Here’s a quick and dirty on water pump rubber-vaned impellers. The function of the cooling system is to  remove excess heat from the engine and other components.  Seawater is picked up from the drive system lower unit and pumped through the engine to absorb excess heat.  Intake & discharge portsThe impeller rotates in an eccentric housing, in which the drive shaft is off center.  The impeller rotates whenever the engine is turning.  The cavities between the vanes pick-up water as they pass over the intake port and as the vane cavities pass over the discharge port, they collapse, displacing the water from the pump, pushing it through the system.  The impeller is lubricated and cooled by the water that it pumps.  Running the pump without water will quickly destroy the impeller.
Volume and Pressure ImpellersThere are two types of impellers.  High pressure-low volume and low pressure-high volume.  Floppy Vane High Volume-Low Pressure ImpellerA volume type impeller will develop lower pump and system pressure, but deliver a high volume.  The impeller vanes have low tension which limits pump pressure.  This type of pump will allow limited pumping of contaminated water and very limited operation with no water present.  A pressure type impeller will develop higher pump and system pressure, but deliver low volume.  These impeller vanes are under a higher tension in the housing which makes the pump more susceptible to damage from contaminated water or dry operation.
Every manufacturer has their own recommended service interval for impellers.  Pressure type impellerIn general, the short vane, high pressure-low volume impeller should be replaced annually/100 hrs; the floppy vane, low pressure-high volume impeller should be replaced every 3 years/300 hrs.  And of course, both should be replaced when cooling problems occur.  If the impeller is damaged with blades missing, make sure that you find the missing blades.  They could be stuck downstream from the pump interfering with proper flow.
Pressure type impellerNow that you’ve got the basics, it’s time to check your maintenance records and the manufacturer’s recommended service interval.  When was the last time your engine’s impeller was changed?  Call now to schedule service!