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