[THEFERMANATOR]

Originally Posted by Destroyer

I don't necessarily disagree with you Ferm, but think about it. ANY pump, at speed, is going to have gallons and gallons of water forced into it simply by the movement of the engine through the water. So just like a rubber pump collapsing (and not doing any pumping at high rpm's) a centrifugal pump will not suffer loss of pressure if it ingests some air, because the pressure is going to be supplied 100% plus from the engine moving thru the water. In fact I'd be willing to bet that over pressure might be a bigger problem. Might have to have some kind of pressure relief valve built into the cooling system.

And there are other pump designs also.. Like gear pumps that don't suffer loss of suction with air ingestation, or moyno pumps or screw pumps. I just find it hard to believe that after 108 years of building outboard engines nothing better has been found. Sure, the materials may have changed from rubber to silicone blends, etc, but the overall basic style of the pump hasn't changed at all... and that bothers me and makes me wonder why.

The water passing over the lower doesn't cause the impeller vanes to collapse, the RPM's and water head pressure do. it is how they self regulate there pressure output is by them collapsing the vanes. If the impeller spins the hub even at high speed, it will stop pumping water. As to the other designs of pumps you refer to, gear pumps work well for il, but water doesn't provide the needed lubrication. And the moyno and screw pumps are basically the same by design, and the biggest drawback I see to them would be size and expense. If the rubber impeller wasn't the best for all round use, don't you think somebody would have come out with soemthing else by now? Look at most any marinized engine out there, it will have a rubber impeller of some sort supplying water to it. Inboard, I/O, outboards, gas diesel, and I believe even some turbine engines ALL use rubber impellers due to there proven track record of reliability, simplicity, and durability, and they just work. Most any engine used on a planing hull will have a rubber impeller supplying it simply due to the fact they work with less drawbacks than other designs available.