bulkhead taping

Discussion in 'Fiberglass and Composite Boat Building' started by tropicalbuilder, Jun 4, 2010.

  1. tropicalbuilder
    Joined: Jun 2010
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    Location: costa rica

    tropicalbuilder Junior Member

    Hi,
    I'm rebuilding a '79 42 feet ketch from the hull..
    I'm planning on taping the bulkheads (3/4 marine plywood) to the hull with epoxy and biaxial tape (DBM 1708)
    How many layers and what size should I use?

    Thanks
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    What is the hull made off? What are the scantlings? Does it have a core? Are the bulkheads structural? If they are, what are they supporting?
     
  3. tropicalbuilder
    Joined: Jun 2010
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    Location: costa rica

    tropicalbuilder Junior Member

    right,
    this is a '79 taiwan built ketch
    the hull is solid FRP no core.
    LOA 41'.1'
    BEAM 12'2''
    displ 31000lbs
    ballast (internal cast iron in full keel glassed over) 8500lbs
    topsides 1/2 "
    bottom 7/8"
    the boat has been abandoned for several years in the tropics (Costa Rica) so basically it was all rotten inside.
    I took everything out and now I'm at the bare hull, and I'm redoing the whole interior, this means all the bulkheads either structural and partial .
    I'm going to use trapeziodal foam fillets at the bulkhead hull joint.
     
  4. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    I don't think that you need 1708 mat to tab in the panels you describe. I've done the same project although on a somewhat smaller boat (27' Silverton). I used 9 oz. per sq yd fiberglass tape. 8" wide to tab in the bulkheads, cabin sole and the floors. Like this:

    http://www.mertons.com/Reinforcements/cloth_tape.html

    You allready know enough to eliminate the hardspot using the trapazoidal footing to form the fillets. I have found the tape to be more than sufficient to stabilize the joint. Once you get the whole interior in and assembled you will find it to be plenty strong enough.

    Regards,

    MIA
     
  5. westsail42
    Joined: Mar 2005
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    westsail42 Junior Member

    Somewhat late to this thread. My Westsail project has similar specs to your taiwan ketch. I did:

    four layers 1708 at 3,5,7,9 inch widths for primary bulkheads.
    three layers at 3,5,7 for secondary semi-structural interior support bits
    two for smaller stuff at 3,5

    While I did use foam fillets for the corners for some extra strength, I don't think the trapezoidal footing fillets are necessary. Not for a solid glass hull at least 1/2" inch thick that is post cured 30 years.

    My $.02
     
  6. tunnels

    tunnels Previous Member

    Keeping the Bulkhead from actually touching the hull is important to stop it from eventually printing and causeing hard patchs where in a situation it becomes a place where it could break and punture the hull . The diameter of the radius in the corner where hull meets bulkhead is all important !! Keeping the ply away allows the radiused corners to absorb a degree of movement ! the bigger the radius the more the spring effect can take place so to speak .
    On hi speed power boats there can be a 12mm gap that totally relys on the glass only so the choice of glass is important to , Double bias is my choice and has never let me down ever , 100% of the glass round the corner is working for you all the time !!:D
     
  7. westsail42
    Joined: Mar 2005
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    westsail42 Junior Member

    Sure, maybe if it was cored with a thin lamiante schedule and was just popped out of the mould in the last couple weeks. But on a 1/2 inch (at least) SOLID glass laminate? Post cured 30 years? Not necessary. That hull will be as solid as a rock. Make your bulkhead cuts as close to the hull and as smooth as possible and you will be ok.

    If there are no stringers in this hull, and it is 70s era, no need for bulkhead foam spacers. They didnt use them then, so you dont need them now.
     
  8. mark775

    mark775 Guest

    I disagree with the lot of you.
    I will say this much; More tape, much more and wider, 3208. If the glass is designed to flex there, it will eventually break (at the very least, it won't do its job) - the radius is absolutely not critical (just don't let it get under the bulkhead at all. The trapazoids are good but are just a mold for a lot of glass and to spread out the stiff area. When taping, don't have a bunch of different widths lying around confusing you. Have but one width, say, a foot and stagger it, e.g., from one side, nine inches, eight inches, seven inches, six inches. It does not matter if the overlap is under or over.
    Let me ask this; does the hull flex? (answer "yes") Then does it not flex at a bulkhead? ("of course not"). Then you have a "seam" right next to the bulkhead where the flex is concentrated. If you do not spread the force out, the original poly laminate will eventually crack there - it will be witnessed by "inverse" radial cracking of the gelcoat, then, one day, a delam. The only reason for the radius itself is so that you don't end up with a line bubble. No radius is necessary for the joint, itself.

