: Hi Mike

: The absolute best material would be 3/4" fir plywood. It will take
: staples, screws, the torque of bending strips around and if you decide to
: go stapless, then you can groove one side of the plywood for some spring
: clamps to grab into and it will hold very well. The particle boards tend
: to break up with use, as does the MDF ( basically just a denser p.b. ) and
: the 1/2" plywood tends to be just a tad too flexible when bending
: things around. Also is a smaller target for stapling. You could go with a
: CDX grade DF and still keep in a good budget
: or go up to an ACX grade and that will give you a good side to do all your
: markings and make notes. I would also recommend the 3/4" DF plywood
: if you plan on making an internal strongback. Although there is a strong
: contingient of metal workers out there that will fight you tooth and nail
: to get you to use the metal studs. Either way will suffice. Welcome aboard
: and keep us informed of your progress.

: Rehd

3/4 inch AC plywood makes really good forms but in anycase use some kind of plywood, OSB and particle board tends to split and flake. 3/4 plywood is pretty expensive. The perfect material for forms would be free. I made one set of forms from 1/2 CDX (sheathing cut-outs for windows in a home addition) which I think you could easily make several boats on. 1/2 inch plywood underlayment is pretty good material. It has one fairly smooth side so it will take a good pencil line, it is 5 ply rather than 4 like CDX and has very few voids.

I have used 1/2 inch OSB for a strongback with OK results, but it's doubled up to inch thickness with offset joints, glued and screwed.