had a pump failure on a main engine except once in the 40 some years of cruising sailboats is that it's been a seal and not the impeller. That's not the case with my generator however. History has shown me that the impeller on my generator will fail at around 400 hours so I replace it at 300. I don't know the reason for the difference between the two but it's significant. I never have a generator impeller around long enough to have to rotate it or upgrade with a new spare. I wish I could nail down a failure mode with the engine pump but there never seems to be any pattern.