: I know 1/2" ( 12.5mm) marine or exterior grade plywood is
: recommended for Yost's boats sections but I found 1/2 birch ply
: (wbp glued) exactly half price than marine ply.

This can be an excellent choice. Klepper has been using baltic birch plywood for their frames for decades. Check out the "technology" section for Klepper @ http://www.klepper.com/en/folding-kayaks/technology.html The picture below is from their site. These guys have been making folding kayaks for about 100 years, and I guess they know something about the materials they are choosing. I haven't seen the specs of these parts at their website, but the last Klepper frame I saw, as I recall, seems to have been made with 1/2" plywood. perhaps someone who has a Klepper will measure their frames and let us know. By the way, they use three coats of varnish to seal the forms.

As fr your materials--big question I'd have is: how amny plies in your 1/2" plywood. If it is 3 or 5 then you are using a lower quality plywood. I'd consider 3-ply to be the worst case, 5-ply would be acceptable (no worse than any other plywood) and 7 or more plies would be most desirable. With so many plies, even, if one layer had a void it would be reinforced by the many other layers.

: I also read the smaller sections can be built using 3/8"
: (10mm) ply, and save some weight there.
: Is it good also using 3/8" birch?

It will work, but I don't think the extra cost is going to be worth it. The weight savings will be negligible, as you'll be trying to cut down on what are already the smallest, and lightest forms. If you cut a 7 ounce for down to 5 ounces, you jsut save 2 ounces on each of those 2 forms.

You should have plenty of extra 1/2" plywood for your forms. I've been able to layout all the forms for two 17' Sea Tours on a single 4'x4' half sheet of plywood. The trick is to divide the panel diagonally (with a pencil line) and lay out the forms with one of them overlapping that line, and extending into the other half of the panel. When you layout your forms on the other side, one of them fits into a wide space which was left on the first part. Looks kinda like jigsaw puzzle pieces, but it can be done, and it will save you on materials if you are making two boats. Why spend money for another sheet of plywood to save the weight of a king-size candy bar?

I have plans for a Percy Blandford folding 12.5 foot kayak which is meant for a paddler up to maybe 190 to 200 pounds. it specfies 3/8" for all the frames, and also has stiffeners (reinforcements) cut from the same panel which make the larger frames stronger. I built a non-folding version of that frame, and I'm not too sure about how strong it will be for someone like me, who is over the design weight range. I may remake it in 1/2 inch plywood. Locally the Menards has some very nice looking Arauco pine plywood in 1/2" BC exterior grade for under $20 a sheet. AC is about $2 more. That would work out to 4 Yost boats for $5 a set for the forms. Once you have the forms traced onto the plywood it takes about an hour with a sabresaw to cut them out. I do a fast and rough cut around a few frames to get smaller working pieces from the 4'x4' panel, then cut out the forms to the lines.

If you use a sharp knife (and straight edge) and go over the cutting lines with a few light strokes before you grab the sabresaw, you'll sever the grain on the surface ply and not tear up or splinter the edges. With a baltic birch with many plies, the surface ply is thin and this goes quickly.

Good luck ith your project. Stick to 1/2" all the way through. You'll be happy with how light the boat is.

PGJ