The main engines and generators in most pleasure craft use seawater to help cool the engines. Just like your auto, marine diesels need cooling. The main difference is that marine engines don’t have a radiator and are in a machinery space/engine room and therefore depend upon external cooling from lakes or oceans. In most cases, the engine(s) have a heat exchanger which cools the engine without allowing damaging sea water into the critical water jackets, cylinder heads, circulating systems, etc. This infinite supply of cold water is brought to the marine diesel engine by a sea water pump. There are two basic types of pumps. One is a bronze impeller type which is not self-priming. The other is a rubber impeller pump which does self prime. Most pleasure craft marine diesel engines of small and medium bore use a flexible vane impeller made of rubber or nitrile rubber compounds.
Sea water impellers made of rubber wear out like all moving parts. According to most engine manufacturers, you must inspect the impeller yearly and/or replace the impeller yearly or after a predetermined number of operating hours. Engine manufacturers, such as MAN, recommends the impeller(s) be inspected yearly or every 1000 hours. We recommend that rubber impellers get replaced once a year with little regard to the operating hours. Engines that sit for extended periods have impeller blades that take a “set” (take the shape of the surrounding housing) and as a result have a reduced life. Impellers also become brittle over time. If you replace them once a year you will have a more secure feeling that you won’t have an impeller fail during your regular boating season.
Remember to check your Impellers or have them checked by a professional
repair shop such as Scott Marine Power on a yearly basis.