Plywood bulkhead - fiberglass hull joint without epoxy

Discussion in 'Sailboats' started by Michail, Jan 14, 2012.

  1. Michail
    Joined: Aug 2011
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    Michail Junior Member

    I got the following problem with my 5 m. sailboat under construction:

    I have a 9 mm. marine plywood structure over which the main fiberglass hull and deck skin will be attached.

    The problem is that the marine plywood I have available seems to be hardwood (800 kg-m3) and oily (it is very resistant to water, I have tried boiling it, it is exccelent). I have run tests with polyester and vinilester resins, and basically the bond is lousy. I can pry it off with screwdriver (applying a lot of force, though). (I have run the same tests on standard pine plywood, and there chunks of wood were ripped out, so the problem is not the resin, but the plywood it seems).

    It is too late to change the design (I need the central plywood cage where the mast, shrouds, and the keel goes, and I need longitudinal stiffeners in form of berths). (There are extra floors and longitudinal stringers with just foam and fiberglass.) I will not use epoxy due to high costs.

    So I have to find a solution. I am thinking about two possibilities:

    Option 1: after forming the joint (with polyurethane foam spacer), I would put two backplates (could be the same 9 mm. plywood) on each side of the joint, and then perforate and bolt them together (something like 5 mm. x 50 mm. stainless bolts every 100 mm. all around the bulkheads and along longitudinal berths. I estimate that I would need about 300 bolts. In literature I have not seen this type of solutions, maybe I am missing something out why it should not be done this way...

    Option 2. Make perforations in the 9 mm. bulkheads before forming the joint, diameter something like 13 mm., maybe bevelled to 20 mm, or slits, something like 5 cm x 1 cm. When forming, the resin would fill in the hole and would bond the two sides of the fiberglass reinforcement. Not clear though how to force fiberglass cloth inside (without fiberglass inside, it would not work, I assume).

    Any input is appreciated.

    (second image shows the plywood structure I need to attach, about 30 lineal meters)

    Attached Files:

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  2. eyschulman
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    eyschulman Senior Member

    Never did this never saw it done. make oval cut outs an inch or two from edge of ply panel and use glass tape through oval holes down to fixation points and cover whole thing in wide tape and epoxy goo
  3. rwatson
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    rwatson Senior Member

    You sure are missing something ...

    .... for goodness sake - for such and investment in time and money you are going to fritz around with a time wasting, unproven method ?

    Just get the epoxy !!!!
  4. Michail
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    Michail Junior Member

    The standard method is to use some resin (epoxy or polyester) without any additions.

    The choise between epoxy and other resins is dictated by the high cost of resin locally, 25 USD per kg. Besides, I have also doubts that obtaining a 100 % bond with the hard, oily wood available locally would be possible even with epoxy.

    Adding a bolted clamping structure at worst would be useless, but most probably will strengthen the joint.
  5. upchurchmr
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    upchurchmr Senior Member

    As I remember, the Gougeons suggested wipeing the oily wood with a solvent (acetone?) letting it dry, then immediatly gluing with epoxy.

    When you realize your actual cost with the strange additions to the joint, you will probably find it cheaper with the epoxy. I suppose weight does not mean much to you given the joints you are suggesting.
  6. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Polyester or vinylester aren't adhesives, so its unsuprising they don't perform in the job. If you don't want to use epoxy then use a polyurethane, or even something old school like aerolite or cascamite.
  7. TeddyDiver
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    TeddyDiver Gollywobbler

    Agree.. :)
  8. eyschulman
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    eyschulman Senior Member

    Since building a boat is often just a form of self torture use something other than epoxy and enjoy the resulting pain
  9. rwatson
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    rwatson Senior Member

    Wow - a true philosopher. Someone who really understands the true purpose of recreational boating. My hat off to you sir.

    If epoxy is totally out of the question , option 2 is the better bet, as I certainly wouldn't like raw foam touching the wood. No matter how oily the wood, moisture would create rot, and water would start ingress down into the foam.

    I would consider a 'one sided' variation of that attachment method, for reliability and easy drainage ( see attached)

    It has the advantage that there is no messy 'filling in gaps' with the resulting possibility of voids and holes.

