This clamp fits around the lower mast and is a really neat way of stopping the rig from coming out of the boat.

Stainless steel lock to secure your mast from coming out of its step when the boat is capsized. This lock has a rubber insert and should be clamped around the mast just under the deck. Optiparts suggest you use the collar in addition to the rope locking system to create double security. The lock will be provided with a line to attach it to the maststep.