Folding Canoe Requirements

Discussion in 'Boat Design' started by ConcertinaBoats, Jul 23, 2009.

  1. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    That looks like a good start. Several problems need to be addressed before going any further, related to the shape of the assembled boat. You've drawn a flat-bottomed hull, rare in a canoe. A boat that is pleasant to paddle needs a rounded or five-sided hull for reasons explained below. This will have an impact on your design. Also it looks like a half-canoe at present: if you plan a flat stern or transom, it will have to be above the waterline or it will create a lot of drag. That's why almost all canoes are double-enders.

    Remember that a short boat is a slow boat if only human power is available.

    Flat canoes are unpleasant to use. The pirogue is a flat-bottomed boat similar to a canoe but it is bigger than a canoe. A flat-bottomed boat has shallow draft and sits high on the water, raising the center of gravity. At the same time it has a high righting moment immediately it is heeled which sounds good. But as it continues to heel the righting moment decreases, creating a feeling of instability that paddlers are not comfortable with. To be comfortable (and safe) a flat-bottomed boat requires enough width that it is difficult to paddle. Pirogue are not usually paddled, they are amost always poled.

    Canoes and kayaks typically have narrow bottoms to reduce buoyancy low down; the bottom flares out to a greater beam at the water-line. This allows the boat to settle deeper in the water for greater initial stability, and also ensures that stability increases as heeling increases, which allows the paddler to concentrate on paddling instead of staying upright.

    Rocker (longitudinal curve of the bottom) will also lower the center of gravity, and helps make the boat easier to turn, but cannot be overdone or the boat will become directionally unstable and the paddlers energy will be dissipated in steering effort. Greater rocker is more common in a rowboat where the oars do not generate the turning force of a paddle.
     
  2. ConcertinaBoats
    Joined: Jul 2009
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    Location: UK

    ConcertinaBoats Junior Member

    I had not considered the effect on drag of a flat back. Thanks for pointing that out. I am modifying my design to make it double ended which means it will be just under 11ft long. I take your advice about a flat bottom but intend to keep my design that way. I have paddled in flat (or close to flat) bottomed canoes before and find them to be very stable.
     
  3. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    If you start with a round-bottomed boat then make it flat with the same draft, you have added buoyancy low down. That is the opposite of adding ballast low down, which increases stability. But the boat rises in response to the added buoyancy, so up goes the center of gravity. Both of these make the flat-bottom boat less stable.

    To restore the lost stability the beam must be increased; for the same draft stability is proportional to the cube of the beam. as the boat gets wider it gets harder to paddle and has more drag. Been there, done that. It will work but it's not a good solution.
     

  4. Squidly-Diddly
    Joined: Sep 2007
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    Location: SF bay

    Squidly-Diddly Senior Member

    I think your fabric will bow in far more than you expect

    and as the ribs are vertical it will make the boat really stick in the water.

    I made a kayak from PVC piping and waterproof nylon. I thought I had it stretched TOO tight and the lengthwise frames were only about 4" apart and it still bowed WAY in.

    The fabric didn't seem as though it had any stretch to speak of which is why I selected it.

    I had 6 frames from bow to stern supporting the 1" pipe and didn't see hardly any bending, but it must have happened and a little bending of the frame inwards allowed a lot of inward bowing of the fabric and great loss of freeboard.

    I ended up laying 1 layer of fiberglass over the nylon.

    Unless you have some framing you aren't showing in that drawing I see problems.

    I was also surprised that the water would push the fabric in so much just a few inches from the surface.
     
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