: Most methods I've seen beginners use, and those using single 2x4s
: create more problems then they solve. (House framing is a
: process of using 2x lumber that will continue (or want to
: continue) to move and twist for years and years, in webs that
: are made straight, true and flat by multiple members resisting
: each other. 'Problems' are worked out with a sledge hammer-and
: then nailed down. The sheathing material finally locks it all
: together. You will never see a single 2x4 used to create a
: straight unmoving element.

Does anybody use a single 2x4 for a strongback?

One book refers to a 'traditional strongback' that's a ladder-like 2x4 structure- but I've never seen that style recommended in the older books I own.
Both of those 2x4 ideas don't sound very practicable to me.

I've been building strip canoes and kayaks (from time to time) since David Hazen's book and the MCA 'How-to' manual, and I've always used a T-type beam. Two decent pieces of 2x6 or 2x8, reasonably dry, screwed together with deck screws make a pretty stable strongback for me. I've been using sawhorse brackets to make quick legs.
However:
I live in an area where the relative humidity doesn't vary much with the seasons, so wood doesn't tend to 'move' much.
I generally don't take very long to get a hull stripped and glassed- things just don't warp and move over a period of a week around here.

Definitely, box beams are stiffer and more stable structures, especially if you use plywood. If you anticipate a long (months?) building project, or if you have big humidity swings, or if you can't get good 2x lumber, that's the way to go, IMO.

But, ;-) it doesn't have to be perfectly straight, no matter how you build it. ;-)