Gluing fiberglass/epoxy skin to pvc core for cabin construction

Discussion in 'Materials' started by JATesvich, Feb 16, 2021.

  1. kapnD
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    kapnD Senior Member

    How about a drawing of your concept of what the windows will look like?
    The speculation here could be narrowed down to more specifics if there were some better guidance.
    I will say that 1/4” square frp tube is severe overkill to support the windows, as the roof and windshield are already supported. A light C channel should suffice, especially when it’s bonded to a foam sandwich.
     
  2. fallguy
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    fallguy Senior Member

    Politely, you have some misconceptions about composites. The issue is not in individual components, but how they combine. Strength adds when things are combined. You don't butt join components.

    you would typically not, for example, lay a window onto the frp tube, but onto the core which is bonded to the tube, but how is the core fastened to the adjoining core? Typically this is done with tabbing.

    Here are some pictures. This is a mid tabbing shot of a cabin front. Tabbingis done on the horizon here, not the vertical. See the vertical recesses? Those are to allow for the tabbing. You cannot do that with an FRP panel.

    784EBC53-34BF-4994-8502-DB585219FCE2.jpeg
    Here is some early fairing work. The recesses allow for clean lines.
    F5B7CFD9-60C7-40EC-98A1-78DEC18D8A86.jpeg
    here a shot of the inside, no recesses were allowed, so we faired and painted around the window cuts, but this requires tabbing in composites construction; yours will as well
    17FD85C9-B6CD-4357-8B83-D6BADB58C4AA.jpeg

    I have another picture to add in another reply.
     
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  3. fallguy
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    fallguy Senior Member

    what he is forgetting is the bonding/tabbing, he has a really nice drawing in a reply
     
  4. fallguy
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    fallguy Senior Member

    Here is a final picture of the results of fairing after tabbing.

    B7358128-7D84-413C-83C9-BF3FE4C36949.jpeg
     
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  5. bajansailor
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    bajansailor Marine Surveyor

    Re looking sleeker and racier, dare I say that the first analogy that comes to mind is that it is a bit like putting a Formula 1 aerofoil spoiler on a family saloon car - I don't think that a lovely motor yacht like yours needs to look racy, elegant yes, and I think that less 'racy' would help her here.

    Re the vertical in front of the windscreen, I am not criticising it - rather, I think that this is really the only sensible thing that you can do here. But the whole side profile would look better if you had more vertical lines to complement it.
     
  6. fallguy
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    fallguy Senior Member

    A one inch core does not require any frp tubing unless I am forgetting about loads above or something.

    Properly engineered, the core and associated tabbing does all the work.

    I will add that if you want the sleek lines; you might want the same angle of rake as the lower windows; if attainable. Personally, I like the drawing you did, bit it needs to go to a naval architect who can design the structure correctly.
     
  7. kapnD
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    kapnD Senior Member

    Sorry, I totally missed the drawings in PDFs.
    That’s what I get for looking at this stuff on a small screen.
    I do agree that reinforcement would not be needed, foam sandwich will easily carry the windows and more.
     
  8. JATesvich
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    JATesvich Junior Member

    Nice work, by the way. Having done my fair share of fiberglass work over the years, I'm well aware of the work that goes into getting the finished surfaces looking like your finished picture. You can consider me a hack but that is exactly what I'm trying to minimize and the reason for my post. In cases where the ultimate strength of a joint is not so critical what options may be used to minimize the finish work needed to make it look good. sketch.jpg

    In the sketch above the FRP tabbing in Option 1 will require tedious and messy inside corner sanding and fairing. The question is whether something like Option 2 is a suitable alternative to attach the wall section to existing structure. Another variant of option two and maybe better would be to use an 1 1/2" open channel that is attached to the existing structure and embed the end of the wall section into the channel filled with thickened epoxy.
     
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  9. fallguy
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    fallguy Senior Member

    I am not too worried about the bond to the existing. I like what you have done, but have a question. I think it can work. I am more worried about how you would achieve vertical bonds at the corners and your bonds at the ceiling.

    I thought all the angles had heavy raking. Would you build the raking into the frp tubing?

    your drawing doesn't show a raked window, for example, when you put all that rake into the thing; suddenly something has to change; so were you planning to make all the frp tube to fit all your angles? So the tube would not be all 2x2 then? You can't putty more than 1/2" thick without smoking the epoxy..
     
  10. fallguy
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    fallguy Senior Member

    Also, with the rake, I don't see how a 5200 seam supports the raked panel. I can see cleating the inside, but the outside concerns me. All the roof load is creating tension there.

    Deaw it up with rake and explain the vertical corners more. Right now, I don't see it unless you customize the frp tubing. And then I still don't think the outside works with roof loading. Keep in mind roof loads on the boat are not static. Hit a wave and the forces become dynamic.

    Above my paygrade, but I see the potential for trouble.
     

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    Last edited: Feb 19, 2021
  11. Rumars
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    Rumars Senior Member

    You would have to glue the fiberglass with thickened epoxy, and weigh or clamp the foam down while maintaining a fair surface.
    In my opinion if one is going to all this trouble, one can also make the panels from scratch and include gelcoat. That way you don't even have to paint the outside at all, and can include rebates for window frames. The inside of the panel can be laminated in place after curving, then you use foam backed fabric, or rigid PVC sheet as a liner.
     
  12. JATesvich
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    JATesvich Junior Member


    framing.jpg
     
  13. JATesvich
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    JATesvich Junior Member

    The sketch of the framing after reconsidering and going with the upright mullions. Basically the wall sections would be attached (glued & screwed) to framing members all around the perimeter and at the corners. The side windshield corners would be fiber glassed to tie into the side wall, and at any butts joining wall sections would be tabbed.
     
  14. fallguy
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    fallguy Senior Member

    I think it is going to be trickier at the joins than simply using an angle.

    the sides can work as described, but the front will be trickier

    you can simplify it with a flat across front window, but not as nice unless bottom window is same (though that is now flat)

    I don't think the seams at the front window bottom and the hardtop will be strong enough in an epoxy bonded joint and 5200 seam seal alone, but I could be wrong.

    If you build it post pics back here, please.
     

  15. fallguy
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    fallguy Senior Member

    Sorry, our posts crossed a bit.
     
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