Actually the old style hulls were lighter than the foam sandwich hulls.

The 5.0 & 5.7 hulls are very different. They are designed with the same philosophy as the Hobie 16, beach-proof. They have a ton of glass in their "keels".

My old style Nacra 18sq, (5.5) hulls are much lighter than a new 5.5. They are also more fragile, but they don't absorb water like the foam core hulls.