Replacing foam core with plywood for small repair?

Discussion in 'Fiberglass and Composite Boat Building' started by teneicm, Nov 12, 2019.

  1. teneicm
    Joined: Jul 2012
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    Location: Chicago

    teneicm Junior Member

    We are repairing a pre-fabbed panel that forms part of the salon wall in our cat. To get at the repair without destroying the woodwork, we cut away the panel from the backside and removed a section of foam core (about 22cm x 60cm) and repaired the fiberglass. Now we are ready to replace the cut away foam core and repair the other side of the panel. Unfortunately, we are in Malaysia and cannot get our hands on any foam coring material. I think it is preferable to use the same material in a repair, but what is the impact if we have to use 1/2" marine ply instead of 1/2" foam to replace the core? Is this an absolute no-no or just a "avoid if possible"?

    Cheers!
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Is it a cosmetic or a structural repair?
     
  3. teneicm
    Joined: Jul 2012
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    teneicm Junior Member

    It's structural. It's one of the transverse walls running across the boat, so it distributes load from the cabin roof.
     
  4. Zilver
    Joined: Nov 2007
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    Zilver Junior Member

    The plywood is stronger than the foam, so that shouldn't be a problem, should it ? Also there's no risk of water intrusion/rot. Or am I overlooking something ?
     
  5. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I wouldn't worry about it. Foam core often has plywood or other dense material incerts.
     
  6. teneicm
    Joined: Jul 2012
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    teneicm Junior Member

    That's my (admittedly uneducated take on it to). My concern was that by introducing dissimilar materials that it might create a stress point in other areas. Sort of like overbuilding a repair with extra glass that then causes issues in the adjacent (and now less strong) original construction.
     
  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    It will be fine done right.

    The plywood panel must be bonded to the neighboring core with a thixotropic mixture and the glass must overlap the neighboring glass; despite cosmetic issues.

    You cannot add plywood and float it from the adjoining core; nor can you just fiberglass the ply and fair the seam.

    You must also avoid dry joint with plywood. To do so, simply provide a coating of resin to the ply and allow it to gel or near gel before bonding it to the glass or bonding glass to it. The edges can also be precoated to avoid dry joint. You can also make your mix a little runnier and that works as well, but is messier.

    If you did not remove glass from one side; you do not need to overlap the seam. Only where glass is removed is overlap needed.

    If you removed tabbing; it too must be replaced and overlapped onto adjoining tapes; not butted.
     

  8. KD8NPB
    Joined: Mar 2018
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    Location: South Carolina

    KD8NPB Junior Member

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