Foam core thickness and skins

Discussion in 'Materials' started by JacquesSparaux, Jul 27, 2022.

  1. JacquesSparaux
    Joined: May 2022
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    JacquesSparaux New Member

    Trying to find resource about usual thickness of foam cores and their inner/outer skins for DIY hulls

    The boat will be a ocean crossing 42 to 50 feet catamaran

    The production mode would be resin infusion in a vacuum bag

    I will of course rely on a engineer for the final calculations but I'd like a rough estimates of the materials used modern hulls to budget my hulls

    I thought about this:
    • Inner skin: 600g 0/90 Carbon fibre cloth + 600g 0/90 Carbon fibre cloth
    • Core: closed cell foam core 80kg/cubic meter; 15 mm thickness
    • Outer skin: 600g 0/90 Carbon fibre cloth + 600g 0/90 Carbon fibre cloth
    Total thickness: 1.2mm + 15mm + 1.2mm = 17.4 mm

    Does that sound realistic? Any advice or resource is welcome
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    No, it is not realistic at all. You can't say anything sensible without having more data and doing a few, very few numbers. But my impression is that 1.2mm skins are too thin and a 15mm core can be totally disproportionate in many places. Just an impression, I repeat.
    The fact that the total thickness is 17.4 mm does not mean absolutely nothing, neither good nor bad, it is simply a figure as good or as bad as 12.3456 mm could be.
    You could have some first calculations, sufficiently detailed, in a very short time. Talk to your engineer.
    bajansailor likes this.
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    That lay-up that you propose 'might' be fine in theory, in ideal conditions - but does it still look ok to you when you are in the middle of the ocean, sailing at 10 knots in hairy conditions, with a good possibility of the hulls hitting driftwood, or a semi-submerged container, or a whale - or even a reef....
    The 'lay-up' that you propose is effectively approx similar to the average beer cooler, if you substitute plastic for the carbon fibre (or vice versa) - would you really like to sail a 50' cat with an exotic carbon reinforced beer cooler lay-up?
    It is always useful to look at the whole picture from a 'seat of the pants' and experience perspective as well.
    Ad Hoc, TANSL and jehardiman like this.
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    If you are doing rough estimates, use a thicker core; estimate epoxy at 125% of fabric weights, and try to come up with the glass plan. Not sure why you want carbon, but that's just me..

    post edit

    I am sorry. See you said infusion and I had wet bagging in my head. For infusion, I'd budget at about 65% of glass weights. The thing is losses and fillets and stuff use some. Rob Denney pointed out my error. Wet bagging is very wasteful I found because you don't have lots of time to play with and all my bag work was done at 110% of glass weights. The amount of epoxy I used for a net result of 50% was rather incredible and I'd only use infusion going forward.
    Last edited: Aug 1, 2022
  5. rob denney
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    rob denney Senior Member

    There used to be plenty of 12m/40' cats with 600 double bias glass and 400 uni fore and aft either side of 20mm foam as the base laminate. None of them broke. I know of one with 300 gsm kevlar either side of 20mm foam. Still going strong after 35 years.
    Impact damage is more likely ripping holes in the boat from junk/fish/logs hitting daggerboards, props and non kick up rudders.
    Dry a big cat out on a single sharp rock and the laminate is almost certainly irrelevant.
    One of several great things about infusion is how easy it is to vary the core weight and thickness and the skin type, weight and orientation.
    I agree with Fallguy about carbon. It's pretty pointless for most of the boat, very handy in unidirectional form for high load areas. It also makes it hard to see what is happening with the infusion. I disagree about resin usage. Infusing, you should use about half the weight of the glass, plus 200 gsm per side of foam and a small amount for plumbing.
    The best way to minimise the laminate is to minimise or concentrate the loads. Cats with forebeams, headstays, swept back shrouds, travellers, roachy mainsails and free flying headsails are probably the worst way to achieve this. Unstayed masts are probably the best.
    Make sure your engineer works from first principles, not from a set of rules which almost certainly will result in a heavier boat than necessary.
    redreuben, bajansailor and fallguy like this.

  6. JacquesSparaux
    Joined: May 2022
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    JacquesSparaux New Member

    Thanks for the inputs, guys
    Carbon is indeed too expensive for overall usage. I'll consider it as reinforcement
    I have to finish my 3D CAD model to get the surface and rough estimates
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