FRP house or house extension on steel deck?

Discussion in 'Boat Design' started by SinkingDream, Feb 25, 2019.

  1. SinkingDream
    Joined: Apr 2018
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    Location: colorado

    SinkingDream Junior Member

    Hi all,

    Just a question, is it common to build any FRP superstructure on steel vessels?

    Would it be a bad idea? Difficult to keep the interface joints water tight?

    Just wondering.

  2. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    It’s becoming more common on large yachts, but has been done for quite a while.
  3. JamesG123
    Joined: Mar 2015
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    Location: Columbus, GA

    JamesG123 Senior Member

    With proper engineering its not a problem. They just become two separate structures bolted together.
  4. SinkingDream
    Joined: Apr 2018
    Posts: 16
    Likes: 1, Points: 3
    Location: colorado

    SinkingDream Junior Member

    Thank you for the replies, Guys.
    I knew plastic has a high coefficient of expansion, hadn't realized it was so much more than even aluminum.

    I did expect there were ways to deal with it, and it seems an obvious way to significantly reduce topside weight.
    Easy to provide insulation in the bargin.
  5. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    I don't know the layout of your boat or the FRP cabin and this may sound totally 'out there', but you may want allow for lightning. Electrons love steel more than FRP so if you have a metal mast (or other metal) on an FRP cabin, on a steel hull (& deck), there might damage during a 'strike'. I may be wrong so contact an expert.
    Last edited: Mar 6, 2019

  6. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    The way it is done is they weld a metal bracket to the deck. The fiberglass cabin is bolted to the bracket with slightly larger hole in the center then progressively becoming more oval as it moves out from the center. Before bolting, seal the mating surfaces with silicon adhesive and use fender washer for the bolt. Do not over tighten it. leave 1/16" or more of the adhesive to move/shear.

    This is a trick used in the early days of manual drafting. A metal strip with three aligning pins is taped to the board and the mylar sheet is placed/aligned to the pin. The mylar has an exact fit in the center but the rest of the holes are oval. This cuts the total expansion by 1/2.
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