Question on Glassing bulkhead “seams”

Discussion in 'Fiberglass and Composite Boat Building' started by massandspace, Mar 16, 2021.

  1. massandspace
    Joined: Sep 2017
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    Location: Salt Chuck

    massandspace Junior Member


    I am building a custom power catamaran and have a question about glassing bulkhead “seams”. Here is the situation:

    I am building the boat from older foam hulls...the old bulkheads were trash and not in the right locations, so they were removed.

    The new bulkheads are being added in. 3/4” foam/18 ounce biax cloth/epoxy. Some bulkheads I was able to build full width and drop them in from the problems....but other I have to build in 3 sections....main bridge deck span, and the 2 hull pieces.

    So there are two “seams” where the main bulkhead has to join the outer hull bulkheads...and that seam is right where the bridge deck joins the hulls. High load area.

    So, what do I need to do to get the final 3-part bulkhead to be the same strength (in tension) as the other bulkheads that were laid up as one piece, full width? Would a “patch” of biax across that seam be enough? And if so, how large of a “patch” and what weight cloth compared to the original cloth? Or should one use unidirectional cloth across that seam to save weight since the tension loads are, I think, only wanting to pull the seam apart?

    I added a picture of one of the full width bulkheads, and drew lines where I had to make cuts in the “3-part” bulkheads.

    Thanks for considering.


    Attached Files:

    Last edited: Mar 16, 2021
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hello David,
    How far advanced is the final 3 part bulkhead now? Have you already glassed the foam on both sides with the required lay-up of glass?
    If you haven't, would it be feasible to just apply saying one layer of glass on each side of each of the three sections, to stiffen them up a bit, and then install them (with epoxy filler on the joints) and then apply the remaining layers of glass on each side of the bulkhead?

    Apologies for the thread drift, but re your photo, was your catamaran originally a sailing cat?
    I see that she has very narrow transoms.
    How large is she, and how much power are you planning on installing?
    Have you carried out a weight estimate to make sure that you have enough buoyancy available for all the weight that will be installed?

    Edit - I just saw in your previous thread here -
    Only one steerable engine on small power multihull?
    that your cat is 28' long, with 2 x 20 hp O/B motors.
    And I can now see the two engines lurking under the cover in your photo.
  3. massandspace
    Joined: Sep 2017
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    Location: Salt Chuck

    massandspace Junior Member

    All 3 parts of the 3-part bulkheads are glassed. Just trying to determine the best (lightest/strongest) was to glue them back together.

    Yes, originally a sailing cat....27 only power and 28.5 feet. The new sterns are MUCH wider and longer.....ignore the old pic of the narrow sterns....only there to show the bulkhead....
  4. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Well, respectfully, you botched it. It could have been better to build them in 3 pieces, bond them together and then glass. Or relief and glass. But by the sounds of it you glassed, cut them up.

    Cut the margins 3mm apart and same all around. Use a batten and ship tape the bh side and screw them to hold flat. Bond seams with fumed silica and epoxy.

    Almost no matter what you do; you will be creating some imbalance in the system.

    What are you using to tab them in?

    I would follow the same tabbing schedule for the seams. For example if you are staggering two 4" 1708 tapes, same for the seams..
  5. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    I disagree, he did not botched anything. It's just like any other core glueing and glass overlapping situation.
    Glue the core with thickened epoxy, laminate over the joint on both sides with the same layup as the rest of the bulkhead skin. 4 inches each side of the cut will be plenty, then grind the edges for a smooth transition.

  6. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Only a botch if you have to fair it all...
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