Multihull Bulkhead Refit Advice

Discussion in 'Multihulls' started by Coastal Ogre, Mar 1, 2021.

  1. Coastal Ogre
    Joined: Jun 2019
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    Coastal Ogre Junior Member

    Hello Everyone.

    I am currently conducting a refit of a Lagoon 380. Many of the plywood bulkheads are attached and tabbed with a methacrylate adhesive that over time has failed in several different modes. I am looking for advice on how best to refill these gaps (see poor quality photos enclosed) between the bulkhead and the hull to facilitate required vacuum bagging for new fiberglass tabbing. The original adhesive has been found to fail (or has already failed) in chunks in some areas. As such, replacing with methacrylate again is not providing me with confidence. My question is: fill the gap with thickened epoxy, 3M 5200, or some other superior adhesive???

    I also notice that several references on this site use the term "Compliant Resin" to be used between the bulkheads and hull. For an un-cored hull and a balsa cored hull; what is a "Compliant Resin"?

    Many thanks!
     

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  2. fallguy
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    fallguy Senior Member

    I do not know what 'compliant' refers to here.

    But I can tell you that I would use an epoxy and thicken it with fumed silica. The rate is about 2:1 by volume or a bit more.

    you need to remove the failed glue fully

    then, generally, fillet at the joint and taping immediately follows; the fillet would be about 1/2" radius or say 3/8"

    I prefer to mix the resin in a tub and then move it to a board and spread it out thin. It must not sag and needs to be mixed very stiff.

    pile it on a board; if it sags or moves after about 20 seconds; it is too thin and must be thickened

    I like to use fast epoxy for the fillets and then wetout the tapes with slow, because the fillets have less time to sag if the mixes are a wee bit thin and I like the tapes to stick and I also don't like a ton of exotherm...a fillet and three tapes, for example gets pretty hot if it is all kicking same time, with fast, the fillets kick before the tapes are applied, but not too hard

    if you don't fillet and tape same day; then you have to sand the fillets and inevitably find some places you might want to fill; with all bonding same day; the tapes move into any voids and humps are pushed down with consolidation roller
     
    Last edited: Mar 1, 2021
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  3. fallguy
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    fallguy Senior Member

    Also, vac bagging those tapes is a bit silly; the gains are somewhat incremental unless someone specified it as a work order/instruction

    Do not use 5200; that is not correct.
     
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  4. Coastal Ogre
    Joined: Jun 2019
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    Coastal Ogre Junior Member

    Thank you for the feedback and the information. This is helpful. As I don't mind the work, I just do not wish to have to perform rework on this at a later date.

    Interestingly enough I have removed approximately 150lb. of cosmetic wood paneling from this front cabin alone. The finished project will be stronger and lighter.

    I found the following two references on older threads here. They both use the term "Compliant Resin" (just fyi).

    Thank you again - Coastal Ogre
     

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  5. Rumars
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    Rumars Senior Member

    In this case "compliant" means approved by the classification society, in the case of your drawings, Lloyd's Register. They have a list with approved resins of all types that changes over time. Just use epoxy and you will be fine.
    You don't need to fully remove the failed adhesive, just enough to make a fillet, the strenght will be in the fiberglass.
     
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  6. Coastal Ogre
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    Coastal Ogre Junior Member

    Good to know. Thank you!
     
  7. gonzo
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    gonzo Senior Member

    I see a gap on the photo, but not failure of the adhesive.
     
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  8. fallguy
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    fallguy Senior Member

    I will echo Rumars with a caveat. Any glue that has failed should not interfere with the fillet and bonding. Basically, you don't bond over a weaker bond. But if there is glue deep between the BH and hull; it can stay there.

    There is a really good book I recommend you read. It is a fun read and educational for your plans. The book has one error in it that bothers me. Sanding fillets is a job to avoid and taping immediately is best.

    epoxy basics by Russell Brown

    Also, not sure what the tabbing schedule is, but do not stack tapes over each other and wider tapes first is best practice to avoid air entrainment on follow on layers. Fiberglass does not benefit from a shingle effect here.

    I am a bit confused after Gonzo points out the missing glue.

    Are these actually structural bulkheads? It seems odd they would not have tabbed them in if so. Or was all that ground away? Etc. a little more detail might help
     
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  9. fallguy
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    fallguy Senior Member

    If I had to guess, those are miscuts or intentional overcuts where the BH did not meet the deck join. And the glue was not capable of filling the seam, but they did not care.

    still seams odd no tabbing was done...

    Are you sure these are not intermediates where they cared less and next bh is done with glass tabbing?
     
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  10. fastsailing
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    fastsailing Senior Member

    There is nothing wrong with using 3M 5200 as a sealant to facilitate vacuum bagging. When cured it will also hold the bulkhead in place to facilitate laminating. And it will provide some protection for the bulkhead against moisture penetration through the exposed edges much better than a stiff fillet would. Any local movement of the bulkhead due to swelling will cause either the fillets or the plywood bulkhead to crack, which does not reduce strength along the strain much, but reduce strength perpendicular to the strain a lot, and thus also shear properties will be reduced. Using 3M 5200 or a similar product from some other manufacturer will help with all that, as it remains watertight regardless of changing dimensions due to a little swelling or thermal expansion, or even flexing of the hull/deck panels.