    Untitlefgwe4gr.jpg

    Of course, if you will never see repeated beatings or don't care about the long-term...proceed as before.
     
  9. westsail42
    Joined: Mar 2005
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    westsail42 Junior Member

    I agree with Mark. The more tape you can bear to lam, the better. And, if you are using bi-directional reinforcement, alternate between 0/90 and 45/45 weaves between tabs is ideal. Mark's graphic illustrates why foam corner triangle fillets are nice in that they can help prevent a pinching situation of the lam. The fillets will also help mold the glass to stiffen the bulkhead against fore/aft movements (foam fillets provide no actual structural strength of course).

    I have seen a number of production sailboats "pop" their bulkhead tapes on simple (non-sinking) grounding accidents. Often too light on the tape lams.

    As for trapezoid strips, sandwiched between bulkhead and hull, besides being somewhat of a pain the work with, aren't necessary for this boat (but wouldn't hurt).
     
  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Bias cloth will be better. All the fibers are reinforcing the seam. Proper taping is done with different with tapes. Start with the widest and then reinforce the seam with narrower tape. It should look like small steps. If the smaller tapes are inside the fibers have to make a zig zag that means they are not aligned with the directioin of the force. Staggering the tape also creates those misalignments which weaken the structure. A foam or other spacer is a good thing to keep the bulkhead from creating a hard spot. At least the first layer should be either mat or stich-mat to help adhesion.
     
  11. AndrewK
    Joined: Mar 2007
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    AndrewK Senior Member

    Based just on the above replies you can see there are a number of ways doing the job.
    Coves are a very effective way to spread the load, long time ago I came across some data comparing bending force required to break a plywood T joint and the radius of the epoxy cove. It was found that at a ratio of just above 2:1 (cove radius:ply thickness) was the transition point. If the cove radius was <2:1 the cove would fail, if above the plywood failed. Taped only joint (no cove) strength was below a 1:1 fillet.
     
  12. tropicalbuilder
    Joined: Jun 2010
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    tropicalbuilder Junior Member

    well
    I think i will definitely use the foam fillets to avoid hard spots and ease the fiber turn.
    Four layer 1708 for main bulkheads sounds solid to me and the point of using the wider first so to avoid the kind of zig zag on fibers of the following tapes really makes sense ...
    Thanks everybody for all the info
     
  13. mark775

    mark775 Guest

    I hate to keep harping on this Gonzo, fellow, but he just knows so much that isn't so (there is absolutely no difference between smaller first or last, I told you an effective way to make it simpler, and the resin is a glue, THINK OF IT AS A GLUE, and you'll see for yourself that "matt first" is no different than any other fiber first if you prepped it to have good contact - it would simply have a higher resin content at that intersection or a layer of non-structural matt. You are using epoxy anyway, so a layer of matt is absolutely weaker/more brittle than resin filling the weave of what is there (with a wet on dry/cured intersection, I defy you to get matt into the weave of the base laminate for better surface contact anyway. Do a practice, then destroy it - you will see.). The non-bound non-oriented fibers of your material are enuf. Make no mistake, matt is pretty much non-structural... and Andrew's epoxy cove has nothing to do with this. Trapazoids are fine but they are not the critical aspect of this, particularly when your tape may as well be...tape. Because you saw them somewhere or someone said they are better does not make an inadequate taping any better.
    You will, in the end, do it how you want rather than how it shud be done. There is no qualifying test before responding on a forum and you will get contradictory advice because some go on gut feelings/wive's tales, just have a need to appear knowledgable (when they are not necessarily) or have not much experience with the sea. "Sounds solid to me" is quite a test. TODAY, I was in seas that wud allow "three layers of 1708" to move enuf that a delam wud surely start, no matter what resin (do yourself a favor - since you are going on feelings, make a mock-up of three layers of 1708 on your trapazoid and 3/4" ply, or just draw it - your laminate will be 3/16"minus. How do you FEEL about that?). Good luck, and congratulations on the fine start you are giving your new boat.
     
  14. AndrewK
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    AndrewK Senior Member

    When using multiple layers of tape, technically it is better to start with the smallest first and finishing with the largest as this minimizes the free edges. But I do agree with mark's recommendation of only having one or two widths and staggering these.
    I disagree that coves have nothing to do with this, making hard 90 degree tape joins is bad practice. I also say do your self a favour, make a test and see for your self.
     

  15. Jasonsansfleece
    Joined: Oct 2009
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    Jasonsansfleece Junior Member

    Are there not issues using a DBM tape and epoxy? Does not the M have binders in it that will not break down in epoxy and inhibit proper wet out?
     
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