Plywood gussets

Discussion in 'Boatbuilding' started by wudenbote, Oct 2, 2008.

  1. wudenbote
    Joined: Aug 2008
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    wudenbote Junior Member

    Is there a generally accepted percentage for the size plywood gussets for sawn frames should be in relationship to the size of the frame members? Thanks Much!
  2. thudpucker
    Joined: Jul 2007
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    thudpucker Senior Member

    Questioning how to support stress joints, you need to know a heck of a lot about the Material, size, angles, and stress's from all four directions.

    I suspect you are talking about supporting Right-angle butt joints with little plywood triangles.

    Maybe if you could provide a sketch?
  3. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Right--- not enough information. Still, when gussets are plywood, think about the fasteners into the frames. Those frames could be oak or cedar, meaning the size of the gusset is also determined by the number, size, and location of the fasteners---- the goal would be to assemble a joint where it would be difficult to predict which would let go first-- the fasteners, the frames, or the plywood.
    Good seat of the pants engineering would be to do a test on the bench, with long frame pieces for added leverage. Seeing where the joint breaks tells you the weakest link in your "chain". Then either strengthen the weak area (frame, gusset, or fasteners) or weaken the stronger areas.
    Over years of experience, some builders can come close to guessing the right combination of parts and processes that create a balanced assembly.
    You will need to either rely on an existing design, test as described, get lucky, or over-build.
    As said already, a sketch would help, and someone here, given enough information about frame dimensions, materials, frame angle, location, and intended use, might help.

  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The average rule is to make the gussets 75 to 100% of the sided dimension of the frame. An example would be if you have 1" frame stock, you'd use a 3/8" gusset on each side of the frame at the joint (totaling 75% of frame thickness), but this wouldn't have a lot of fastener holding power, so you'd up to 1/2" (100% of frame thickness) on both sides. If you have closely spaced frames, the 75% dimension works okay and keeps things light. Use 100% if the frames are widely spaced and/or you need the heft.

    The heights the gussets travel up and down the frame futtocks (frame segments) should be at least 2 times the molded dimension, preferably three if you have the room. This is because plywood isn't as strong as solid stock longitudinally and needs the extra length (purchase). An example of this would be 3.5" molded frames would have a minimum of 7" of gusset on each side of the futtock joint, preferably 10.5"

    Ideally, you'd scale the gussets to the load requirements as part of the frame engineering, with what ever safety margin you elect to incorporate.
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    To avoid over- or under-designing why not build and break some test pieces? That'll cover the whole gamut of what could go wrong from glue bonding to Euler buckling.
  6. Lawrence101
    Joined: Oct 2011
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    Lawrence101 Junior Member

    I used a "brace" piece of hardwood across all frame joints, and "sandwitched" it between 2 pieces of 3/4 ply, pre-drilled and screwed together using 2" decking screws .The whole assembly was soaked in epoxy. This made for a very solid joint.

  7. Lawrence101
    Joined: Oct 2011
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    Lawrence101 Junior Member

    EEk! , just saw how old this thread is .:eek:
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