There was a thread on this repair a few years ago. I have repaired 4 Hobie 18 Magnums, all of which cracked left front and right rear under the crossbars, both inside and outside lips. All Magnum sailors should check this.

These boats were not designed for such a load- the excess torque of wings-- sometimes with 4 adults on the weather side wing. Only the early 1978-79 Hobie 18s cracked here (or pulled the decks off) before wings: the D-shaped reinforcing plates then became essential to long Hobie 18 life. I still have 2 of these ironclad beasts!