there was an old black and white diagram of what happens to impellers based on the amount of time they are run dry, for instance, 5, 15, 30, 60, 90 seconds. It showed the progression of degradation. Frank I remember you being in on that thread. The guy had a driveshaft that had worn a hole in the water tube in the exhaust housing, even though there was water coming out of the exhaust relief holes.