    Also, most of the vertical layers of fibreglass ( green ) can be laid up against the plywood bulkhead, providing an easy application method. The foam ( grey) makes it easy to build a ridge to receive the plywood.

    Any moisture will drain into the bilges, and if the frame does rot, it can be easily replaced.

    Attached Files:

  10. gonzo
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    gonzo Senior Member

    Have you tried degreasing the wood? A mix of acetone and alcohol works really well. Soak the area with a wet rag and wipe it with a dry clean one.
  11. Michail
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    Michail Junior Member

    Indeed, it is a self torture, especially since I have decided to do it my way taking into consideration the limitations of the materials available, and doing everything almost from scratch... In fact, I am finishing the mold of the hull and the deck, and I have spent about 120 hours working with tools and materials, about 50 hours trying to obtain the materials, and about 250 hours figuring how to do it, searching the internet, literature, and designing and working in Autocad and Delftship, both of which I have never used before... It is fun, but I doubt that I would embark on another project, and in fact, if I knew what I am getting into, I might have reconsidered it.

    The original idea was also to reduce costs. It is not that I am totally out of money, and I could buy epoxy, but then, it might have been a better idea to get a used boat for about 12.000 USD (microtoner).

    Weight is an issue, but secondary. I want to have a boat which would be sturdy, relatively slow, but safe in open sea. So stregthening mechanically a questionable joint seemed to me a natural step to do.

    I have not tried yet degreasing the wood, although I have heard about it, and I will run a new test. Sanding did not affect the joint strength.
  12. tunnels

    tunnels Previous Member

    something else to try !!

    :confused::pIs the veneer on the outside the same as the core plys ? Use a grinder and take the top ply off to the one below . resin will only penetrate to the glue line so remember that Another one is to use Colalt and acetone mix 50/50 and paint it on the area and alow it to dri Then glass with a generous resin layer of poly or vinylester . both work in varying degrees . of goodness !!:p
  13. peterchech
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    peterchech Senior Member

    This is similar to the method in "this old boat" by don casey However he suggests using unidirectional glass instead of tape (the horizontal threads would contribute nothing to the joint if using tape)

    The book is really great and worth having anyway, but he suggests some similar methods for re-attaching old plywood bulkheads that have delaminated from the tabbing in older glass boats. The bolts and 3m 3500 method is suggested too, but those bolts are gonna add up price wise I think (unless you use galvanized but even then)...

  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Bolting isn't the reasonable option and can introduce point loading if not well backed.

    The choice of resin is up to you, but all three of the typical players can be used, given that epoxy will be the strongest, vinylester next and polyester at the bottom of the list in bond strength and stiffness.

    First, wash the area (oily wood) with a mixture of 33% acetone, 33% denatured or isopropyl alcohol and 33% toluene. This will remove the tannins and surface oils in the wood. If using polyester or vinylester, wetout the area with at least 2 coats of resin to seal the wood, then apply a 45/45 biax tape or fabric as the tabbing to the hull shell, to a schedule that's suited to the loads involved. Naturally, good building practice would require a tapered isolation piece, between the hull shell and the plywood bulkhead. This can be foam or other suitable material and should have a reasonable taper, to prevent the tabbing from making a hard turn at the intersection. You must error on the too much side with this type of laminate, as failure is almost always because insufficient resin and/or tabbing fabric was employed. Unidi fabrics wouldn't be a reasonable recommendation for tabbing, except in hi end racing hulls, where every ounce of weight is accounted for. Just do difficult to keep the fiber orientation, perpendicular to the joint. This is the biggest benefit of a 45/45 biax material, besides it's obvious strength.

    Epoxy techniques are very similar to the poly, but 3 coats of wetout to seal the wood are recommend and you don't need quite as much fabric in the laminate.

    If you do elect to hard fasten the bulkhead, you need a tabbing flange built up which will use twice as much laminate then a more conventional bonding approach. The fasteners holes should be bonded with at least a 30% larger (then the fastener shank) hole and the fasteners well backed and bedded. This seems like a lot more effort, cost and could be prone to several issues. I would reserve this type of assembly, for things that need to be removed reasonably often.
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