Plywood Bulkheads vs. Composite plywood vs. composite glass

Discussion in 'Materials' started by CatBuilder, Dec 13, 2010.

  1. CatBuilder

    CatBuilder Previous Member

    I have a wide open field right now as to what bulkheads to install.

    I have plans for the wood/epoxy version of my boat as well as plans for the all glass/foam version.

    I asked the designer and the bulkheads are interchangeable.

    My choices are:

    1) 3mm Okoume - 1/2" balsa - 3mm Okoume

    2) Triaxial/uni - 3/4" 100kg foam - triaxial/uni

    3) 10mm Okoume


    Which would you choose and why? I'm leaning toward the 10mm Okoume for simplicity and to speed up the build. Also, I think a little bit of wood might look nice. So far, it was going to be 100% gloss white inside.

    What are the weight differences of these?
     
  2. rasorinc
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    rasorinc Senior Member

    I think 10 mm ply is 1-32" over 3/8s ply so should be about 38 lbs. a 4x8 sheet. Cannot help you with composit weights.
     
  3. CatBuilder

    CatBuilder Previous Member

    Thanks. I'll work some of this out right on the forum to help with future questions.

    For #1: 3mm Okoume + 1/2" balsa + 3mm Okoume

    Ok, so a 3mm Okoume sheet is 11lbs from a supplier website. I have two of these in the #1 option, so we start with 22lbs. Now we add in the balsa's weight:

    I know balsa has a density of 9lbs/cuft, typically.

    If I assume we are just looking at a 4x8 sample sheet of any of these options, there is 32 square feet in a 4x8 sheet. If the balsa is 1/2" thick, then I have 32 square feet x 1/24th of a cubic foot (that's what the half inch thickness is, we've already converted to square feet, now going to cubic). That 1.33 cuft of balsa in a 4x8 panel, or 12lbs of balsa.

    Total weight of 4x8 panel of 3mm okoume + 1/2" balsa + 3mm okoume?

    11+12+11= 34lbs.

    Option #3: Straight 10mm (has to round up to 12mm or 1/2" because they don't make 10mm Okoume): 38lbs

    Option #2: No clue... but quite a pain in the *ss to do, comparatively.

    Looking at the total area, all my bulkheads add up to 470sqft of material.

    Weight of my bulkheads using 10mm is 38lbs / 32sqft per panel = 1.188lbs per square foot, times 470sqft = 558lbs TOTAL for all bulkheads in 10mm

    Weight of my bulkheads using Optoin #1 is 34lbs /32 sqft per panel = 1.063lbs per square foot, times 470sqft = 499lbs TOTAL for all bulkheads done in 3mm okoume - 1/2" balsa - 3mm okoume

    So, about a 60lbs difference. WOW! Not very big difference for the difference in price and extra hours of labor.

    Now I wonder about the glass/foam bulkheads. Probably not much lighter than the balsa ones, I assume?
     
  4. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Regular 3/8s marine ply is 25lbs + 12.5 lbs = 37.5 lbs most any ply except teak and mahogony even Meranti (Hydrotec BS 1088) falls in the same weight area. I prefer that ply
    with epoxy encapsulation over composite. Just my 2 bits.
    this site has specs and retail prices on several marine plys. They offer volume discounts.
    http://marine-plywood.us/ the meranti has 2 a-grade faces.
     
  5. sabahcat
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    sabahcat Senior Member

    I used 12mm okoume (gaboon) for all b/heads
    Main bulkheads done like this with 100 x 21 Kiri planks

    [​IMG]

    All foam or Balsa bulkheads I have seen or had anything to do with have had a massive ring of uni's embedded in the outer edge and around door cut-outs.
    Something in the vicinity of 50-75mm x b/h thickness of solid uni's weighs quite a lot and is very labour intensive.



    Some builders are now doing bulkheads in strip plank
    [​IMG]
     
  6. CatBuilder

    CatBuilder Previous Member

    Wow, strip plank bulkheads. That seems very labor intensive.

    Glad to see others are using 12mm plywood though. I think I'm going with that.
     
  7. rberrey
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    rberrey Senior Member

    One other thing you can do is hold off on the bulkhead material untill you see how much core cell if any is left over from the hulls. rick
     
  8. sabahcat
    Joined: Dec 2008
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    sabahcat Senior Member

    Compared to what?

    You dont have to rout out the foam edges and put in a few klm's of uni roving for starters
    You dont have to scarf sheets of ply together
    You dont have to make a pretty lattice work of structural timbers
    You dont have to do several coats of resin, inside and out on the ply

    Just glue some timber strips together on a flat floor and glass
    Easy
     
  9. CatBuilder

    CatBuilder Previous Member

    Well, maybe it is easy. I don't know. I'm just learning. :D

    It thought it looked difficult compared to plain marine plywood bulkheads, but I've never done either.
     
  10. rxcomposite
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    rxcomposite Senior Member

    The more you should use the Laminate Properties I posted. The spreadsheet adds the weight of the glass and resin depending on the process and proportions used. Just add the weight of your foam or plywood afterwards.
     
  11. rxcomposite
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    rxcomposite Senior Member

    Door jambs or door cut outs require local stiffening. Also on the door where the hinges, and the door knobs will go. I usually embedd high density kiln dried wood in local areas. So far so good, 12 years now and has not failed.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Lets see, $60 bucks a sheet for Okoume 9 mm (3/8") or hundreds of dollars for all the other options . . . hummmmmmm . . .
     
  13. CatBuilder

    CatBuilder Previous Member

    Yes, sorry, RX. I had forgotten about using that sheet for this. I had forgotten a tip or two from Andrew as well. Sorry about that. It's information overload on this end.

    You'd be surprised how much one has to learn to go from not knowing how to build a hull to having a basic understanding. :)

    I'll go plug this stuff into that sheet and give it a try. Thanks!
     
  14. CatBuilder

    CatBuilder Previous Member

    I do need 12mm, but yeah, that's what I'm thinking. The weight difference (very important to catamaran building) just doesn't seem to support the cost difference or huge difference in hours of labor.

    Seems all bulkheads should be marine plywood in boats. Very hard to see how spending so many more dollars and hours for so little weight difference would be worth it.
     

  15. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Its Ok cat. You are trying to learn all at once. Me, I try to make things simple. These are the things I do everyday so I hate repeating it. My excel is now so complex and large to the extent that while I am designing the boat, the other spreadsheets is computing the BOM and material cost, labor cost, weights, ect.
     
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