Great book. A real classic.
The way a composite sandwich structure (= a surfboard) break under flexural strains is well known : under strain the overall structure flex, skin that take compression load elongate negatively so it buckle, shear load core that try to keep it in shape. When core is overload and break, skin debond, and break. this is the end...
Every thing that increase flexural stiffness increase flexural strengh: stiffer foam (or foam with stiff stringer), stiffer skin, etc...
Édit: ... but a thin wood stringer that cross middel of board is not the most effective way. For sure it increase overall stiffness but it's not so effective to prevent local buckling, and because glass contact surface is reduced it's not effective to keep thin skin to buckle.
If I "get" your last comment you suggest multiple stringers?
You omit to mention the part the rider plays in the deal. Some guys put their boards in harms way because of the way they surf. I snap most boards landing free fall take offs or late re-entries in punchy beachbreak conditions, but I've blown the bottom out of a few boards tuberiding.
In particular kneeboards are a different animal because of the fulcrum effect of the knees parallel at the half way point of the board and the relative flatness across the deck (because of the width)
I have come to the conclusion that, for me, a decent stringer is a good option. Just off the back of 6 months travel in west Aus (Granites, Streaky Bay, Cactus, Red Bluff, Tombstones, etc.) and the stringer board is still holding together.
Great comments, thanks.
I'll post some pics as work progresses.
As always getting the pads and glue off is a major part of the job.
Just because I can I'll put the board on a rocker bed and use the CNC to route it. Could be a disaster, lol.
For sure kneeboarding is a little différent. I would suggest you two stringers, one under each knee. Wide stringers in balsa or hard "strucural" foam. Cap them both top end bottom with ud glass end use multi layers in multiaxis of glass under your stance to spread load to those stringers. Finish laminate as usual. An alternative build can be the "old" Classic windsurfing pvc sandwich build with T or Omega stringers on top, it's still the best weight/strengh build when well make, but far to be the easiest to do well, and still expensive. As a cheaper alternative you can use balsa for sandwich core, but at least 2mm.