Hi,
I did a fair amount of strength testing re. butt joints for a Pygmy kit that I am starting in the Spring, (having played with puzzle joints too when I contemplated using plans for a different kayak). I also created a jig to do splined joints in the ply so that I could get a "smoother" outside. I then tested the strength of various combinations of methods using various glues including the Pygmy joint method of glass on both sides. I posted the results compared with a solid piece of ply with no joint (page 2 of this forum) but to save searching they are are as follows:
- strongest was the Pygmy method - the joint did not break at all - it broke outside the taped area
- next was a splined joint using Gorilla glue - no FG either side (note that I forgot to dampen the wood too!)
- next was a splined joint with epoxy - no FG either side

All joints were clamped then tested in exactly the same conditions till they broke.

I have a pdf file with all the results and pictures of the jig etc but I have no idea how to post it as an attachment

Hope this helps.

Geoff