Transom space filling

Discussion in 'Boatbuilding' started by billy241072, Sep 29, 2010.

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

    PAR Yacht Designer/Builder

    A 1" gap is huge and unacceptable. Typically for a transom of that HP rating you'd have 2 pieces of 3/4" plywood, which as you probably know doesn't bend worth a darn. In most cases on curved transoms, you'd use 3 layers of 1/2" plywood or 4 layers of 3/8" if the curve is really severe. You have two choices, fill the gap with more wood and hope you have enough outboard clamping range to accommodate a 2 1/2"+ thick transom (maybe not) or you'll need to get that plywood to conform.

    From experience I can tell you that you're never going to put much more then a slight bend in 2 layers of 3/4" (17 mm) plywood. This means you need to switch out and use 1/2" (12 mm) or possibly 3/8" (9 mm) depending on how much curve you need.

    Now, I'll bet you're thinking, "I can just cut a bunch of saw kerfs three quarters of the way through the plywood, then bend in it place, right." From technical stand point, if the kerfs were filled with a healthy mixture of thickened epoxy, then the two pieces bent and bonded all in one shot. You'd have a good chance of a solid, curved transom. Reality says, you'll never get all the kerfs filled, the two pieces of plywood well curved, bonded to each other and the hull shell, all at the same time. It's just not going to happen in someone's driveway. I could manage it, but this wouldn't be my first picnic either.

    The best you can hope for is to get it done in stages. Bend and bond the first layer of plywood to the hull shell. It's very likely you'll have to screw it on from the outside, just to hold it in place as the goo cures. If you don't want all the screw holes, then you'll have no choice but to brace it into position from the inside. Then back fill the edge of the plywood with thickened epoxy to fillet to the hull shell. Next bend and bond the next layer of plywood (which can be temporarily screwed to the first layer to hold it for curing) to the first and again back fill around the edge, filleting this piece to the hull shell. This sequence continues until all the plywood pieces are installed and filleted to the hull shell. The next step is to tab the new curved core into the hull shell with biax, by at least 6", 9" would be better. Three layers or more of 12 ounce 45/45 biax. Lastly, bond the liner to the core with more thickened epoxy and crack open a beer, because this is a hard day's work, I've done it more then once and you'll have earned it. If you brace things from the inside, you'll have to stop at let the epoxy cure, before removing the braces and installing the next piece of plywood. Once you're done, you'll understand why we drink and charge so much.
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  2. billy241072
    Joined: May 2010
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    Location: South Africa

    billy241072 Junior Member

    Ttxs Par for the detailed advise.Bottoms up.
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Great and very informative post PAR. Thanks! :)
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  4. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    You should go with PAR's advice, Billy. I believe he assumed the fiberglass hull scenario. This repair is so common here on the forum that you will learn a lot by researching here using the search feature to visit those threads.

  5. Vulkyn
    Joined: Jun 2010
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    Location: Egypt

    Vulkyn Senior Member

    CDK is one of the most helpful members in the forums and his point of view is true you need to provide more info to get better help ...
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