best practice for tabbing bulkheads...

Discussion in 'Fiberglass and Composite Boat Building' started by robwilk37, Sep 19, 2012.

  1. robwilk37
    Joined: Nov 2010
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    robwilk37 Senior Member

    im replacing all the bulkheads and bracing in my yorktown 39. these boats where sold as kits and the original owner(s) had used CDX with 1 layer of mat/polly and no fillet or spacer. the hull is solidly braced to preserve the shape, ive made full-sized templates and, since the beam is 11.5' and close to 7.5' high from the floortimber/step, ill have to piece the main bulkhead together. the plan is to essentially make 2 complete bulkheads in 1/2" marine doug fir and laminate them together with thickened epoxy staggering the seems by at least a foot and rabbiting/taping the seems with 4" 17oz biax/epoxy. im stepping the over-all thickness up from 3/4" because i plan openings in the bulkhead to allow for better airflow in the v-birth. the mast is deck-stepped with a compression post so the main bulkhead is primarily stiffening the hull and not taking the entire rig load.

    so...

    im assuming 1" OA is sufficient. what is the best practice for the tabbing? is tabbing directly to the outer veneer acceptible or do i remove it and bold to an inner lam? or do i remove/taper so the tabbing bonds to more than one inner lam? is there any benefit to tabbing both sides of the first 1/2" layer and then the outside of the second 1/2" layer...essentially having three tabs holding the bulkhead in place? (now that ive typed that out it sounds like over-kill).

    which is better... tab the bottom edge of the panel to the top of the floor timber or fasten mechanically to the face of the floor timber?

    and while im on the subject. is there a benefit to cutting out the individual pieces so as to orient the ply lams on a 45 rather than straight up and down?

    of course ill seal all the pieces/edges prior to assembly...

    anything else i should be thinking about? thanks, as always, in advance...
     
  2. robwilk37
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    robwilk37 Senior Member

    seriously...im asking.
     
  3. PAR
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    PAR Yacht Designer/Builder

    I'm not sure about the construction of your boat, but generally you'd be advised to tab the bulkhead to what ever it was tabbed to previously. Since the boat has been modified, who knows how it was done originally.

    [​IMG]

    Without pictures it's difficult to visualize your situation. Personally I like to tab to the hull shell, but if the liner is well bonded and not just floating, bonding to it will serve as well.

    Doubling the plywood with staggered seams is the proper way of spanning your boat's beam. The extra 1/4" of material will not affect anything, though don't go crazy with additional penetrations or holes. If you need more access, heavily radius the corners of any holes cut in the bulkhead, no abrupt direct changes, just nice sweeping radiused turns.

    You don't need to tab both sides of each layer of 1/2" plywood. Install the bulkhead (laminate the piece together) in position, then tab the bulkhead as a completed unit on both sides. You can reuse the old tabbing or part of it, if you trust it's integrity, but you be wise to bulk them up with more biax, to insure they stay stuck.
     
  4. robwilk37
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    robwilk37 Senior Member

    thanks par...

    the boat is bare to the glass inside other than temporary bracing to preserve the shape. ive lightly ground adhesive residue so have clean toothy glass for bonding. the hull is 1/2" thick roving with no mat/chopper gun. where the bulkhead meets the hull im planing to use a closed cell foam spacer. where the BH meets the underside of the decks ive got the plywood to bond to.

    i saw or read once about tapering or at least relieving the tabbing edge of the ply, any need other than cosmetic? also, where the new bulkhead meets the floor timber/step... is it a better practice to land the BH on top of the floor and tab both sides or down the face of it and fasten mechanically? and lastly... (3) layers of 6" 1700 biax/epoxy, all seams staggered several inches ?

    pics are stuck in the old computer, im working on it. thanks so much as always...
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Generally, you taper the old tabbing to receive new. You can rabbit the plywood edge to receive the tabbing but you are cutting into the veneers of the plywood to do this, so I don't recommend it, but a varnished bulkhead could benefit from this treatment, with the warning mentioned. Yes, it's just an aesthetic consideration.

