: I'm either having a brain fart or made a mistake. 1/3 plywood is OK
: for cross sections... right???

Asuming you did both, and mean 1/2" ply:

Probably fine for a solo kayak up to maybe 24" wide. Where the forms are smaller you could use 3/8", and save some weight there. For a tandem, or an expedition-strength boat, I'd go with 5/8" or even 3/4" plywood for the center form, or just add some extra material to stiffen long stretches of 1/2".

There is some really cheap 3 ply 1/2" plywood out there. It is not good for this, despite it's size. On the other hand, there is some 7- and 9- ply 3/8" birch which is beautiful and maybe better than common 1/2" exterior plywood.

You can always build these light, then when you start to skin the boat you can see if tightening the skin deforms the frame. If so, glue and screw on some 3/8" or 1/2" solid wood strips to reinforce things where necessary. You'll only need a few feet of this stiffening stock. If you don't want to rip it from boards yourself, you can buy the clear trim moldings sold at most lumber departments in big-box stores. The "D" section moldings look nice and add strength.

PGJ