The most vulnerable part of your lower unit is the skeg. It often ends up damaged, requiring repair or replacement. Now, the revolutionary Blackfin provides a breakthrough in skeg protection, repair, or replacement. Blackfin is made of a rugged, nylon composite, and it's easily installed, saving the time and expense of a trip to the marine shop. Unlike typical skegs, Blackfin offers performance enhancing features that provide better handling, quicker plane times, and more steering control. Comes with all you need for a do-it-yourself application, including special marine adhesive and complete instruction. Sizes are determined by your lower unit gear housing measurement: S(10-1/2" to 12-1/2"), M(13-1/2" to 14-1/2"). Exception: Mercruiser Alpha One measures medium but requires small instead.