

    I can give you one data point. 9mm of okoume is roughly equivalent to 3/4", 5lb foam with 34 oz skins.
     
  3. gonzo
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    gonzo Senior Member

    It depends on the type of plywood and foam. Gerr's book addresses sandwich construction in an easy way.
     
  4. tsanakou
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    tsanakou Junior Member

    CatBuilder: thanks for the data;5lb foam though seems to be very light.
    gonzo: Gerr's 'Boat Strength', chapter six is referring to cored constructions, but doesn't compare it with plywood.
    I am quite sure that I have seen a spreadsheet about that somewhere, but I dont recall where. Will keep searching.
    Thanks for your help! Tsan
     
  5. groper
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    groper Senior Member

    You do realize you are trying to compare apples with oranges dont you?

    Regardless, this might help you - http://www.atlcomposites.com.au/files/products.composite_panels.duflex/duflex-foam.pdf

    It shows a comparison of a 600gsm glass laminated 20mm thick, 80kg/m3 foam sandwich with plywood, aluminium, steel and solid glass... its a shame the comparison is not of equal thickness`s to give a better idea of the strength vs thickness in materials...

    So, the 13mm plywood is already twice the weight of the 20mm foam sandwich and its only 1/6 the strength... you could extrapolate this very roughly by following the rule of thumb - strength proportional to thickness squared, stiffness porpotional to thickness cubed and re work the numbers for 20mm plywood if 13mm ply = 5mpa then 20mm = 11.83mpa. compared with the foam sandwich of same thickness with 600gsm epoxy/glass skins 62-64% fibre content (so infused or prepreg) = 30mpa... you get - roughly 1/3 the strength in plywood of the same thickness and 3 times the weight.
     
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  6. CatBuilder

    CatBuilder Previous Member

    Doesn't matter if you think it "seems light" or not. Those two item (9mm of Okoume) and 5lb corecell with skins of 34oz on each side are equivalent in terms of load carrying ability.
     
  7. groper
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    groper Senior Member

    No way cat builder, i find that impossible to beleive... when you refer to the 5lb/cu.ft corecell thats A500 3/4" or 19mm thick right? that density is around 80-90kg/m3 yes? 34oz glass on each side = 950gsm, assuming its a stiched fabric and not CSM etc. - would give a MUCH stronger and MUCH stiffer panel than a tiny 9mm of okume plywood by an order of magnitude.
     
  8. rberrey
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    rberrey Senior Member

    Cold mold =CM
    Single fiberglass skin=SFGS
    SFGS ,single skin refers to hulls that are built up of fiberglass laminations only,no foam.
    Formulas, cold mold plywood to foam sandwich construction.
    A: Ratio 1 CM to .66 SFGS. B: SFGS x 2 = core thickness C: .7 of SGFS = Total FG laminate. Example using two 1/4" plywood skins, equaling 1/2"plus the glue and fiberglass hull cover of say 1/16", which gives .25 + .25 + .062 = .562" of CM.
    1. (A) Ratio 1 to .66, or .562 x .66 = .376 SFGS
    2. (B) .371 x 2 = .742, or 5/8" or 3/4" core thickness
    3. (C).7 x .371 = .259" total laminations thickness.
    4. .259 divided into two surfaces = .129" thickness of fiberglass laminate per side.
    The conversinn formulas here are not to supersede your designer specifications. Formulas are presented as a guide only. Formulas may be on the heavy side as they are oriented to the use of mat and woven roving in the layup. This is taken from Ed Horstman,s book Foam Fiberglass Sandwich Construction. I cant type fast so thats all you get . Buy the book it,s well worth the money and time spent reading it. Rick
     
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  9. CatBuilder

    CatBuilder Previous Member

     
  10. jim lee
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    jim lee Senior Member

    Hw can the corecell be "too light"? He must mean something else.

    -jim lee
     
  11. rberrey
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    rberrey Senior Member

    Using Ed,s forumulas I came up with 5/8" foam for 3/8" ply, so 3/4" foam for 9mm is about right, maybe a little oversized . We dont know the desighed stress loads or any other factors taken into consideration when the designer converted the scantlings from ply to foam. I,m sure the designer had a reason to call for 3/4" foam. In Ed,s book ,rule of thumb , approx pvc core- 41' to 50' boat length 3/4",FRP thickness- .290, 51'to 60' boat length pvc core 3/4", FRP thickness .350. Length of Cats boat may be one reason the designer calls for 3/4" foam. Bottem line most of us are not NAs or designers, we are lucky there are some and we all know they dont always agree. Designing or converting your scantlings on your own is risky and maybe costly. Rick
     
  12. groper
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    groper Senior Member

    Gentlemen, i think i can see the problem here...are we referring to using ply as a core material or plywood on its own? I think this clarifies the point....

    Which begs the question, Catbuilder, does your data point specify the same thickness of glass skins for both types of panel, foam and ply?
     
  13. gonzo
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    gonzo Senior Member

    In smaller boats, wood produces a stiffer panel unless you are using some exotic laminates. People that go from wooden boats to fiberglass often complain of the spongy feeling of decks and the give of the hull.
     
  14. Sand crab
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    Sand crab Junior Member


  15. CatBuilder

    CatBuilder Previous Member

    It specifies a lighter skin on the ply, mostly for protection from impact damage: DB 1708 or something like that, along with the 9mm of Okoume.

    These are the hull panels used on the boat I'm currently building... it comes in two flavors - ply/epoxy/light glass or foam/glass.
     
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