Although billions of kayaks have been built with internal strongbacks ... I'd never use one unless it was super rigid and super stable. I'd certainly not use 4" Sch 40 plastic pipe if that's what you're considering ... it's like a noodle.

For me ... it's an external plywood box beam strongback ala Ted Moores ... had mine for years ... was dead nuts stable. They're simply not as wiggly as internal strongbacks.

Ken Blanton

: I have been considering a 2 X 4 strongback made of plywood, a 2 X 4
: inch aluminum extrusion and schedule 40 plastic pipe.

: The 2 X 4 plywood is pretty straightforward and inexpensive. It
: would not take too long to make--maybe a couple hours. Some care
: has to be done to make sure it is straight.

: The aluminum extrusion is straight and ridgid, but will probably
: approach $200. If anyone used a 2 X 4 rectangular extrusion, let
: me know your experience. If other than 2 X 4 extrusion is
: suggested, I would like to hear your experience.

: I have heard of people using schedule 40 drain pipe (4"?) as a
: strongback. It would seem to be a good idea. It is quite ridgid,
: VERY cheap (less than $10 for a 10 ft section). I am not sure
: the best way to attach the forms, but it does not seem like it
: would be a big deal. The 4 inch sched. 40 is a little over 4
: inches in diameter and would require some modification to the
: strongback dimensions in any plans I bought. If anyone used
: schedule 40 as a strongback, I would be interested in their
: thoughts on how it was modified to attache the forms and stem
: pieces, if 3 or 4 inch was used and any other feedback.