How to fix deformed hull

Discussion in 'Fiberglass and Composite Boat Building' started by mikesuperrc, Aug 25, 2012.

  1. mikesuperrc
    Joined: Aug 2012
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    Location: Queensland, Australia

    mikesuperrc New Member


    I'm currently rebuilding/customizing and old Flying11 (sadly wont sail any more, will be my fishing/runabout boat:p)
    I am nearly finished with sanding all the layers upon layers of old paint off (including house hold enamels :confused:) and will be making new buoyancy tanks, seats, etc. for it next.

    But before I get on with that there is a dint or depression in the hull were it had been sitting on the trailer, it is about 300-400mm long and 100-150mm wide running length wise. You can push it back to shape but comes back in as soon as pressure is release from it. I've tried putting weights on it and left them there for a week or so and that worked for a day then it went out of shape again:(.

    So my question is, What is the best way to fix this problem?

    Once the boats ready for a trailer again it will be on a proper cradle to support it more evenly so this doesn't recur.

    All help would be great.

  2. mikesuperrc
    Joined: Aug 2012
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    Location: Queensland, Australia

    mikesuperrc New Member

    ps. It's a single skin hull, no deck.
  3. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    You can try two things. First, what has happened is that the fiberglass has creeped under pressure (weight of the boat on the pad) and probably done so on a really hot day. The laminate has stretched a bit.

    One solution is to reapply pressure and heat. Try placing the weights inside again to press the dimple back out like before, and then use a hair dryer to heat the surface of the hull until the temperature on the surface exceeds 140-160 degrees F (55-65 deg C) (but less than 200 degrees F or 85 deg C) for a good while, say half and hour to an hour. The limit temperature (55-65 C) is roughly the heat distortion temperature of the resin, the point past which the resin goes slightly plastic and creeps easily. After the one half to one hour, remove the heat but keep the weight on the dimple until the fiberglass cools back to ambient temperature. That might do the trick.

    If it doesn't, then solution #2 is to add a brace to the inside, a partial frame, if you will, made of plywood and fiberglass. Cut a partial frame out of 1/2" (12 mm) plywood that is the original shape of the hull that is right in way of the center of the dimple. You can use the opposite side side of the hull as a template. The height of the frame can be about 4" or so (100 mm), and make it symmetric so that the length that stretches over and beyond the dimple on one side, and equally as far on the other side. Sand down the inside surface of the hull with coarse grit sandpaper so that you have a good clean and rough surface to bond new fiberglass to. Place this plywood frame in the hull over the dimple and press it down with braces or an internal jack until the dimple pops back into shape. Use some Bondo (polyester auto body putty) to glue the frame to the hull and fill in the slight gaps between the frame and the hull. Screed nice thumb-radius fillets of Bondo along the joint both sides of the frame. Let the Bondo cure. Sand the fillets smooth. Use fiberglass cloth and resin to glass over the frame with a layer or two, spreading out the layers onto the hull about 2-3" (50-75 mm). You'll probably have to do this in two stages, the first being in a few areas or patches while the jacks or braces are in place as the first overwraps of fiberglass cure. After the first patches cure, remove the jacks or braces, and this frame should be holding the dimple in place, hull surface fair again. Finish the job with a few more layers of fiberglass over the frame and tabbing onto the hull up to about 4" (100 mm) either side of the frames. Let that cure. The hull and frame should be rock solid. You can sand down this new fiberglass slightly and paint over the repair so that it looks halfway decent, if you care.

    I hope that helps.

  4. mikesuperrc
    Joined: Aug 2012
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    Location: Queensland, Australia

    mikesuperrc New Member

    Thank you for the fast reply Eric.

    I should be able to give it a go in the next week or two.
    I was thinking about heating it but wasn't sure if the resin would like heat, but now I have some temps to go by I'll give it a try. Failing that the second option wouldn't be an issue either because the back seat will cover were the fix is so a lump wont be a problem cosmetically.

    Thanks again, I'll probable have more questions through out the build and now i know were to come to :)


  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I wouldn't use heat on your boat. I see this sort of thing all the time and without some experience "moving plastic" with heat, you'll probably move a lot more then just the area you want to affect, creating more work for yourself.

    If it's a shallow dent, then just fill it and fair to the surrounding area. If it's substantial and will need a gallon of filler, then consider a brace, bulkhead or partition to wedge it down to where it wants to be. This way you can precisely control how much you've moved it, provide internal support and lock it down too. In time, the hull will "remember" it's adjusted location and local stresses will relax.

    You likely have the same issues we face here in Florida and many older boats suffer from this. It can happen from heat, but usually just from point loading, caused by an improperly fitted trailer. The under side of a hull doesn't see much direct sunlight and generally is cooler then the temperatures necessary to cause creep in the laminate. 'Glass has a "memory", amazingly enough, and if held or forced into a distorted position from a trailer bunk, roller or whatever for long enough, it'll "take a set" and remain this way.

    You can remove this memory with heat, but it's not easy to do well. I've had some luck using a temporary mold, usually a piece of plywood, bent to the approximate shape and wedged under the hull. Heat is applied inside the hull and braces push it against the plywood, until it cools. To be honest, it's a hit or miss kind of thing and more often than not, filler is used, if shallow enough or an internal brace arranged on the deeper ones.

    Lastly, arrange trailer bunks and rollers directly under internal supports, such as longitudinal stringers. These can bear the weight and will prevent future issues like this.
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