Modern sheet material designs

Discussion in 'Multihulls' started by bruceb, Aug 6, 2021.

  1. Russell Brown
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    Russell Brown Senior Member

    Interesting discussion. I did a lot of work on a 50' cold-molded monohull that was built in the 80's, raced everywhere, and then left on the hard closed up with lots of water inside for 12 years. The deck and house was shot and had to be replaced, but the hull and bilges (where the water had been) were fine. I finally had it explained to me that it's water vapor that can penetrate epoxy, not water. Water vapor seems to collect on the undersides of decks when a boat is closed up with some water source inside. Decks are also kind of an obvious place to use foam core, because you can make it thick and get rid of a lot of framing.
     
  2. Skyak
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    Skyak Senior Member

    Bruce, please give some consideration to laying up your own sandwich.
    -far better material utilization
    -easy shaping with no residual stress
    -layup for exactly the right properties
    -higher quality materials (likely)

    With one sheet of melamine, some plastic sheet, a shop vac and some tape you already have the ability to make better material yourself. Options range from hand layup and vacuum bag, to full infusion, flat or shaped, shaped from flat (KSS), cylinder mold....
    Intelligent Infusion – Harryproa
     
  3. bruceb
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    bruceb Senior Member

    Yes, it is always the water vapor! I have hatches in all of my multi floats, and even when the bilges are dry (not that often ;( ), moisture beads up and drips from the underside of the clear hatches and decks, and frost forms in winter. It is a real test of sealing, and the water almost always wins. I have both foam core fiberglass and fiberglass over ply float decks, the ply float decks have lost the battle, the fiberglass over foam boat seems to have no issues in the same conditions.
    Skyak, I have all the equipment including a proper vacuum pump for vac bagging and I know how to use them, plus a fair amount of experience, but I am looking for quicker/easier/more cost effective methods. Always!
    I rebuilt a set of floats for my Dragonfly 25 last year, I removed the decks from the floats, removed the crumbling core and vacuum bagged new corecell and glass to the undersides and refitted the decks. I wanted to preserve the appearance of the stock decks and not add weight, they came out almost the same as the originals. Not a fun job though, and on my ply decked 33, I certainly am looking for something easier.
    I am also somewhat limited by access to materials. I live in Atlanta, a major city, but there are no local sources for either boat building grades of ply or marine grade foam core so everything has to be shipped in if I don't need pallet sized quantities. I do as much as possible of my boat projects at home, but I am limited to a max of about 22' for layup area and anything over 12' is a challenge. My boat and marina are 45 miles away from my shop/house so pattern cutting sheet material and then taking it to the boat to install is a much easier and faster way to work.
    I am semi-retired, but I still find my time quite limited and making cored panels from scratch is still very time intensive and expensive as compared to taking a jig saw to a pre-made panel off the stack. Once you add in the labor and material for prepping for holes and fasteners in foam cored panels or ply, a material like Coosa that in many cases needs nothing extra and can be directly fastened to for light loads really looks tempting.
    Bruce
     
  4. bruceb
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    bruceb Senior Member

    FWIW, The web site with technical data is "coosacomposites.com"
    I started this thread to try to better understand the differences and limitations of using a material like Coosa or other panel product instead of or as a direct ply replacement.
    I have no commercial interests in Coosa nor have I purchased or used any, yet.
    Bruce
     
  5. bruceb
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    bruceb Senior Member

    Also an engineering question. Any thoughts on a proper butt joint between sheets. Since most of the strength is in the skins, should they be scarfed or would a butt joint with several layers of glass on each side be better? Or...
    B
     
  6. gonzo
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    gonzo Senior Member

    There is no need to add layers at the butt joint. Simply glue the edges.
     
    redreuben likes this.
  7. Skyak
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    Skyak Senior Member

    Thanks for taking the time to ease my mind that you know what you are doing. I don't believe it will be easier but you are the best judge of your time.
    BTW, about finding materials -ask around here. I have never seen them stumped on a request for a skid of boat materials.
     
  8. bruceb
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    bruceb Senior Member

    I am curious too Skyak ;). I expect I will purchase enough to do my side decks (a couple of sheets of 1/2") and see how it goes. They are not structural so almost any !/2" panel will work.
    Gonzo, I don't think you understand the question. Two 1/4"or 1/2" panels cannot be edge glued to each other and expected to withstand much load. There has to be fiber across the joint. Plywood can be scarfed at about 8 x 1 but with a material where all the strength is the fiberglass layers on each side I don't think a scarf is the answer. I will try to contact Coosa and see if I get an answer. As far as I know, only metals that can be welded can be butted together with no loss of strength.
    The skid of materials is the problem, I don't need that much.
    Bruce
     
  9. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Butt joints in sandwich core is the accepted practice.
    You are free to scarf or lap joint if you wish.
     
  10. bruceb
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    bruceb Senior Member

    The Coosa board is the structure and core in one panel. In some uses they just paint it, but for my intended use I will need to join two or more sheets end to end, and those joints are the ones I am concerned with.
    B
     
  11. Blueknarr
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    Blueknarr Senior Member

    Since the skins are the structural element. The core can have whatever joint one desires. The skins are scared with new fiberglass and epoxy. The same as any structural fiberglass repair.

    I personally like to halflap the core. It provides reference to maintain a consistent core alignment.
     
  12. bruceb
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    bruceb Senior Member

    I like the half lap, and it is easy to do. Much easier than good scarfs.
    B
     
  13. fallguy
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    fallguy Senior Member

    Core joins work well with thickened epoxy. You can use plastic staples of you want to maintain a hold.

    Coosa does not have the same flexural rigidity as plywood.

    I have had trouble finding technical resources to explain it quantitatively.

    I would say it is about half as stiff, based on my experience. That said, I have not used the BW26.

    But, I would laminate the underside with biax or uni and the thickness depends on spans. Probably something like db1700 or so for up to 20" spans. For the top, a minimum of same like a db1700 or a hexcell 18 oz uni.

    If you have longer spans in some areas, you can also i beam under with a coosa 1.5"-1"-1.5" i beam with 2" tapes each side to cut spans down to 20". For much larger spans, probably doubling the glass on the bottom.

    Foams really require glass and coosa is no different.
     
    Last edited: Aug 12, 2021

  14. bruceb
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    bruceb Senior Member

    Fall, Thanks for posting that, that's info I can use. I had wondered if I would need glass on the bottom. I had expected to glass the top surface, and they both help with the butt joints. It also sounds as if it will be easy enough to bend it over a small amount of deck camber. My deck beams are about 18" x 15" apart so the somewhat stiffer Bluewater 26 should be stiff enough. Also, some flexing when walked on shouldn't hurt anything. I can always add an extra beam under the areas that get much foot traffic.
    I will probably try the 1/2" BW 26 first, but will also plan on a sheet of 1/4" to experiment with.
    I expect to pick up the material next month and then get started.
    I am still listening!
    B
     
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