Strut backing material

Discussion in 'Inboards' started by 7228sedan, Apr 11, 2013.

  1. 7228sedan
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    7228sedan Senior Member

    What are some accepted options in backing up a strut? My 28 Luhrs has a simple painted 3/4 inch plywood backing used to support the bolts on the shaft strut. I am looking to re-do this set up as the wood is a bit soft. Are there any synthetic materials which would create a better, more hydrophobic backing? Or should I use new ply and glass it in place?
     
  2. PAR
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    PAR Yacht Designer/Builder

  3. 7228sedan
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    7228sedan Senior Member

  4. dinoa
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    dinoa Senior Member

    Would coosa core be a good choice. I've read about it but never seen or used it?

    Dino
     
  5. 7228sedan
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    7228sedan Senior Member

  6. PAR
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    PAR Yacht Designer/Builder

    Bingo. Not the cheapest stuff going, but easily bonded or fastened, great compressive strength, quite stiff, etc., etc., etc. You could look into a a hunk of flat stainless plate stock, but you'll find the cost equally as uncomfortable, plus corrosion is an issue with metals, unlike G-10.

    The cheap way is another hunk of plywood, which works well. If the fastener holes where bonded and the whole piece epoxy encapsulated, you'd have much less difficulty with plywood, as the moisture couldn't get at it.
     
  7. 7228sedan
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    7228sedan Senior Member

    For the G-10 or Ply is 3/4 sufficient in your opinion, or should I go with a full inch?
     
  8. PAR
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    PAR Yacht Designer/Builder

    1/2" of G-10 will do, but make the plate as big as practical, considering the hull's shape limitations. Also make sure it's well bonded to the hull shell (epoxy), with substantial fillets to ease stress risers along the edges..
     
  9. michael pierzga
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    michael pierzga Senior Member

    The problem is getting your hands on a small piece of G10.

    A piece of ply could be used if it was correctly detailed.

    Recently I replaced a strut on a small low power boat.. The detail was marine ply 18mm seated against the inner hull into a bed of thickened epoxy.... fastening holes overdrilled and plugged with thickened epoxy...then an aluminum backing plate.

    It is important that the exterior strut to hull joint is perfect. I prefer the grind back the exterior hull skin untill I reach the substrate...dry assemble to prove alignment... dissamble, cover the foot surface of the strut with mylar packing tape, then seat the foot of the strut into a bed of thickened epoxy. This achieves a perfect mating surface and assures that load is transferred evenly to each strut fastening
     

  10. PAR
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    PAR Yacht Designer/Builder

    G-10 is available in 12" squares. Grinding away the hull shell to the backer seems self defeating Michael. It's much better to use the bonding technique on the exterior, rather than grind through the hull. You just cast a base, possible using the strut, like you describe as the lower portion of the mold.
     
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