72' Billboard Skinned Longboat/Umiak

Discussion in 'Boatbuilding' started by charlesakeem, Jan 29, 2020.

  1. Rumars
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    Rumars Senior Member

    Just to give you a picture of how the structure looks, pay close attention at minute 5 when they are lashing the frame. Observe the stringer and frame spacing, they can barely fit their fingers in the resulting space. This will be the same regardless if you lash, rivet or glue the structure. If you go to much deeper frames you can increase frame spacing but not stringer spacing (stringers need to suport the skin). A boat with a 6:1 L/B ratio will also need diagonal strapping to counter torsion loads. Please keep in mind this is a 36' boat, imagine the wood dimensions for a 72' one.

  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Structurally, the skin isn't acting as a brace, and that requirement has to be met in other ways. Then you have the possibility of chafe, as the skin moves relative to the framing. But I do like the idea of those cheap signs as tarps !
  3. charlesakeem
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    charlesakeem Junior Member

    @Rumars Thank you for the video, good to see a nice visual on something. Man with all that lashing they did surprised they didn't just build the thing out of wood. But I'm assuming historically in that area much easier to harvest and lash thin split saplings and such than cutting and planing good usable straight lumber.
    @Mr Efficiency Hadn't considered skin chafe. Can wrap my mind around the bracing structures though I hadn't given as much thought to it's stiffening effect. Can probably get much of that strength by making braces on the corners with ply but that opens up the whole build thing thing with wood.
    On the frames @gonzo or one of the math types probably knows exactly but how does the bracing effect scale with ply thickness What I mean is does a 1/4ply brace 30% of what 3/4 ply would on say the walls of a Trilobyte type boat or is it something like any bracing will generally get you 80% of the way there as far as strength is concerned?
  4. gonzo
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    gonzo Senior Member

    The structural calculations are far more complex than a simple percentage. You would be better off using an existing plan.

  5. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    @charlesakeem I am curious about how do you actually picture your boat. Wide expanses of unsuported fabric are not going to work, you will need a structure similar to what you saw in the video.

    If you don't like plywood, double diagonal would be a good option for a square boat.
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