Impellers, thermostats and water pumps are essential components; they regulate efficient working temperature in your marine engine. Marine engines invariably use the sea surrounding the boat to cool the engine. If it is raw water cooled, i.e. passing seawater directly through the engine, it will inevitably require service. Other system types use the cooling effect of sea water indirectly via a heat exchanger or keel cooler. Regardless of type, simple routine maintenance of cooling systems will assure your vessel is ready for its next adventure.
Good habits and preventative maintenance are your best ally in keeping the engine running at peak performance. Raw water systems like those on outboard and i/o engines should be flushed on the trailer after each haulout or at minimum the end of season. Flushing out seawater can be improved with salt removal detergents added into the water supply. Failure of these components will overheat an engine, possibly leading to costly damage of core components.
Water pumps with rubber impellers burn out if run dry. This could be caused by simply forgetting to open a seacock. Another common cause at sea is weeds or a plastic bag stuck in the seawater intake through-hull. Having spares on hand is highly recommended. As impellers age they lose there flexibility and should be replaced.
The thermostat is a simple valve which opens once the engine has warmed, thus allowing the coolant to circulate. On most engines the thermostat will pop at 160° F and be fully open at 180° F. To troubleshoot, remove a suspect thermostat and place it in a pot of boiling water with a thermometer. A malfunctioning thermostat must be replaced.