    Obviously fillets providing rounding for the taping should not be done with 3M 5200, the joint formed by sealant alone will be weaker than the plywood in all directions. Taping also needs a stiffer substrate. As long as the taping is strong enough for the loads applied, brittleness of fillets made from thickened resin is irrelevant. But 3M 5200 between the fillets will increase the capability of the panels to handle bending loads a little, by making the joint slightly a more flexible and thus reducing the peak bending load of the panels at the bulkhead position which functions as a stiff support.

    Vacuum bagging the tapes does save some work after the lamination is cured. They can simply be painted, with no smoothing out needed first.

    Of course none of this matters if the bulkhead is still attached to the boat, this is only relevant if the bulkhead is presently not attached at anything at all.
     
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  11. Coastal Ogre
    Joined: Jun 2019
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    Coastal Ogre Junior Member

    Thanks for the assistance in this everyone!

    Failure modes seen thus far with the original adhesive include: shearing (cracking) at high stress points, failure to bond on the polyester & vinyl ester hull, bonding varnished plywood to the hull (where the varnish fails and thus the joint fails), and chunks coming off when chiseling out the original methacrylate tabbing. The void in the photo shows where a sizeable chunk came out due to (most likely) a failure to bond to the hull.

    I plan on mechanically removing any observed suspect areas. But some of it is still in good shape, as when it has not failed it is still tenacious stuff...

    This is a 18 year old hull. And there are three tabbing methods seen thus far: fiberglass tabbing, methacrylate tabbing on most everything else (with a methacrylate standoff), and a methacrylate tabbing with the plywood edge resting directly against the hull.

    The photos show the forward bulkhead in the forward cabin. So it divides the forward cabin and the watertight bow compartment. Inside the bow compartment it is tabbed with fiberglass. Inside the cabin it is methacrylate. Since the plywood is still dry and in good shape I am planning on sheathing it with fiberglass or CF (pulling a vacuum, since that's how my OCD runs) and tabbing with fiberglass.

    The forward main bulkhead is tabbed with fiberglass starting from under the compression post, across the bridgedeck and down the inside of the hull to the keel line. On the outside of the hull of the same bulkhead, you guessed it; methacrylate only.
    The rear main bulkhead is methacrylate inside the aft cabins and fiberglass in the engine rooms.

    Regards - Ogre
     
    Last edited: Mar 2, 2021
  12. Coastal Ogre
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    Coastal Ogre Junior Member

    Thank you. I noticed in a 2011/2012 build post of an Outremer 5X that a white adhesive was being used to hold bulkheads in place temporarily and provide a gap. Thus my comment about the 5200. As I seem to have a penchant/luck for sailing in rough conditions, flexing is something my 'girl' does.... And I am very concerned about making things too stiff on this production boat. Is making a 38 foot cat 'too stiff' possible?

    My wet layup skills are marginal to good (only). So using the pump is like a godsend for the finished quality. I was amazed at the consolidation of the 1208 biax that I recently used compared to the same 1208 the factory used 18 years ago.

    The bulkheads are still firmly in place. Cutting those out scares the bejesus out of me...

    Regards!
     
  13. catsketcher
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    catsketcher Senior Member

    Gday - I would never use a 5200 adhesive on a bulkhead and I would very much ensure that epoxy is used under the bulkhead as well. Epoxy filler under the bulkhead gives a good glue surface that helps spread out in line bulkhead loads into the skin. If done well you make a continuous glue bond from one fillet, under the bulkhead and through to the other fillet. It is normal boatbuilding practice to push epoxy glue under a bulkhead and the fillet out the sides. Then glass over the fillet. Epoxy glue is very strong but weak in shock loading. Glass fillets increase the shock loading ability of the hull/bulkhead joint. Read the Gougeons book or Russ Brown's book but please use epoxy glue under the bulkhead where possible.
     
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  14. fallguy
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    fallguy Senior Member

    No tabbing can be done over varnish. If you have more of that grind it out. I know the bond strength of polyurethane is pretty high, but unlikely it would be keyed correctly and then all those bonds are secondary with no key or little key and esters; not epoxy. Surefire failure. Good thing you weren't in some really, really bad seas.

    All surfaces need mechanical key of 40-60 grit sanding; not higher. Wire brush is also suitable, but rather hard to do well. I use a festool rts400.
     
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  15. Coastal Ogre
    Joined: Jun 2019
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    Coastal Ogre Junior Member

    Thank you. Currently the issue is that most of the bulkheads have fiberglass tabbing on one side. Therefore pushing epoxy under and through is not feasible. Unless of course I go to the extent of cutting out the 'good' tabbing too. And that would also require removing the methacrylate still adhering the bulkheads. Is it time to get 'medieval' on these bulkheads and remove them, or is a one-sided repair acceptable?

    Regards!
     
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