I built my strongback from one sheet of 1/2" OSB and a couple of
stud-length 2x4s.The depth is 8". The top is 6" wide and overlaps the
sides. There are 2x4 spacers every 2 feet. Overall length is 12 feet. I
used a two foot section to overlap the joint. Everything is glued and
screwed except the overlap so I can break it down into one 8 foot and
one 5 foot section.
I gave it a couple of coats of urethane varnish so I could use it
To get a straight keel line I stretched a string end-to-end, then
sprayed black paint over the sting. Pick up the string and the shadow
of where it was is a permanent straight line.
Here's an overall view.
And here is a detail of the overlapped section.