Yo Jeff

I made an mdf strongback for my expedition single. It was ok and definately straight and square. But it weighed alot and sagged under its own weight. So I added some reinforcing angle steel bracing to both sides of the bottom. This straightened the sag up, but gave the strongback a memory - If you lay it unsupported on its side you got a permament curve in it when you put it upright again. But you could bend it straight manually. I think this would have worked well if I had the space to set up the strongback and forms and leave them in place on the stands (bolted to the floor). But I don't have the space and the project had to be slung into the rafters (where it is now) for periods when I couldn't carry on construction.

Figuring I was looking at a long build and wanted everything as square and true as possible, especially for the expedition strongback which is almost 17' long I bought some fairly basic grade 12mm ply for not alot of $$$ and ripped it with my skill saw with its little guide. This strong back was lighter and stiffer than the mdf. So that is what I used. Easy to make, square and straight.

The common 4 x 2 in NZ is pinus radiata and it would be incomprehensible to find a piece that long that is and would stay straight. Pinus radiata is also heavy stuff and I would still have had the sagging under its own weight problem.

Having faithfully followed Nicks form lines; my artificially made 4x2 box beam made of ply and fairly tight tolerances meant that it was pretty easy to line everything up, prior to stripping. There is no doubt that you could line all your forms up with a quite curly strongback, but it would take a little longer.

Cheers Pete