Ideas for repair and fiberglassing structure under decking

Discussion in 'Boatbuilding' started by mydauphin, Mar 28, 2012.

  1. mydauphin
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    mydauphin Senior Member

    I am fixing a front deck for a friend on a 65' foot boat. The hull is fiberglass and unfortunately the structure is plywood covered over with fiberglass. The area under forward deck in front of bridge has been retaining water between the deck and ceiling of the fore room. So the structure which is made of 2x6 and plywood under the fiberglass deck is rotten. I have removed the rotten wood and trying to see the best way to fix it. I can rebuild it with 2x6s from underneath. I was thinking of shooting the underside with a chopper gun for that. Not sure how to fiberglass the underside of a deck. Any ideas are welcomed.
     
  2. alan white
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    alan white Senior Member

    Fiberglass inside is something you'd need to do for strength when spanning a few feet, in order to add tensile strength to the underside of the plywood, preventing sag. Otherwise, a glass skin inside is a good way to trap water that seeps in from above. I wouldn't glass the inside unless it appears to be adding needed strength as mentioned.
    If this appears to be the case, the first order of business is to prepare the deck surface glass for new plywood underneath. You'll need somehow to make sure you maintain the original deck crown/shape.
    It might make sense to do a section at a time, depending on the size of the foredeck.
    Even if you do glass the inside, don't use a chopper gun. They create a very resin-rich skin that has low tensile strength compared to glass cloth. You's use a lot more resin, driving costs up as well. You save on labor, but the job will be inferior.
     
  3. Petros
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    Petros Senior Member

    You might consider getting some guidance on sizing the structure for wood framing, and than just fiberglass the outside to make it weather tight (plus UV protective finish). Than just paint or bright finish the inside, presuming that forms the ceiling of a finished room.

    This would prevent trapping future leaks, and would allow the owner to detect the leaks sooner, and would likely last much longer even if it did leak a bit since it can dry out.

    The other alternative might be to vacuum bag the ceiling with a portable vacuum pump. That is the only way I can think of to make epoxy and glass stick to the ceiling.

    but I think I would do without the inside layer. If it is not needed for strength, there is really no reason to have it. Saves costs as well.

    Good luck.
     
  4. Landlubber
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    Landlubber Senior Member

    ...if I was replacing the deck and cabin liner, why not just glass both sides of the plywood with epoxy resin and cloth, that way the boards remain stable, the ceiling can be filled with phenolic resin balloons and sanded flat really easily and then flo coated woith epoxy white flo coat. This is then sanded and polished for a fantastic, easy to clean ceiling (deck liner), the glass cloth on the deck makes great non skid surface as it is, so just needs light painting. Laminated deck beams will form the shape for the plywood decking, do the bottom side first and they will naturally bend that way ready to lay on the deck and glass the top.
     
  5. mydauphin
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    mydauphin Senior Member

    Thank u all for your answers. Clarification the deck is fine, having a sandwich of teak. In between fiberglass, I don't want to cut it out because it would be over 200 sqft . I redid bow of boat for same reasonbut that was smaller area over non living spaces. People live on this boat and cutting deck open during raining season is not a good idea. I willpost pictures later.
    The best idea so far is to vacuum bag small sections att a time.
     
  6. mydauphin
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    mydauphin Senior Member

    Not sure how to do vacuum bagging upside down, or is epoxy better than fiberglass for this. Fiberglass cures faster and more stiff, easier to bond layers. Epoxy no smell, better moisture resistance, more working time. Ideas welcomed, this is what is great about this board. Great to get other opinions.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Overhead work is never fun, particularity using your bald spot as a squeegee, as I have had to in the past. Epoxy would be my choice, considering you have wooden elements in the mix. Yes, bagging works upside down, though it may not be completely necessary to do this.

    Most just arrange a jig, that is propped into place once the laminate is positioned and wetted out. This can be as simple as a plastic covered piece of plywood and a 2x4, wedged against the sole to hold it up against the overhead as the goo cures.
     
  8. mydauphin
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    mydauphin Senior Member

    So break it down into specific questions:
    1. Should I removed all the rotted wood, or will letting it dry and painting it with antifungal paint and then covering with Epoxy do.
    2. Can the wood be cleaned with something like sandblasting or water pressure? I hav e used a water pressure machine before useful but never upside down inside a boat.
    3. How do I do upside down vacuum bagging?
    4. I would to get ride of wood all together. But I think I might be build 2x4 wraparound with fiberglass then bonded in place with epoxy.

    thanks
     
  9. alan white
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    alan white Senior Member

    I wonder why you need to glass the interior. What is the advantage over a wood structure that's painted?
    Fiberglass is a fast construction method but not necessarily the best repair or rebuilding method. You need the kind of strength you get from athwartship beams that have enough depth to support loads from above.
    obviously you have enough headroom for 6" beams and the question is, what spacing do you use?
    How thick is your plywood? Plywood 3/4" thick can span a couple of feet, especially if the deck is reasonably thick fiberglass.
    I don't believe you need any glass whatsoever inside. As you know, the original design failed due to trapping water between the glass layers. If fasteners can be driven into beams from above, the job will go quickly compared to glassing inside.
     
  10. mydauphin
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    mydauphin Senior Member

    I think Alan you may right,. Perhaps I replace plywood with tongue and grove cedar decking and 2x4 with cedar 4x4. Use fiberglass only at ends.
    Working with plywood sheet is going to be down here.
     

  11. alan white
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    alan white Senior Member

    I agree the ends should be well tabbed. As far as the rest goes, that would have to be figured out based on your particular boat's design. Now's a good time to have a good boat carpenter analyze the situation. It's a major job and it can't be done from here, except for noodling several different ideas and "digesting" them before commiting to a final plan.
     
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