Best Method To Prop Up Hulls?

Discussion in 'Boatbuilding' started by CatBuilder, May 4, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    I'm building like this, for those who haven't seen my posts:


    I currently have the two hull halves joined, as you see in the photo, but I have only some temporary half bulkheads on the upper half of the hull. There are no bulkheads yet in the bottom half. It is supported by the mold/form.

    A couple of the temporaries have popped off their feeble little mini-coves when I dropped the hull yesterday. No damage! Whew!!

    Anyway, I'm trying to figure out how to keep the hull shape perfect, given that my real bulkheads cannot touch the hull anywhere, as per orders of the designer. They must all "float" on the bog.

    How can I be sure of a good hull shape?

    What keeps the uppermost part of the hull in the picture (the topsides of the boat) from sagging?

    What have any of you done at this stage?
  2. rberrey
    Joined: Oct 2010
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    Location: AL gulf coast

    rberrey Senior Member

    Cat, if I,m correct you should have about 1/4" between the hull and bulkhead and bog between.Trace the bulkheads and cut temps 1/4", 1/8" larger , or whatever bog space you have between the hull and bulkhead. Use cardboard to trace out what you have now for a hull shape and lay it against the true bulkhead shape. Rick
  3. CatBuilder

    CatBuilder Previous Member

    Thanks, Rick.

    Are there faster ways?

    I'd like to minimize extra work, since this project is really, really long and difficult.

    Did you get started on yours yet?
  4. rberrey
    Joined: Oct 2010
    Posts: 443
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    Location: AL gulf coast

    rberrey Senior Member

    They just put me on 12hr days , at least you have a good start on yours. I'll think on it a little and see if I come up with something else. If you want to hold the hull true then the form you use has to be true. Maybe use true bulkhead forms at just 3 or 4 stations. rick
  5. cthippo
    Joined: Sep 2010
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    Location: Bellingham WA

    cthippo Senior Member

    Can you cut some pieces 1/4" wide and temporarily attach them to the perimeter of the bulkheads? For this it seems like even duct tape would work as they are just spacers.
  6. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    I normally use a full cradle bonded to the hull with car body filler to ensure correct fit with the molded deck. I am quite finicky about this and take measurements along the width from time to time because the hull can distort in so many ways.

    Yours is a half finished, half glassed hull. I suggest you screw rectangular pieces of plywood (1/2 x 2 x 4") along several points at the outside of the hull in bulkhead intersection. Bond this to a 3/4" x 4 light wood using car body filler. The temporary outside frame does not have to be pretty, As much break as practicable to approximate the hull shape. Edges overlapped and screwed. I have posted something similar before.

    Measure, align, and screw the ends to the bottom jig.

    Now, when fitting the true bulkhead, Attach with quick dry epoxy square pieces of foam along the inside bulkhead line. 1/8" to 1/4' thick (depending on the thickness of the bog your designer requires) along the line of the bulkhead. Width should be the same as the thickness of the bulkhead. Make it square for simplicity. Do not overdo as we are already "cheating". Just enough points to ensure a no contact design.

    Now fit the undersize bulkheads to see if all points are in contact, otherwise, remeasure as your hull could be sagging. This "raised points" are to ensure your bulkhead are floating when you attach them. Make sure you have clamping points on the top edge (deckside) so that you can work hands free when you are ready to attach the bulkheads.

    Once satisfied, lay on the bog to the thickness of the raised points. Fit in the (primed) oukume bulkheads, clamp securely, and fillet the bog with your fingers or a big popsicle stick.

    I normaly glass over while the bog is still soft to spare me the trouble of sanding high spots.

    Watch out, the bog will pull during curing. Luckilly, you still have the foam unglassed on the other side.

    I am attaching a design detail from Lloyd's. I think this is what your designer want.

    Attached Files:

    1 person likes this.

  7. CatBuilder

    CatBuilder Previous Member

    Wow, RX. You're good!

    That attachment is *exactly* what my designer specifies and yes, I do put the cove in and also some biaxial to tab it.

    Interestingly, I will be also doing a couple of those "hat sections" you show in the bottom of the attachment, so it is nice to see those somewhere outside my plans as well.

    I have combined two of my problems into one solution:

    Problem #1: This thread. How to prop up the hull correctly.

    Problem #2: Another thread - How do I cut out bulkheads, projectors, etc?

    I am going to just bite the bullet here and get my bulkhead patters plotted out on a 1:1 scale. It might cost money, but it will solve both problem #1 and problem #2.

    I will then use the foam wedges between the correctly sized bulkheads and use the real bulkheads themselves to keep the shape of the hull for final joining of hull and bonding of bulkheads.

    I have no clamping surfaces at all... I will have to wedge the bulkhead in with foam. The boat is already completely enclosed, deck and all. The deck and hull are done at the same time in the form/mold.

    Thanks for the help on this thread everyone. Even if I don't do the external frame, I have learned more techniques along the way.
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