Sierra 18-3214 Impeller Kit for Mercury A boat impeller is a part that is primarily made from rubber, which projects as a series of vanes that are molded around a metal hub. The tips of these flexible vanes wear down over time through use or if the boat is used in waters with a sandy bottom. It is the abrasive sand that essentially grinds down the vanes causing the impeller to wear out quickly. When that occurs, during pump operations noises such as rattles, grinding, or knocking, signal that th