Hello All:

Just posting a slightly different strongback approach. I really did like the steel (and aluminum) square tube variations posted recently, but as comfortable as I am working with steel, my problem is that I needed a mobile strongback, and therefore it had to be extremely rigid both in bending and in torsion. I have a pretty extensive shop, but alas, too crowded for kayak construction. So I must share a garage workspace with my wife's car. So you married guys will appreciate the priorities here.

The boxbeam itself is 16ft long x 8" x 9". The whole affair can be made from two sheets of 1/2" ply (although the one shown is made from one sheet of 1/2" CDX, and some misc other stuff on hand.) One of the tricks in box beam construction is to have well fitting blocking inside, (in the one shown, there are five blocks) one at each joint and each end. And it must have four sides. Before I put on the fourth side I was able to twist the beam quite easily.

To test how rigid when completed, I hung two 100 pound buckets of scrap metal from the ends of two pipe clamps approx two feet from the center of the beam and opposing each other at opposite ends of the beam, and I was unable to detect any measurable twist with a fairly precise level. So testing it successfully with approx 400 ft/lbs of applied torsion I felt fairly safe in mounting casters and rolling along.

Happy trails
ron