Sakari says:

I do have a vague idea of doubling; in other words, using 12mm strips
instead of 6mm plywood, say.

You need to carefully assess how much of the plywood is providing strength in
the 90degree direction, and apply a similarly-strong layer of strips in that
direction, as well as the obvious similarly-thick strips in the zero-degree
direction to replace the outer lamina of the plywood.

Steve
Stephen C. Baker - Yacht Designer
http://members.aol.com/SailDesign/pr...cbweb/home.htm