: Not sure if this will be of much use or interest to people but I've made up a
: plan for an external strongback that I'm using for a kayak build. The
: thing is that I'm building it with doubled plywood strips that I've gently
: bent into shape. In bending them I realised that it was possible to put
: uneven stress on assembled planks if I wasn't careful. Obviously it will
: depend on the type of wood you're using and the overall length but in my
: case, doubled okume 12mm ply planks, 5m long, I found it stressing
: significantly as I bent it in from a maximal width of 0.60m at the middle.

: To try and even up the stress I worked out widths along the centerline such
: that the strongback edges follow the line of a circle, and hence have even
: stress. Without a picture it probably doesn't make much sense but I have
: put a lot more effort in trying to explain it with some diagrams on my
: blog site. I'm planning to finish the strongback this weekend so if it all
: works I'll follow up with an image.

: Cheers,
: Bohdan Szymanik

I applaud your use of geometry, which is a wonderful thing and sorely lacking in the current crop of engineers. However, the wood itself is capable of evening the stress over its own length, without resort to mathematics. If you attach the ends and put in a centered spreader to give the maximum beam you require, the wood will assume a natural equilibrium position. Scribing additional spreaders to fit within the existing spaces will lock in the shape.

True, the ends should theoretically be free to rotate, but that can be taken care of by clamping during the bending process and connecting afterwards. What the geometric method doesn't account for is variations in the properties of the wood over the length - however, I assume the lengths of the scribed spreaders would match your theoretical values within a few percent.

The real advantage of the scribing technique is that the bevel angles at the ends of the spreaders is available directly and you can mark it with a pencil. I'd wager that a theoretical formula to find the needed bevels on the spreaders will be much harder to come by mathematically! If you figure it out, I'll look forward to seeing the derivation.

I like that stressed kayak-shaped strongback - too bad I saw it yesterday after I had already started my own conventional lumber strongback. It's made of three 14 foot 2x4 with 2x4 spacers to give a total width of 7.5", with birch ply top and bottom. It's glued and screwed, and oh my god, it weighs a ton! To paraphrase Darth Cheney, "you build the boat with the strongback you have...."

Cheers, Carl