Mast reservoir for Optimist dinghy. Optimist Mast Lock Optiparts DESCRIPTION Stainless Steel Lock to Secure Your Optimist Mast from Coming Out of its Step When the Boat is capsized. This mastlock has a rubber insert and should be clamped the mast just under the deck. We 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 masststep.