Repairing dent in foam cored hull

Discussion in 'Fiberglass and Composite Boat Building' started by abourgault, Mar 16, 2016.

  1. abourgault
    Joined: Jul 2014
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    abourgault Junior Member

    Hi all,

    I want to repair the hulls of a beachcat that have a series of large dent probably caused by contact with round rocks of about 6 to 10 inches diameter. They are not deep, most of them less than 3/32 of an inch. They are all located at the bottom. The hulls are foam cored. The hull is made of 1 layer of fiberglass cloth, 1/4'' foam core and 1 layer of fiberglass cloth. Resin is vinylester.

    I am wondering if I can repair those just by filling with gelcoat or I need to use a fairing filler then apply gelcoat? It is probably related to the depth of the dent so what is the maximum depth to use only gelcoat?

    Thanks for your help,

  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The crushed foam is structural. Most likely the sandwich construction is delaminated. It would be better if you have someone knowledgeable look at it.
  3. abourgault
    Joined: Jul 2014
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    abourgault Junior Member


    I have checked the integrity of the lamination by pushing on the hull over the dents and it is as strong and rigid as the undammaged portion. I don't think the dents are deep enough to have cause delamination. And since those sailboat are very light, less than 400 pounds, the loads transfered to the hull are not really high. Nothing like on a motor boat. On my other beachcat, I have replaced a large portion of foam because of a bad repair from the previous owner as you can see in this link.

    I don't want to do more than necessary on this one. The dents are shallow and varies from 1 inch to 2 inches in diameter. I was just wondering if adding gelcoat would make it more prone to crack.

    Thanks for your help.

  4. Tungsten
    Joined: Nov 2011
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    Tungsten Senior Member

    I had the same dents as you describe,the foam gives but the cloth seams to still stick.I filled with epoxy and chopped glass sanded and painted.
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you have a dent, you have localized delamination at the very least. If they're fairly shallow, just fill with putty and hope for the best. Without knowing how deep they are or how large an area they occupy, it's tough to offer more than this. BTW, pushing on the area, isn't going to tell you much.
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is the core material, exactly ?
  7. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    I'd do similar to above but just key up the dents with coarse paper, mask the perimeter & fill with gel/flowcoat & if a bit deeper a couple of hits or add a bit of colloidal silica- tool off with a broadknife, sand, polish & sail & maybe not even bother with any of that & just sail the hell out of it like I stole it... it's a beach cat, just hot dog it close to shore & have fun, swim home if you need to & buy another...........................;)

  8. abourgault
    Joined: Jul 2014
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    Location: canada

    abourgault Junior Member

    Par, The size of the dents are specified in my 2 post.

    Mr Efficiency, the core seems to be divinicel.

    The boat is a Nacra Inter 18. Maybe some one knows the kind of foam they used.
  9. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    The classic way of dealing with these light dents is to heat the area lightly with a hair dryer/paint stripper to expand the foam core a bit. Sometimes, if really lightly dented, and say foam 12mm thick, the shape returns, so worth a try. As long as the glass is still tight to the core you can usually skim with gelcoat. However like PAR says, it is a bit tricky without seeing and feeling the damage. Any damage to the glass for instance, would indicate letting in new glass and replacing the damaged core with a filler first.

  10. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    I agree with SukiS. We used HT divinycel and prepreg sandwich on a panel and somebody knelt on it. There were dimples on the surface but before we could repair it, it popped out to its original shape after a day in the sun.

    Not sure about the low temp divinycel but if it does not pop up back, the core is probably damage and section needs to be replaced in the approved method of cored laminate repair. Low temp DV is known to crush.
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