Question:

[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.

[reelapeelin]

Originally Posted by Destroyer

and the impeller is made of some kind of hard plastic.

I'd go w/a softer material over hard plastic any day...there is no give w/the harder material, so more prone to nick the edges as sand and other hard debris are pushed between the impeller tip and the housing...chinking away the edges will eventually lead to loss of pressure/volume...

In a perfect world where nothing foreign could get into the pump, maybe the harder material wins, but for real world use, the softer rubber's gonna offer better/longer performance.

On a global basis...I'll be willing to bet there are more (many more) rubber impellers out there in use than harder nylon...and that's because, I think, one word...reliability.

[Destroyer]

Originally Posted by reelapeelin

I'd go w/a softer material over hard plastic any day...there is no give w/the harder material, so more prone to nick the edges as sand and other hard debris are pushed between the impeller tip and the housing...chinking away the edges will eventually lead to loss of pressure/volume...

In a perfect world where nothing foreign could get into the pump, maybe the harder material wins, but for real world use, the softer rubber's gonna offer better/longer performance.

On a global basis...I'll be willing to bet there are more (many more) rubber impellers out there in use than harder nylon...and that's because, I think, one word...reliability.

Really, I don't know what these impellers are made of, I just know that when I hold it in my hand it's hard, and it's not metal. Nor is is Bakelite or any other fibrous sort of material. It looks like nylon, but for all I know it's ABS plastic or some variable in between. All I know is it's hard, it's some kind of plastic, and it works. I't's the only type of pump that British Seagull Outboards used throughout their history. And even though they are no longer made, they are still very much prized outboards, especially by sailboat owners as a sailboat outboard for moving through dock areas, etc. I own 2 of them and can attest to their claim as "The best outboard ever made". They are reliable, simple to operate and fix and are ideal for rowboats, sailboats, etc. And I've never had to replace either impeller in the ones I own from sand, debris or anything else being ingested into the engine, even though I use both of these outboards in rivers and lakes only and have kicked up tons of sand, mud, leaves and twigs and crap over the years. All I can tell you is that they are centrifugal pumps, and they work.