Plywood can be used but might warp. It also may not hold a staple as well as particle board. CLC used P-board and routes in the tick marks. When I cut my own forms I use 1/2" MDF (medium density fiberboard). 3/4" boards may throw off the curve of the deck.

Dan

Hi   I'd say from my limited experience of building a wood duck hybrid and a petrel play, that plywood is fine, and if anything better than MDF or other softer materials because the staples grab better to plywood, which in turn will make the deck fairer. I'd stick with the 1/2 inch though as using thicker forms will subtly change the shape of the deck because the strips only touch one edge which will have moved by 1/4 inch at every form and you might get unexpected bulges or dips.

Pete

you can use 3/4 ply if you want.

i have built with both. for me, i use MDF becuase its cheaper.  if i had some ply lying around, i probably would have used it.

on the staple issue, the MDF is softer so a staple under a given pressure goes in easier...and typically holds. however, sometimes when a lot of pressure is necessary, MDF cannot hold a staple....but its relatively rare....and plywood would be better....but then you have to have a staple gun that can push a staple hard enough and some of the less expensive guns might not be able to do the job.   

you also have to think about being able to remove the staple without breaking it.  and i have found when i get a staple with a super solid hold, it's easy to break the staple trying to remove it and then its a big hassle to dig it out.

so it's a bit of a balancing act on staples, staplers, holding power and removal..  most of my work, candidly, has been with MDF....and it seems fine and i have it worked out..  at the same time, i know people who get the job done with plywood too and they seem happy.

with respect to the forms width, the key thing is the alignment of the form on the reference line for its size/station -- which is typically the forward face of the form in the front of the boat and the rear of the form on the rear of the boat.

if you use 3/4 inch forms, the spacing between the forms (e.g., the space of the exposed strongback between two forms) changes a bit (compared to 1/2 inch forms) but the spacing between the face of the form that aligns with the station stays the same (typically 12 inches)....so depending on how instructions are written, it can cause confusion.  but if you are careful and understand what you are doing and make the right adjustment, you can do 3/4 inch with no problems.

i hope my comments are helpful.

h