What is an impeller and why do I need to have it changed on an interval? An impeller is basically a rubber propeller that scoops the water into the engine from the body of water you are navigating. Without a good working impeller, the water can't come into the engine to help cool it off. Most manufacturers such as Mercruiser or Volvo Penta recommend your impeller be changed every 3 years or every 100 hours whichever occurs first. Here are some impeller examples that I have serviced. Don't forget to have Torque Marine service your impeller today!