You are probably right, I used too many. On the earlier frames I didn't use any, even when I was doing doublers. But as I got to putting three layers of solid wood in one go I used them to position and hold it while the glue went off. This frame divides a waterproof section behind it to the transom, the plans said doublers all round screwed on 100mm centres but I think it was probably just around the outside?
Went a bit mad with the drill, but I will have no worries about the frame being stiff and holding its shape! I take your point and think you are probably right, one or two per section 30mm x 45mm and a few in the solid wood to hold it to the ply.
The transom is covered in a layer of 12mm ply and the Charlie Wipple build thread on John Welsford's site shows photos with a similar screw pattern where it meets the solid wood frame, I will check it.