A good article construed from information a fellow surveyor shared.
Impeller Fails
Ever wonder what goes on inside your raw-water pump? We didn't either. At least not until a surveyor sent us these pictures. The first shows a brand-new impeller. One thing you might notice is how squished the vanes are. When your boat's not used much, say over winter, these poor things stay folded over for months. After a while, they take a "set," which means they stay a little bent over like most of us would if we'd been hunched over for a whole season. This makes the pump a little less efficient, and every year, it pumps less water.
The other picture shows what happens when you ignore your impeller too long. Those poor vanes finally gave up and broke off. Actually, you'd be fortunate if they just broke off; what usually happens is that they get carried downstream in the cooling system, where they can clog your heat exchanger, or if you don't have one, clog the cooling passages in your engine. Either way, it can be a big job to retrieve them, and retrieve them you will — otherwise you'll be fighting overheating problems forever. This spring, replace your impeller(s) if they're over a couple of years old. It's one less thing you'll have to worry about.