[Destroyer]

Originally Posted by mawshj

the correct direction is obtained by rotating the impeller in the normal direction of shaft travel and let the blades curl back. the blades are designed to act as a displacement pump at low speeds and to flex under pressure at high speeds and act as a centrifical inpeller and thus relieve some water pressure. a sump pump only runs at one speed. I have rarly seem an impeller worn out unless someone was plowing with it for a long time. the cost in fixing a water pump is the labor ,so always replace it when you have the engine open

Exactly, it's that centrifugal action that got me thinking about this in the first place. A sump pump impeller IS a centrifugal impeller. So if the rubber impellers are designed to flex under pressure and become centrifugal impellers anyways, why not just start out with them like that in the first place. The speed of the impeller really doesn't matter, as long as it doesn't run faster than whatever it was designed for. In the case of my British Sterling outboards they run anywhere's from 700 to 4500 rpm. About the only thing I can even think of that would be a logical arguement against using a centrifugal impeller is the displacement properties of the rubber impeller at idle speeds. A displacement impeller provides a positive fluid flow, whereas an impeller at low speeds looses some of it's head pressure. The question is how much pressure do they loose at say....what?....500rpm? At higher speeds they actually would provide more fluid flow since the faster you turn them the more they pump. Which might also be a problem, but I'm sure that the water passages could be designed for improved water flow. After all, an automobile's engine uses a centrifugal impeller and it does just fine at varying speeds. And for that matter, I've seen some inboard and I/O engine's that use a raw water pickup use exactly the same type of impeller also.