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woodstrip form conversion

Discussion in 'Wood and Canvas' started by Mark Engler, May 12, 2009.

  1. Mark Engler

    Mark Engler New Member

    After successfully putting together a woodstrip boat, I'd like to tackle a wood-canvas canoe. Has anyone used stripper plans to make a wood-canvas form? I know the stations need to be reduced, but by how much? 3/4" seems about right to me. Any advice or tips would be appreciated.
     
  2. Scot T

    Scot T LOVES Wooden Canoes

    Interesting! I had thought about this a while back as I have a couple nice stripper forms gathering dust. Here's what I came up with.

    For the sake of easy measurements lets assume the cedar/canvas canoe ribs are 5/16" thick, the planking 3/16" and the sheathing for the form is 3/4"(12/16) which gives us a total of 20/16 or 1 1/4" inch. The stripper forms I have (and most I believe are the same) are designed to use 1/4" strips so subtract that from the 1 1/4" and I came up with 1"...now if you take into consideration the thickness of the steel backing bands then one might want to remove (1/32 +/-) that as well.

    Now, if my measurements are all wacked I'm hoping someone, more learned than I, will pipe in and correct me.
     
  3. peter osberg

    peter osberg LOVES Wooden Canoes

    Notch some ribands into the forms and you have a skeleton to bend the ribs on. then the stems need to be notched in (as well as the keelson) and the planking applied. There is enough space between the ribands to clinch the copper canoe tacks, The steel clinching bands are unnecessary unless you are into production mode. This has worked for me, 8 canoes so far (5 patterns).
    Peter
     
  4. Todd Bradshaw

    Todd Bradshaw Sailmaker

    If you want the design to truly be accurate in shape to the original, any deduction made for the thickness of the hull has to be graduated slightly, depending on the angle that the hull material crosses the forms at. Assume, for example, that we determined that the difference between the thickness of the new w/c hull and the original stripper hull required a 1" reduction in the size of the station forms to make a boat the same size. That's fine at any spot on the form where the hull material crosses our station forms at pretty much a 90 degree angle (as it will on much of the bottom and almost everywhere on the middle station). Figure A in the drawing shows such a hull/station form junction.

    However, if we have a strongback with parallel, crosswise stations set up on it (as we would have had for a stripper) and you start approaching the tapered, pointed ends of the boat, the hull material is meeting the stations at an angle (just as the strips did).

    If we simply deducted our 1" allowance from those stations, it wouldn't be enough and our finished boat would actually gain girth slightly in those areas. This can either produce a boat that's a bit fuller in places than the intended design, or it can possibly make one that is not a fair shape and has unwanted bulges or hollows here and there. As you can see in drawing B, a 1" deduction in places where the hull crosses the stations at an angle may noy be quite enough if you really want accuracy.

    To prevent this problem, the deductions to the station forms need to be made taking the angle at which that particular portion of the hull meets the station in mind. Designers and builders us a home-made gizmo called a bevel board to help make accurate deductions. They are also often dealing with hulls having frames (ribs) and planking that may total 4"-6" thick - a situation where the errors are much larger than they would be on a canoe hull.

    I doubt you'll need a bevel board for a canoe, but as you plan your deductions, keep the angles in mind. If you have spots where the angles are in play, you may want to deduct a bit more in those areas to get a fair and accurate hull. Some station forms, like those a few feet from the end, may need no additional deduction on the bottom where the planking meets the form at about 90 degrees, but may need an additional small bit removed on their sides where the planking meets the form at a pretty steep angle. Is all this critical on a canoe hull? Probably not. But it never hurts to have a good understanding of what's actually going on and to be able to compensate for it if and when it's needed.
     

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  5. Louis Michaud

    Louis Michaud LOVES Wooden Canoes

    What deduction should be used to compensate for the rebound of the ribs (mostly along the bottom I figure...)?

    Louis Michaud
    Rimouski, Quebec
     
  6. peter osberg

    peter osberg LOVES Wooden Canoes

    clamping the steam bent ribs to the ribands allows them to harden up nicely in the shape desired. If you have a longer flat area on the bottom clamping a straight board or brace allows it to keep that profile. I would clamp initially (I don't have that many), then often use long fine screws and washers to keep the ribs in their shape on the ribands and take the screws washers off as I was planking. Sanding seems to make the screw holes really hard to see (and a little wood swelling once on the water).
    To keep the form making simple, the depth of the notching of the forms and the dimension of the ribands will give you the compensation for the thickness of the ribs and planking (may vary with the design or wood chosen).
     
  7. Douglas Ingram

    Douglas Ingram Red River Canoe & Paddle

    You need to make adjustments for rib springback only on the flatter sections of the canoe bottom. Any section where it is still rounded won't have any springback. The wider the canoe bottom is, the more allowance there needs to be. For a 36" canoe you can expect about 1/4"-5-16". Keeping the ribs wedged to the form as long as possible reduced the amount of springback, as does using thicker ribs.
     

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