water impellor direction

[Destroyer]

Originally Posted by THEFERMANATOR

Your leaving out one KEY component in your equation that REALLY changes things, and that is speed. Your small engines are designed for engines that don't run that fast, and probably never experience cavitation while running. Now lets say you have that solid impeller and hit a wave and suck air through your water pickup. Now you've lost prime and quit pumping water, yet your still going on plane. Now you have to come off plane for it to pick up water again, and then you get that cold water hitting the hot cylinders that were just left dry for a few seconds and you get that nice steaming and stressing of the metal. I see what your point is, but in a high HP planing application the rubber impeller is king.

Actually I had thought about speed but was hoping with a redesign of the lower unit that the impeller pickup could be placed somewhere where a wave/blowout would not result in anything more than the normal shot of air that even rubber impellers get occasionally. I'm sure there are good sound engineering reasons for using the rubber impeller over a solid type. (But I'd still love to see factory data)....

Looking at it from a different angle though, here's a thought/question I've always had. At speed, how much water does a rubber impeller really pump? Think about it. At low speed an impeller pumps whatever water is in the pickup. But at high speed, when you're up on plane and skimming along the surface like there's no tomorrow the water that's under your boat is hitting your lower unit water pickup with more pressure than a fire hose. Does the impeller continue to pump water at all, or does the extreme pressure flex the vanes out of the way and go directly to the engine all by itself? ... or maybe it is a combination of the two...pumping and pressure?

Originally Posted by spareparts

keep in mind, you need to drop the lower unit every couple of years just to grease everything up, if you don't, when you really need it to come off, it won't.

Excellent point also.