Id seriously consider what Etienne suggested, just id place the stringers at 45 degrees, in a wide net fashion or VVV fashion, and use polyurethane foam strips (construction material density) rather than wood. That to count on the "corrugation" created by the internal glass rather than the foam itself. Actually 4 stringers going middle keel-top bow and middle keel-top stern (basicaly a wide V on either sides) plus a longitudinal at the waterline should be quite enough to stiffen the hull, obviously you should have an internal keel, foam again, to form a stiff system and a foam sheer clamp wouldnt hurt either.

: An extra thought on the subject. In order to stiffen a very thin hull that
: uses light glass as well you may consider internal longitudinal stringers
: (half round) in the very lightest material you can find. Wooden or carbon,
: or foam or whatever, running lengthwise along the hull three inches apart
: or so. These would go between the glass and the wood and would seriously
: stiffen the hull by adding effective corrigations to the structure of the
: glass epoxy matrix.

: Etienne