Transom Foam

Discussion in 'Materials' started by Narwhal55, May 18, 2016.

  1. Narwhal55
    Joined: May 2016
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    Narwhal55 New Member

  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    PVC on its own lacks strength. I built a boat once with PVC foam cored GRP sandwich transom, the only concern is compression of the core from tightening bolts that secure the outboard, that was easily fixed with a thick marine ply pad under the engine bracket, and very large "washers" on the inside. Never had a problem. Naturally there was enough glass in it to supply the strength needed, much more than you'd have with ply.
  3. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Don't forget the structure must support the thrust from the motor to the hull properly. By this I mean across the transom to the sides and bottom of the hull or it might decide to crack somewhere towards the edges.

    It will also depend how much horsepower your looking at transferring, you don't say so could be 4, 40, or 140?

    I quite like using an alloy bent plate or right angle wide extrusion (modified) on the top of the transom, local to the engine mount bracket. This acts as a scuff plate and support, on the inside for the upper bolts, especially if thicker than 2mm. You need support for compression, as Mr E wisely points out, so you can use end grain soild timber as well as ply. Both should be sealed prior to encapsulating in glass. This should spread reasonably to progressively support the thrust and take the local bolt pressures. Best if you do use ply/timber to drill the mount holes oversize, fill with epoxy and microfibres and then redrill through that at correct size. Assuming 12mm bolts say drill at 15/16m first, then seal the core, redrill at 12 and your done. Fat 'penny' washers should be used and tapered custom washers as well IF the hull is curved on the inside face.
  4. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    What is it that concerns you about Coosa Board, I sell it to several customers that use it in transoms, they've used it for years without any problems.
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Cossa does make a good transom core material, once you get over the cost.
  6. bryanemer7
    Joined: Aug 2016
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    bryanemer7 Junior Member

    I would look at using SpaceAge Thermo-Lite Board. It is similar to Coosa in terms of the foam and fiberglass but much better quality, consistency and service.
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Really, you've joined today and dropped 6 nearly identical plugs for "Thermo-Lite", but the site has no real data other than it being a 9 pound urethane foam. How about some pricing compaired to similar core products. Lastly, how can you compare a foam core product with Coosa?

  8. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    Coosa is great for Transoms!
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