Repairing cracked frames

Discussion in 'Wooden Boat Building and Restoration' started by partgypsy, Aug 4, 2009.

  1. partgypsy
    Joined: Jan 2007
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    partgypsy Junior Member

    A boat I'm looking at has two cracked frames at the turn of the bilge. The frame radius is about 14". I can access the hull from the bilge to about 4' up the side and about 2' across the bottom. I understand about sistering frames and making them as long as possible.
    Is there any added advantage to installing additional plywood gussets edge-on in the shape of the curve of the bilge to span the bottom and side for about 12" and tying them in with the new sawn frames? I know they won't take screws from the planks, but they should take up some of the stress imposed on the new frames. Or is this over-kill? Is it common practice to epoxy new frames in place or to just use bedding compound?
    Thanks
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Your plywood truss is over kill and possibly harmful, creating hard points and of course more fastener penetrations into the frame.

    This is the way I like to do them, as it retains the weight and the look of the original arrangement. You can cut a piece to fit or laminate then mill to fit.

    Sisters work too, though some times you have issues with butt blocks or other interference.

    If your frames are oak, consider a different adhesive then epoxy, as prepping oak for epoxy isn't for the novice. Plastic resin, resorcinol, TiteBond III and PU are options to consider.
     

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  3. Zappi
    Joined: Oct 2008
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    Zappi Senior Member

    Par, could you explain the oak and epoxy issue in more detail. I too have the same issue on my 1960 Pacemaker. Was thinking laminating bent strips onto the top of the cracked frames with epoxy but apparently thats not such a great idea. Thanks so much!
    Aaron
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you have considerable epoxy use experience, you can probably laminate your rib repairs. If not, you'll have a percentage of failures you can expect. It's all in the prep and understanding what and how the bond is being affected. With this understanding you can address the issues surrounding them. What level of epoxy experience do you have?
     
  5. partgypsy
    Joined: Jan 2007
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    partgypsy Junior Member

    Hi PAR. You're sounding mysterious. A quick look through the wild web tells me that oak and other woods can a) soak up a lot of resin due to their porous cells, thus leading to resin-starved joins, and b) don't bond well because of chemicals in the wood which can interfere with the epoxy curing process and c) need a glue with somewhat elastic properties as these species tend to move with envirinmental changes.
    In any case, this issue seems to be well-understood by the major manufacturers and they have developed products which they claim will overcome this issue.
    Assuming these products are the state of the art, and that I follow the instructions on the can, and use the product in the right heat and humidity conditions, what is the correct method of preparing the wood itself?

    Thanks
     

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

    PAR Yacht Designer/Builder

    It's just those generalities that will cause your epoxy on oak laminates to fail.

    All hardwoods move from environmental changes more then softwoods. It's not an oak issue. Yes, the stuff in some species do affect epoxy's cure.

    With the exception of one manufacture, I don't know of a single brand that directly offers a product to address oak laminations. The one manufacture that does make this assertion has a product that isn't waterproof, so there's a rub.

    My point is simply, if you're new or relatively inexperienced with epoxy, making these repairs will be problematic at best, perticulay if you follow advertising on a reformulators site. If you know what chemicals are screwing with your bonds, how to deal with them, have established procedures and techniques to contend with the difficulties and can adjust your environment, prep and resins to suit the demands of this type of repair, you success rate will be high.

    This is enough of a concern that you've read about the failures, so there must be something to it. The goo manufactures can't do much to there products, it's more about prep and environment then anything. They could make changes to the their products, but then they'd have an extremely limited line of goo that doesn't sell, except to those making these types of repairs.

    Again, what is your epoxy experience? I ask, because there are things I routinely do now, that I couldn't dream of years ago.
     
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