A couple of layup questions......

Discussion in 'Fiberglass and Composite Boat Building' started by Anthony212, Nov 22, 2019.

  1. Anthony212
    Joined: Nov 2019
    Posts: 4
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    Location: Australia

    Anthony212 New Member

    Hi, I am sure these questions would have been asked and answered already, but I am unsure on what search terms to use to find the info I am after....

    The first question is as follows; I am doing a large repair to the outer skin of the transom on a 22 foot fishing boat. Due to damaged glass I have removed a large section back to the plywood core. I have already purchased chopped strand mat, double bias cloth and vinyl ester laminating resin to complete the job. My understanding is that I need the the chop strand mat between the plywood and the DB and between each layer of DB in order to provide 'tooth' for the DB to adhere too. Do I need to lay the chop and DB in one go? Or can I get away with laying a layer of chop, allowing it to go hard, then laying the DB? I only ask, because I have been having trouble getting everything done before the resin starts to gel on some other repairs I have been doing (its pretty warm here in Australia right now). I am concerned that I wont be able to get 2 layers down, properly wetted out and then roll out any excess resin / air before I run out of time.

    Second question, and along similar lines, if I am adhering two sheets of ply together my understanding is that I need chop strand between the sheets for similar reasons as I have mentioned above. One of the sheets already has 2 layers of cured chop / polyester glass (laminating), do I need to do another wet chopped strand layer before sticking them together, or can I just make a thickened vinyl ester paste and clamp them together?

    Thanks, I know that is a pretty long winded first post, but any advice is much appreciated

    Anthony
     
  2. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    1. Yes, you need to apply both the CSM and the DB at the same time, anytime the surface cures you need another layer of CSM to achieve a good bond.

    You can reduce the size of each piece of glass so you can more easily manage it in the time allowed before it cures.

    2. It’s better to have a layer of CSM between the sheets of plywood, but not required in all situations. How large are the sections of plywood being bonded together. Also are these sections of plywood being through bolted in some way after being installed.
     
  3. Anthony212
    Joined: Nov 2019
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    Location: Australia

    Anthony212 New Member

    Thanks for your quick reply.

    Is there any difference in strength between several smaller section (with adequate overlap) and 1 larger section? How large should the overlap be?

    This section won’t have any through bolts when finished, why do you ask?
     
  4. ondarvr
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    Location: Monroe WA

    ondarvr Senior Member

    Through bolting clamps the skins on each side against the core, so the bond sees less stress.

    The difference in strength won’t be great. But again, the size of the repair comes into play. Are we talking a 6” square, or 6’ square. Even if left alone and not fixed the 6” square probably wouldn’t be a problem, the 6’ square will have a much greater impact on the overall strength.
     
  5. Anthony212
    Joined: Nov 2019
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    Location: Australia

    Anthony212 New Member

    The fibreglass repair would be roughly 7’x3’. Essentially the entire outer skin of the transom apart from 4-5” around the edge (which I have feathered and will stagger my repair as per all advice).
     
  6. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    You can do 1/2 or 1/3 at a time, stagger and butt the joints. An overlap would be nice, but is less necessary when doing multiple layers.

    Finishing the outside with one layer of CSM allows you to do fairing/sanding into the glass without sanding into structural fibers.
     

  7. Anthony212
    Joined: Nov 2019
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    Location: Australia

    Anthony212 New Member

    Thanks again. I actually have a feeling that my fairing will end up going through more than 1 later of reinforcement, partly because I am starting with an imperfect surface and partly because I will have a few overlaps along the way. I am not concerned about that though; the original skin has come straight out of a chop gun (perhaps there is a layer of woven roving there somewhere) so my layup will be ALOT stronger anyway.
     
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