So, 5mm vs 4mm is 25% more wood. Wood is a big percentage of the materials in you yak. Materials: wood, epoxy, fiberglass, varnish, a seat... If you don't mind a slighly heavier kayak then use the 5mm plywood. I would bet such a yak is at least 10% heavier maybe 20% heavier worst case. CLC perdicts a CH16 built of 4mm plywood will weigh 42lbs. so +10% is 46lbs and 20% would be 50lbs. I would guess you end up somewhere between those weights. If that is OK for you, build away!

Some folks might worry about the different bend capability between 4mm, bendier, and 5mm, less bendy, when trying to shape the yak. I would guess that a CH16 has very subtle bends and the 25% more wood will not impact the attempts to get the plywwod to properly become a kayak. Of course, John at CLC might have something to say about this.

Also don't forget your time. You are going to spend many many hours buiding this yak, don't skimp on materials.

Have fun building!

Joel

I built an Eastport Pram with parts that were laminated from 1/4" plywood instead of the 6mm and 9mm specified.  I actually like the extra beefy feel.  And for me, heavy is heavy.  A few extra pounds is not an issue, especially if you get more strength and better wear and tear. 

   5mm is 5-ply, versus 3-ply for 4mm and smaller. This makes the 5mm stuff something like 2 or 3 times stiffer across the grain. The thicker ply won’t bend over the CH-line’s radius deck- the 4mm just barely does.

Options, if 4mm just isn’t in the cards (I have to drive 6+ hours each way, or pay about $100 freight for the first sheet of ply -I know the pain of being far from marine sources!), then you can strip the deck -the forms you would need to make are all the same radiused top shape; or use some locally-sourced 1/8 luan/doorskin that will be glassed both sides.

Good luck!