TECH TIPS PAGE FIFTEEN
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Design is a subject that has been covered before but is so key that it warrants more comment. Key to this discussion is how to get the finished canoe to look like the one you saw in a some picture.
Edwin Tappan Adney in his book, The Bark Canoes and Skin Boats of North America has documented the process of building and the dimensions of the completed historical specimens. However, I have never seen anything on how one arrives at those finished dimensions. Click here. and look at measurements and photos of canoes that I have done to get an idea of how design points are arrived at.
The key points to design are:
Sheer - This is the line of the gunwales when looking at the canoe from the side. Straight sheer would be a gunwale profile that doesn't curve up from the middle to the end. This is created by the height sticks (see below)
Rocker - This is the line of the bottom of the canoe when looking from the side. Using a narrower frame and placing the gores (the cuts in the bark that allow the bark to be folded around the frame) about 12 inches apart will give you less rocker. Most birchbark canoes will have a bit of end rocker.
Bilge - When looking at the hull shape this is the bottom of the canoe. A rounded hull has a full bilge. This is established by the ribs. For a flat bottom canoe only place hot water on the place where the bend will begin to the end of the rib. By not heating the mid part of the rib it will not want to bend. For a shallow arch hull (slightly rounded) put some hot water on the entire rib. Even though you don't bend the mid part of the rib it will want to bend once in the canoe.
Chine - This is where the hull curves up from the bilge to the side. Flat bottom canoes usually have a sharp chine whereas rounded hulls have a more moderate chine. This too is created by the way you bend your ribs. Start your bend the distance of 4 fingers placed parallel to the inside gunwale. For a more rounded effect start the bend just inside that line. For more tumblehome (where the sides curve inboard) start just outside the line and continue up the sides.
Stem - This is the wooden piece that forms the end of the canoe. For accuracy in stem height make sure the bark does not sag near the ends. A sharp bend in the bark from the end of the frame to the end of the bark is crucial for this.
Height Sticks - These are sticks that hold up the inside gunwales. Once all the bark has been sewn together, the inside gunwales, spread apart by temporary or the permanent thwarts, are dropped in and held in place by the "height sticks". They establish the "sheer". Therefore you will need to establish the measurements of the height sticks in order to establish the sheer. For relatively flat sheer I typically use the same measurement for the height sticks that are next to center thwart and the intermediate thwarts.
Depth - The depth is determined by the height sticks and shape of the ribs. Using the same height sticks and the same width of frame and gunwales you will have a deeper canoe if the bottom is rounded as opposed to flat. Therefore if you found a canoe that is too deep simply heat the ribs and straighten them. Then reheat them but only from the line of bend to the end and you will arrive at a shallower canoe with a flatter bottom.
More on Design by John Lindman
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