Building a cutout transom materials methods

Discussion in 'Fiberglass and Composite Boat Building' started by motorbike, Mar 4, 2021.

  1. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    I have 1 1980’s 34ft yacht that I intend to convert from a closed cockpit to a walk through transom. This has been done very successfully multiple times on this particular design so the dimensions are reliable with regard to structural integrity .

    The current construction is solid glass, including the transom so there is considerable weight savings to be had if I use foam. The approximate dimensions of the job is a 1.5 x 1.0m box if you will to get an idea. The question is what is the best method to employ since the majority of the work will be flat panels apart from the transom door itself. Plywood has been recommended, epoxy coated on the inside with a heavy biax on the exterior, the reason is the simplicity and ease of working plus the ability to screw into it. My resistance to ply is that the boat is glass and I would prefer all composites, that ply needs to be well glassed- essentially treated as core material otherwise there is the every present risk of water intrusion.

    The second option is to use corecel or similar, in which case what thickness and density would be recommended, and as I would be planning to laminate one side on the bench would it be advisable to vacuum bag it. Weight is not critical. I am reasonably capable have built a few wooden boats in the past but not much foam work apart from rudders.

    Thanks and on another note to the old timers, Par is still sadly missed
     
    Last edited: Mar 4, 2021
  2. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Plywood is so easy to work with, so intrinsically strong, that I recommend it.

    PAR is absolutely missed, and he passed away before I joined. I read his stuff regularly, and I wish I could talk to him.

    John
     
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  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Motorbike, would you like to post a photo or two of your cockpit and transom please?
    Have you managed to find out what the scantlings are on any of the previous conversions that were carried out on this class of yacht?
     
  4. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    I crawled all over several conversions and it appeared to be foam and 19mm thick, another was done by making a mould and going the solid laminate way. So there have been different and successful methods. The sole will be about 1.0m wide and say 800 long, will need to check and my previous mention of a box shape still holds but I am debating how big it will eventually be. On the foam conversions there are gussets from the top hat stringers to the underside of the sole.

    The question isn’t about whether it can/should be done, or saving the last gram as the solid layup is very heavy and any cored method will be an improvement but the about what’s the minimum density/dimensions of foam. Is there any reason that either ply or foam would be preferable in relation to durability, ease of building etc
     
  5. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Ply wins it hands down in durability over foam core. Point load from impacts... I've seen the claws of a hammer go right through an extremely strong composite structure. In one side and out the other. Admittedly I was holding the hammer, but I didn't think I was overdoing it. With plywood, no chance.
    Easier to build too, but not as dramatically so. And plywood is cheaper. The plywood can't compete in strength vs weight, or sophisticated shapes, but you did say a heavy box.
     
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The edges of the door opening would need to be glass wrapped. Whether foam or plywood, glass wrap is needed. You radius the edges and wrap the ends anf probably overlap the glass. You could probably relieve the dimensions both sides like a drywall joint to simplify (electric plane off 1/8").

    A transom conversion to open requires due care. It has to be made strong enough for the transom to not fail, of course.

    There are significant differences between coosa, plywood, and lower density cores. All of them would have different glass requirements. Ply the least, but ply also has risks of rot if any ingress. Low density core most.

    But to do it right; you need it specified to make sure the tabbing to the sole is sufficient and the glasswork enough.

    Also, always preferred to maintain the existing transom exterior if at all possible.

    Avoid cutting anything to you have the job fully planned.
     
  7. fallguy
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    fallguy Senior Member

    Add pictures. The most likely option is going to be coosa or corecell, because if you already have solid glass; it is pretty strong as is..and you may only need thickness to stiffen things up.

    specifying the layup is out of my wheelhouse
     
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  8. motorbike
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    motorbike Senior Member

    Many thanks for you replies, here's an example.

    IMG_5130.jpg
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Can you tell us please what the make of the yacht is?
    And perhaps post some more photos, including a 'before' photo of your yacht and her transom, and another photo or two of 'after' photos on other boats.

    Re your photo in post #8 above, do you have a 'before' photo for this boat as well?
     
  10. fallguy
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    fallguy Senior Member

    Hard to gauge anything by that picture, except it looks like the transom is gone.
     
  11. gonzo
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    gonzo Senior Member

    If you cut the section of the transom carefully, the panels can be reused. Only the door frame and the edges of the door need to be fabricated.
     
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  12. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    Thanks for your responses, its a pretty straightforward job and because there have been a number of successful conversions then cant see too many structural or design issues. As I mentioned its more about ply vs foam. Ill keep you posted.
     
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