Definition
An impeller is the flexible rubber vaned rotor inside a marine raw-water pump that draws cooling water through the engine, and is a consumable item that hardens, cracks and sheds blades with age and use.
The impeller is the heart of the raw-water circuit and the most frequently replaced part on most marine diesels. Its rubber vanes flex against an offset housing to pump water, and that constant flexing is what eventually destroys it.
Why it is a consumable, not a component
It hardens with age and heat, takes a set when the engine sits, and cracks at the vane roots. Running dry — even briefly — kills one immediately. It is replaced on an interval rather than on failure, because failure means an overheat at the worst possible moment.
The rule about missing blades
Every missing blade is downstream and must be accounted for. They lodge at the heat exchanger inlet or in the cooler tubes and cause a restriction that shows up later as unexplained overheating. Count blades against a new impeller — see how to tell if an impeller is failing.
Where you meet it
On every service schedule, in every spares kit worth carrying, and in most overheating diagnoses.