flat panel foam sandwich stregenth

Discussion in 'Fiberglass and Composite Boat Building' started by pescaloco, Aug 17, 2007.

  1. pescaloco
    Joined: Feb 2006
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    pescaloco Senior Member

    hi, have a different post with no replies.

    maybee someone can help me with this (more general question)

    when using diab or corecell 5lb density foam as core material with std. poly resin / no vacumm bag / hand laid using say 1.5 oz mat 18 oz cloth 1.5 oz mat glass schedule. how important is the core in the whole stregenth equation ??

    if a person took a 2ft X 4ft flat panel and buted to another 2x4 to make a 4x4
    or 8x8 what ever the case might be. is the butted joint a serious weak point or does the glass provide the majoity of the stregenth ??

    thanks for any insight you may have

    Mark
     
  2. KnottyBuoyz
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    KnottyBuoyz Provocateur & Raconteur

  3. kach22i
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    kach22i Architect

  4. pescaloco
    Joined: Feb 2006
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    Location: so. california

    pescaloco Senior Member

    thanks

    knotty, thanks for the reply and link

    do you know in my application with 3/4 in foam and the glassing schedule that i mentioned ( the cloth being 0-90 knited ) would this achive the type of stregenth required for decking and cabin top roof ( my concern is ) the but joints being the weak points. meaning would any load carying member need to be aligned with the but joints (as in the grid that would support deckin)
    my concern is that I will have some fairly large spans that would be unsupported due to fuel and engine room desgin

    thanks mark
     
  5. KnottyBuoyz
    Joined: Jul 2006
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    Location: Iroquois, Ontario

    KnottyBuoyz Provocateur & Raconteur

    Hi Mark. I'm not a designer or structural engineer.

    You should, with the advice of the foam manufacturer, be able to bond the individual panels of foam together without loss of integrity. The foam, from my limited understanding, does not provide any (well very minimal) structural
    strength to the panel. The strength comes from the fiberglass laminates on either side of of the panel. One side will be constantly in tension and the other in compression as you put a load on it. The opposing forces via the I-beam qualities of the panel is what provides the strength. The thicker the core the stronger the panel.

    You could do a test. You can also lay in some support battens to help bridge those large spans.

    [​IMG]

    You can also buy pre-formed composite parts that are ideal for reinforcing long span panels.

    http://boatbuildercentral.com/proddetail.php?prod=Preform_TR4007

    [​IMG]

    I'm pretty sure your panel is going to be plenty strong. Just how strong I can't say. On a critical part such as a roof I'd use biaxial or triaxial fabrics, they are stronger. When I build mine I'll use foam for the roof but will rely on the designer to provide the scantlings for such. I'm assuming 1/2" corecell or divinycell with a 1708 top and 1208 bottom (possibly 9 or 10 oz 0-90 or 45/45) but that's just a guess right now. I definitely don't want any flex in the roof and I'm a svelt 240lbs plus the weight of a dingy & motor.

    Good luck!
     

  6. pescaloco
    Joined: Feb 2006
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    Location: so. california

    pescaloco Senior Member

    panels

    thanks, for the reply I was reading in the pdf's you linked and saw the "i" beam theory. also appreciate to pictures

    thanks, mark
     
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