Dick,
You will pay a premium of about 40% for 10 foot sheets of plywood. Buy 8 foot sheets of Bruynzeel plywood. Being a product of Nederland it is obvously superior. Scarf two sheets of Bruynzeel together and build any length kayak you would like up to about 15 feet.
A twelve foot kayak is normaly intended for a child or some-one who has never tasted bier und wurst.
If you must build it to 12 feet make it very wide and be prepared to go very slow.
Greets
Charlie

: 8 foot sheets here in the US too. I went to great pains to match my grain
: when building two boats simultaneously (my sole boat building project to
: date), and wouldn't ya know... the finest, most invisible joint of all
: somehow ended up INSIDE the hull. Doh! I still laugh and shake my head
: every time I look at it.

: I found beveling scarf joints with a block plane to be a pleasant task, by
: the way. Nuthin' to it.

: Cheers,
: Kurt