Foam cored stringers?

Discussion in 'Boat Design' started by bigdawg10132009, Jul 28, 2009.

  1. bigdawg10132009
    Joined: Jul 2009
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    bigdawg10132009 Junior Member

    I have seen on a couple posts where folks have used the foam core house insulation from Home Depot or Lowes or what have you to use for the core material for stringers. Is this a good idea? I know that plywood adds a strong core to the fiberglass in the stringers but I have also seen the youtube video where Mas epoxies placed foam cored stringers in a boat that was at least 25' long.......I do know you would have to add extra glass with the foam core than you would for the plywood but just how much more and what type??
    I am starting project "ebbtide" and have a few pictures to post and will be doing so on a fairly regular basis. Floor and anything wood is totally shot! But, it came with a good running 150 Evinrude (so far) and was dirt cheap....
  2. alan white
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    alan white Senior Member

    In a case where foam is used for a stringer, the strength is in the glass. You can form a stringer from lots of things--- plastic sheetrock corners come to mind (though any empty space should be open at the end to drain, especially if below the waterline).
    It's all engineeering. Does the molded shape do what you want it to do by virtue of the skin alone? Could you analyze it?
    Nowadays, wood still makes the most sense to replace wood since it has a strength and stiffness by itself, and when epoxy-encapsulated, it is even better than it was when new. Had the original wood been epoxy-sealed, there never would have been a need to replace it.
    Use foam and other handy shapes to make stiffeners and such elsewhere such as in making a deck (though the savings is a small amount of weight, and on some boats, small increases in weight are negligable).
    The savings is likely to be time, since wood takes longer to shape.
    No oiece of wood, no matter where it's found on a boat, should be sealed in unless first epoxied heavily. Wood will rot even up high, like at the sheer, under the right conditiions, especially if sealed in an imperfect membrane such as polyester resin.
  3. Jimbo1490
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    Jimbo1490 Senior Member

    Rubber hose is an old trick for 'form in place' stringers. Garden hose works OK. The foam companies like DIAB make foam 'hat section' (a sort of trapezoidal cross section) just for this purpose.

  4. bigdawg10132009
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    bigdawg10132009 Junior Member

    thanks guys.....everything i put back will be epoxy least three coats.....maybe four......just to be safe....and then primered / sealed and painted above and below as to prevent the whicking of water
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Christmas gift wrapping paper tubes, slit one one side, cut in half, etc. are good too. Corrugated cardboard, chip board, your mother in law's re-fried beans the day after the BBQ or whatever will hold a shape until the goo kicks off.
  6. gonzo
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    gonzo Senior Member

    Epoxy coating is not going to make the foam a structural member.
  7. bigdawg10132009
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    bigdawg10132009 Junior Member

    epoxy coated glass to clarify........not just epoxy.....roving / heavy cloth

  8. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    A book by Dave Gerr " The Elements of Boat Strength" should be available through your public library and would help you determine the amount of glass and the height of the stringer. The book is written in such a way that the do-it-yourselfer should be able to understand and make use of it.
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