The raw water pump and impeller are vital parts of the marine diesel engine system - supplying cooling water either directly to the engine (direct cooled) or indirectly via a heat exchanger (indirect or fresh-water cooled).

Impellers can give many hours of reliable service; however, the rubber/nitrile is easily damaged by the engine being run without water flow, sand and grit in the water and periods of non-use while fitted in the pump housing.