skeg

A skeg (or skag) is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard. In more recent years, the name has been used for a fin on a surfboard which improves directional stability and to a moveable fin on a kayak which adjusts the boat's centre of lateral resistance. The term is also often used for the fin on water skis in the U.S.A.