Water Pump Impellers
Nearly every boat engine and/or engine exhaust is cooled by raw water. ("Raw" water is lake, river or sea water.) The raw water side of the cooling system is almost certain to have a flexible-impeller type pump. A flexible-impeller pump is able to pass twigs, sand and tiny fish, but quickly fails when the water flow into the pump is interrupted. When the impeller fails, cooling water is not supplied to the engine and overheating begins.
Interruption of water pump flow can occur when the intake is blocked by plastic debris - the ubiquitous plastic bag loss. But a common interruption is running the engine with the boat out of the water. Boats that are regularly trailer loaded and launched frequently have damaged impellers from dry operation. Tests indicate only 90 seconds of dry operation can turn an impeller to toast.
A healthy water pump impeller is required for proper operation of the pump. Flexible (usually Neoprene) impellers are fragile and have a relatively short service life. In operation, it is necessary for the impeller vanes to compress and then spring out with each rotation of the pump. As impellers age the vanes take on a set (bent over shape) and do not actively spring out straight. This results in a reduction in the pump effectiveness and increasingly higher engine operating temperatures. Eventually, the impeller looses all flexibility and the vanes will break off rendering the pump completely ineffective and often resulting in massive overheat to the engine. The reliable service life of a water pump impeller is a matter of varying opinion. Impellers fail from age deterioration when the boat is not used for long periods of time at about the same rate as they fail from regular use wear and tear. Knowledgeable boaters change the impeller frequently.
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