Most boats rely on raw-water pumps to circulate coolant (lake water) through the engine. At the heart of the pump is the impeller, a flexible rubber part with a huge responsibility. It draws lake water up into the engine to keep both the engine and the exhaust components at a safe operating temperature. Raw-water pumps can pass some sand and other small objects churned up from the bottom of the lake, but as vigilant as you may be, you're still dealing with a part that continually deteriorates over time. Vanes of the impeller eventually crack and break off. In some cases the body of the pump erodes away from passing sediment and debris. Either situation results in pump deficiency which sooner or later leads to starving the engine of coolant.

Raw-water pumps usually fail when you least expect it and rarely at an opportune time. Failure usually causes excessive engine and exhaust temperatures and if not noticed quickly, can lead to expensive repairs, not to mention being stranded. As a result of this, periodic impeller replacement is necessary. Some boaters replace their impellers every season as cheap insurance, but most manufacturers recommend replacing them every other season or roughly 200 hours.