Scantling question for beginner

Discussion in 'Materials' started by Brands01, Nov 23, 2006.

  1. Brands01
    Joined: Nov 2006
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    Brands01 Senior Member

    Hello, I'm new here.

    I'm interested in boat design - read a few books (Nature of Boats, Elements of Boat Strength, Devlin's boatbuilding, Designing Power & Sail, Understanding boat Design, along with anything I can get my hands on on the net, ie lectures on the westlawn site etc .. ) - I've got others on order, but in the meantime I've got a few questions.

    After all this reading, I thought as an exercise, I would try and design myself a boat from start to finish. Obviously there is heaps I don't know, but I'm finding the process very interesting and rewarding. I've taken some time and drawn the preliminary hull on freeship.

    I have settled on a hard chined epoxy plywood displacement powerboat - around 9 metres LOA, 3 metres max beam, designed for weekend coastal, harbour and river cruising. I've settled on the plywood for ease of construction and price, and I've built a small stitch and glue dinghy, so am familiar with the methodology.

    I've been using Dave Gerr's Elements of Boat Strength for scantlings.

    After this big intro, I have a fairly simple question. Gerr covers scantlings for exterior sheathed ply, cold molded sheathed inside and out - or composite construction. He doesn't cover plywood composite - so plywood sheathed with glass on the exterior and interior. My assumption is that I could use scantlings for foam sandwich construction but using plywood as the core. However, my experience with boats is that if you assume, things go wrong.

    Can I substitute plywood as the core for foam sandwich scantlings?

    Many thanks in advance for any advice:)
     
  2. gonzo
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    gonzo Senior Member

    Use the scantlings for strip planked with fiberglass in and out.
     
  3. Brands01
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    Brands01 Senior Member

    Thanks for the quick response Gonzo.

    Does this mean for plank thickness I use:

    1. strip planked scantlings with interior/exterior sheathing; or

    2. Strip planked scantlings for wood-epoxy with exterior only?

    Option 1. has a thinner plank thickness.

    Thanks again for the quick response!
     
  4. Tim B
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    Tim B Senior Member

  5. CapKos
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    CapKos Junior Member

    Sorry for the question, am not designer either, but why sheathing the interior? This is substantial work, especially when considering the sanding involved.

    All the best,
    CapKos
     
  6. Brands01
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    Brands01 Senior Member

    Hi Capkos,

    This could be a case of the blind leading the blind, however I think with sheathing the interior the structure becomes a monocoque structure which relys less on the strength of internal stiffeners (such as frames and bulkheads) than the hull structure itself. It also means I can use fewer bulkheads, and theoretically should be lighter.

    If I just use exterior sheathing on plywood, I will need heavier hull planking, and more heavier bulkheads = heavy but weaker boat.

    Any more experienced people on this forum might comment to confirm or set me straight!
     
  7. Brands01
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    Brands01 Senior Member

    By the way, Thanks TimB for the pointer to the ABS site - very interesting - that should keep me busy for a while!
     
  8. Willallison
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    Willallison Senior Member

    The ABS rules will almost certainly give you a lighter structure, particularly for a boat of the sort of length you are considering. (Elements of Boat Strength ers on the conservative side).
    Having said that, I wouldn't bother with the complexity of the calculations used in ABS rules for your project - I'd just go with Gerr's stuff.
    Having said that, they will give you a better understanding of the stresses involved and the way in which scantlings are calculated.

    As Gonzo suggests, you would use the scantlings for strip plank, just substituting ply for planks. So - assuming you will be applying FRP to both sides, that's the system you should use
     
  9. Brands01
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    Brands01 Senior Member

    Thanks Will, that is exactly what I was after. I think I'll save the ABS stuff for a rainy day :)
     
  10. Willallison
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    Willallison Senior Member

    Don't get me wrong - there's nothing wrong with using the ABS rules - quite the opposite in fact. If you were trying to build a lightweight planing hull, that's what I'd suggest.
    But for your displacement boat, Dave's rules will give a strong, conservative structure.
     
  11. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Regarding ABS, I found a lot of interesting pdfs there, but nothing about plywood or sheated strip planking.
    Is it there for free download (after registration)?
     
  12. CapKos
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    CapKos Junior Member

    Hi,
    Plywood is one of the best materials on stiffness/weight ratio basis, so I don’t think you could improve the weight here. Gougeon Brothers made some tests and found that sheathing the interior improve greatly the impact resistance, which is important, but they said the weight (as well as the price) of such construction should increase, which indicate they didn’t consider decreasing in the scantling.

    All the best,
    CapKos
     
  13. Brands01
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    Brands01 Senior Member

    Will - I notice you're here in Australia - do you know of a similar site for Australian standards?

    Re Dave Gerr's rules, a strong conservative structure is exactly how I would want to build a boat of this type, so that suits me well - his book's alot more user friendly than the ABS documents as well :)

    Thanks again for everybody's input.
     
  14. MikeJohns
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    MikeJohns Senior Member

    Brands


    The Oz rules are Called Uniform Shipping Laws (USL) We can use DNV or ABS here as well. All about to change to a new system.
    USL is free to download but you can also pay for it....funny system. Search for USL downloads. USL is pretty much the same as ABS.

    Always make sure you use current rules as older ones were often defficient and have been corrected in light of earlier failures.
     

  15. Brands01
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    Brands01 Senior Member

    Thanks Mike, free sounds good :) I'll try and track them down.
     
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