[THEFERMANATOR]

Originally Posted by Destroyer

Ferm, I know that you know your stuff, and everything you said it true,
but just for the sake of arguement, let me put forth the following. Both of my Seagulls pump a ton of water when they run. One is a 2 1/2hp and the other is a 4hp. Both have solid impellers similar to the kind found on just about any pedestal sump pump. I have a pedestal sump pump, and looking at it I find I was wrong, it's a 1/2hp motor, not the 3/4 I thought it was. Regardless, it still pumps to a 25 foot head. At 3 feet of head it will pump about 4000 gph. According to it's specs, it's designed to pass up to 3/8" solid debris, which is far more than any rubber impeller will.
You say that rubber impellers are self priming by nature, but if submerged, so is a pedestal pump. You would probably have to redesign the lower unit a little to get the pump impeller under the water intake ports, but other than that the self priming feature is pretty much a wash.
As stated above, the solid impeller will pass debris like sand and seaweed about the same as a rubber unit... perhaps a little better. Finally, my pedestal pump impeller is direct coupled to the motor via a shaft, turns at 1725 rpms and pumps 4400gph at zero head. That's a lot of cooling water at idle speed.

I know that this is all just conjecture, but it would seem to me that there's really no good reason to not use a solid impeller, especially given the fact that the British Seagull engines, (considered by many to be the best, most reliable small outboard engine ever built), use a solid impeller in their design. Really, I cannot see why they could not be used on larger engines. I wish some engineer could give me some hard facts documentation as to why the rubber impellers are used. (Other than the fact that it's just good business to make a unit that needs yearly replacement at about $35 per unit)... Multiply that by how many hundreds of thousands of outboards in use and that's really big business.

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.