One of the challenges of building things that are all curves and no squares is figuring out how to bring all of the parts together without an advanced degree in mathematics. The front three deckbeams of this kayak are curved and they are tenoned into the gunwales which are sprung out to shape at a 25 degree angle.
A couple of minutes with a scrap of wood, a bevel gauge, and a pencil gave me the offset that I needed to be able to scribe the shoulders of my tenon with the deckbeams clamped in place.
A spacer creates the offset and extends the angle of the gunnel across the deckbeam.
Repeat the process on the other side, connect the lines from one side to the other, and lay out a 3/8" tenon to end up with something like this.
Saw away the waste...
And a little paring leaves a tenon that is ready to slide into it's mortise after rounding the corners.