Just started cutting wood for my 11' SUP and just wanted to pint out to others starting this project that the materials list calls for 6" wide ply (150mm). I actually think you need slightly wider (6 1/2" would work). Being frugal with my marine ply I cut 6" strips (150mm) as per the instructions then after scarfing (but before glueing luckily) I dry assembled and layed the spline template on it, just to make sure - and it didn't fit.
I scratched my head, checked my measurements again, checked the materials list again but it definitely wasn't wide enough. I lined up the very edge of the tail and nose on one edge and the underside hung off the board - you need 6 1/2" - 7" wide board (I even checked the file in photoshop with the rulers).
I was able to get around it by slightly kicking out the short board at the scarf joint.
Just a heads up for anyone else about to cut their precious marine ply...
I am in the process of evalating a suitable plan for an SUP right now. Still not building but hanging around to see how things work
One thing that might be interfering with your work can be the scale of the drawings - might they be made to be printed in a standard american paper size (letter or 8 1/2 inch paper) and you are printing it with an A4 printer?