    Usually there isn't enough surface area on top of a floor to rest the bulkhead and have sufficient tabbing area, before having to 'round over the floor's edges. Bond it to the face and through bolt if desired, though an epoxy bonded bulkhead will technically not need the hard fasteners. Heavily tab the floor to the bulkhead on the other side and tab well to the hull shell on the plywood side.

    3 layers of 17 ounce biax is the bare minimum, consider 4 or 5. Naturally all layers of biax have staggered seams. You seem to have a much better idea of what's going on than the usual guy doing this. Keep at it and post some photos when you can.
     
  6. robwilk37
    Joined: Nov 2010
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    robwilk37 Senior Member

    thanks so much...

    5 it is... the floor timbers (laminated ipe) are 6" wide so 2 1/2" either side for the tabs does seem a bit skinny, didnt think that one through.

    and just this morning stumbled on a good deal on nidacore. ill probably use it for secondaries, furniture, hatches and the like. whats your feeling replacing the ply for the main BHs for nidacore ?
    i like to save weight when i can, and this product does that, but im familliar with good ply so the "exotics" are a bit outside my comfort zone at the moment. any thoughts?

    then again... adapt or die, right ?

    best
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The exotics just force you to play with goo a lot more, not to mention the cost compared to plywood. The few pounds you'll save will go completely unnoticed on your boat, unless you replace the deck and interior furnishing with it.

    You could be like a few here and willing to die, standing on what you know.
     
  8. robwilk37
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    robwilk37 Senior Member

    agreed. playing with goo will be inevitable on this job though...

    so i stumbled on a bunch of nidacore 3.5' X 10' 18mm thick. $20 a pop. so for the price i thought id try to find a use for it. like i said, the boat is empty to hull glass. so im thinking the 3 primary BHs in ply, massively tabbed and sealed. im just more comfortable with those structures in good-ol known and familiar marine doug fir. but intermediates, partials, furniture, the new cockpit, the new v-bearth, the raised cabintop (8' x 9' - 4" bump for headroom) etc. seems like i can save some real weight in the ends especially. i already have the cloth, epoxy and time, so... worth it? eyeballing it id estimate 200lbs out of the ends and another 300 midships. i dont have a way of judging if thats an appreciable amount or not in an 9 ton boat.

    often this is the point where people stop me and say something like "why spend sooo much time building when there are a ton of sail-able boats available cheap?" well... i like this kind of work, i have the time, space, most of the materials and the skill set to do it. and this is the boat i have for better or worse. and i think the boat has a ton of potential to be a solid cruiser. and i love an underdog. and thats all i have to say about that....

    still trying to tweez the pics out of the old pc. will post some soon

    thanks again
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Instead of tabbing in a plywood bulkhead, you'll be laminating several layers of fabric on the core stock, then tabbing these in. Your goo factor goes up exponentially, are you prepared for this? You can save some weight, but considering how substantial the hull is and it's shape, you're not going to do anything for her preformance, because you'll still have to sail her on her lines (you'll be putting the weight in anyway).

    If you want to use this stuff, go for it, but it'll be easier if you don't have the manufacture the panels, but just cut them from plywood. Lastly, what laminate schedule will you employ for the various panels? In other words, how much of what will be used on each side of these bits of foam . . .
     

  10. Herman
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    Herman Senior Member

    I have built complete boats from Nidacore. Not that I recommend that option, but for secondaries the stuff is OK. Just keep in mind that Nida is less stiff than foam or wood, but 18mm should keep you out of trouble.

    Procedure is roughly the same: make panels, cut and fit them, and tab them in.

    In all tabbing, make generous fillets (radius dependent on load). Over-laminate all high loaded fillets. (not needed for small stuff, like drawers, unloaded furniture, etc)
     
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