F18 rules

D.3 HULL SHELLS
D.3.1 MATERIALS
(a) The hull shells shall be built from polyester or vinylester resin, glass
fibres, core of PVC or balsa or felt. The combination of wood-epoxy,
injected plastic, which shall not be altered, other than locally for fittings,
and passage of equipment and normal reinforcement. Epoxy glue is
permitted for joining components. Every material that is not expressly
permitted is prohibited.

Since the world there's some rumorous about shockwave..

Shockwave core is Corecell.
Corecell isn't PVC or balsa or felt
So Corecell is prohibited in F18 rules.
Corecell marketing is that it is a better alternative to PVC.