: Summing up
: - rigidity of the beam (assembly if necessary) whether internal or
: external is fundamental.
: - Straightness and twistyness of the beam is unimportant unless one
: uses the beam for alignment.
: - Alignment of the forms is fundamental.

Thanks I agree! I don't really understand the apparent need for a strongback beam to be perfectly straight, although I imagine it would save time in the form alignment process. Being *stable* is certainly necessary! I left mine supported for a few days before putting on forms, to check it had warped to its final shape.

But once those first few strips are on, horizontal flex in the 2x4 internal strongback is practically non-existent. Vertical flex I found was no problem either with 4" of beam height. I should think that vertical flex is an issue though with a softer material like PVC pipe. Incidentally, I found that aligning the end forms was a much bigger challenge than ensuring form alignment on the 2x4" beam was true.

I built a 2x4 strongback using cheap framing timber, which had a horizontal variance of about 1". Note the curve in the pink beam in the pic, and the centre line drawn almost to the edge near the middle. A year later, I've recently been able to remove the strongback having done the glass fibre work, and I'm glad to say it's still as true as it was when I started!

http://ianjohnsonz.wordpress.com/category/guillemot-kayak/3-strongback/