Plywood/foam equivalency?

Discussion in 'Materials' started by SpiritWolf15x, Jan 11, 2018.

  1. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 276, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    That's a pretty good summary of the task, but the hard news is , it ain't going to be easy.

    Multihulls with potentially very large point and slamming loads would be a major rewrite of the plans. You won't be able to do an easy substitution.

    If you had a Naval Architect who was interested in the project ( say Mike Waters, who has a similar trimaran on his design list) W17 Study Profile http://smalltridesign.com/W17/study-profile.html

    .. and has owned Buc 24's, he might be able to do you a competitive quote for foam.

    This project is too big for reliable "guesstimates"
     
  2. fallguy
    Joined: Dec 2016
    Posts: 2,070
    Likes: 127, Points: 63, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    You need to find a good naval architect to help you or make some guesses based on intended use since the designer is no longer with us. I would prefer the former.
     
  3. rberrey
    Joined: Oct 2010
    Posts: 441
    Likes: 19, Points: 18, Legacy Rep: 112
    Location: AL gulf coast

    rberrey Senior Member

    Ed Horstman,s book " Foam Fiberglass Sandwich Construction " has Wood to Foam conversion on pg. 9.3 . It should be a good rule of thumb , but keep in mind it might be better to go up 1/8 th inch as he listed 1/2 on material list for the 31 , but 5/8 on actual . Rick
     
    fallguy likes this.
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    These conversion lists are fine for a replacement panel or odd job, maybe some furniture upgrades, but not the whole plan set. It might be best to find a similar design in the method you'd prefer.
     
  5. SpiritWolf15x
    Joined: Apr 2010
    Posts: 194
    Likes: 1, Points: 18, Legacy Rep: 15
    Location: Vancouver, Canada

    SpiritWolf15x Senior Member

    All this wouldn't be a problem if wood was actually a suitable building material. Lol.
     
  6. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 276, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    There are more wooden boats in the world than any other material.

    Wood is fine, if you know how to build and maintain it.
     
  7. fallguy
    Joined: Dec 2016
    Posts: 2,070
    Likes: 127, Points: 63, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    See my private message for some ideas. This boat was done in foam by someone before and I gave you a possible lead on it.
     
  8. rberrey
    Joined: Oct 2010
    Posts: 441
    Likes: 19, Points: 18, Legacy Rep: 112
    Location: AL gulf coast

    rberrey Senior Member

    I would have to compare the foam and wood plans for the bulkheads to tell if Ed followed his Hull Conversion formulas . He advises to go with wood for the bulkhead,s but has plans for both . I would think converting things like hull and stringers would be fairly safe .
     
  9. freddyj
    Joined: Oct 2013
    Posts: 259
    Likes: 13, Points: 18, Legacy Rep: 37
    Location: kansas

    freddyj Senior Member

    Having built a wood Buc, if I were to do it again. I'd use foam instead of plywood. Though I would still wants the frames to be wood.
     
  10. redreuben
    Joined: Jan 2009
    Posts: 1,670
    Likes: 69, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

  11. Steve W
    Joined: Jul 2004
    Posts: 1,806
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Don't forget that if your foam/glass skin thickness ends up being thicker which it probably will, you need to allow for this in your station frames as all calculations are to the outside of the skin.
     
  12. TANSL
    Joined: Sep 2011
    Posts: 5,689
    Likes: 172, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    If I have not misunderstood, I think that statement is not correct. A well-calculated compound should be made by calculating the efforts in all its thickness, not only on the outside. It is mandatory to calculate that each of the layers that compose it is subject to admissible stresses.
     
  13. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 276, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    TANSL = he means all STATION DIMENSIONS, not stress calculations !
     
  14. TANSL
    Joined: Sep 2011
    Posts: 5,689
    Likes: 172, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Be quiet, Rwatson, quiet, that is going to give you a heart attack. As far as I know "all calculations" is not the same as "all STATION DIMENSIONS".
    Wow, what aggressiveness! Breathe deeply a couple of times, before writing, please.
    The measurements of the frames are usually taken inside the skin.
     

  15. Steve W
    Joined: Jul 2004
    Posts: 1,806
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Ok, perhaps I shouldn't have said all, what I meant was hydrostatic calculations, so, as rwatson said, if you end up with a thicker skin you would reduce the station dimensions of the building jig by a like amount if you want to retain the designed displacement. Of course its not quite that simple as if you are trying to substitute a foam/glass panel for a ply one you will not be just comparing to a ft2 of 1/4" ply but rather a panel that includes the stringers. I don't know the scantlings of the Buc 24 but my guess is it has bulkheads gunwhales, chines and then fairly small stringers on maybe 6" centers, whose job it is to break up the panel to give adequate stiffness with that skin thickness. Without the stringers the ply would be much thicker, so, you need to come up with a foam/glass substitute that is equal to the stiffness and weight of 1/4" ply including the stringers, not just the plywood alone. Most of that stiffness is going to come from core thickness more than the laminate so you will likely end up with a 10 or 12mm core so you will need to reduce the stations to compensate. I remember reading an article many years ago by Lock where he stated that (at that time) the only boats of his designs that came in at designed displacement were the wood ones, the foam ones were all heavier. That was of course before we were infusing panels on a table with better control of weight

    Yes, the frames are of course to the inside of the skin, the hydrostatic calculations to the outside, which is why if the skin is thicker you need to reduce the frames by the difference.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.