and are actually quite a bit more durable than they used to be. I used to change out the impeller each spring on Yanmar engines. I now believe the manual is right in recommending a much longer change out period. I have seen engines that have been running the same impeller for ten years and a 1000 hours with no apparent problem - that particular engine was from the Great Lakes and had always been run in clean fresh water. Impellers run in salt water (and especially where fine sand and mud are in the mix) are a different story. Last winter in the Abacos the unusually high and constant winds kept enough fine sand in suspension that it wore grooves in our pump cover and you could see the abrasion on the impeller blades, and this after only 200 hours (fortunately the Yanmar pump cover is reversible, after polishing off the paint, so that was a quick fix). There is a small lobe inside the pump body that one impeller blade comes to rest on, and should the engine remain idle for a long period of time that particular blade will take a "set" and be more prone to failure and might even cause some internal leakage past the impeller. That is just one of the reasons I make it a point to hand turn my engine every month during the lay-up period (this will also make the piston rings less likely to become sticky in their grooves, among other things).