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.

[mawshj]

the theory is that the vanes act as a positive displacement pump at low speeds and flex more and more as speed increases there by regulating water pressure so to protect gaskets and hoses. also allows sand to flow thru . at w.o.t. the vanes do not touch the outer pump housing but act as a turbine pump. I do not know if it really works that way but that's what I was told by O M C man.

[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.

[Destroyer]

Originally Posted by mawshj

the theory is that the vanes act as a positive displacement pump at low speeds and flex more and more as speed increases there by regulating water pressure so to protect gaskets and hoses. also allows sand to flow thru . at w.o.t. the vanes do not touch the outer pump housing but act as a turbine pump. I do not know if it really works that way but that's what I was told by O M C man.

I'm a mechanical engineer by trade.. (or at least I was until I retired a few years back) What you were told by the OMC man is correct as far as how they operate in principal. I've never questioned that. All I'm asking is is there anything else that might work instead.

And by the way, to point out to everyone else, a turbine pump is, in fact, a form of centrifugal pump in that the blades do not touch the outside of the pump body at higher rpm speeds.. so all the arguments about them passing sand and debris etc. fall exactly into the same category as a centrifugal pump. They both will do the same thing, in the same manner. It is only when the pump is operating at lower speed that it acts as a positive displacement pump... and that is exactly what the lower speed British Seagull outboards do with their centrifugal pumps. So if a centrifugal pump can operate at low speed and provide enough cooling, and a turbine style pump can operate at higher speed, why not combine them into something that does both jobs and isn't made of a material that can self-destruct if run without water in less than 20 seconds.

[Destroyer]

Originally Posted by phatdaddy

i am glad that you need to change it annually, that insures you can get the lower unit off when you need to.

Now there's an argument that I can agree with 100%.

[Destroyer]

And lets take this a step further... WHY is it necessary to regulate the water flow pressure? I mean, think about it.. does it really matter what the pressure is, as long as the volume is sufficient to cool the engine? We're talking about metal engines, and I seriously doubt that additional water pressure will cause any harm. So you build up a higher head pressure with a centrifugal pump. Who cares???? The temperature of the engine will still be regulated by the thermostat, regardless of what the water pressure is. Automotive applications (like your car) have used centrifugal pumps forever, and they seem to run just fine with them. So why the big whup over higher pressure in an outboard? What would it hurt?

[smokeonthewater]

Higher head pressure is higher parasitic drag, less mpg and less power to the water.

[THEFERMANATOR]

Contrary to what you might think, higher head pressures CAN cause problems inside the engine. The gaskets and such for the cooling system would have to be completely re engineered to handle pressures in excess of 20 PSI. The only sealing surfaces meant to hold higher pressures are the sealing ring for the head gasket in the cylinder area only. Also higher pressures can have a negative effect on cylinder liners not only from abrasion, theres also cavitation, and micro bubbling at the cylinder liners from the engines combustion process. You are going to get micro hot spots which form tiny air bubbles along the cylinders in the cooling jackets, and these air bubbles under pressure can act like a snad blaster when a combustion event occurs. It is like a hammering action when the cylinder fires, and that hammering action acts upon the cylinder wall in the cooling jacket. As pressures increase, this event is worsened. Believe me, there have been attempts at finding something better, but if it existed don't you think somebody would have come out with it by now. The engines you are talking about with hard impellers osund more like tender engines used on non planing hulls where ingesting air at speed, high RPM useage, and other factors are not a concern, hence why they can use what they use.

And engineers have come up with a pump that can do multiple applications, it just so happens to be the rubber impeller that has proven itself for over a century now.

[Destroyer]

Originally Posted by THEFERMANATOR

Contrary to what you might think, higher head pressures CAN cause problems inside the engine. The gaskets and such for the cooling system would have to be completely re engineered to handle pressures in excess of 20 PSI. The only sealing surfaces meant to hold higher pressures are the sealing ring for the head gasket in the cylinder area only. Also higher pressures can have a negative effect on cylinder liners not only from abrasion, theres also cavitation, and micro bubbling at the cylinder liners from the engines combustion process. You are going to get micro hot spots which form tiny air bubbles along the cylinders in the cooling jackets, and these air bubbles under pressure can act like a snad blaster when a combustion event occurs. It is like a hammering action when the cylinder fires, and that hammering action acts upon the cylinder wall in the cooling jacket. As pressures increase, this event is worsened. Believe me, there have been attempts at finding something better, but if it existed don't you think somebody would have come out with it by now. The engines you are talking about with hard impellers osund more like tender engines used on non planing hulls where ingesting air at speed, high RPM useage, and other factors are not a concern, hence why they can use what they use.

And engineers have come up with a pump that can do multiple applications, it just so happens to be the rubber impeller that has proven itself for over a century now.

Ok, I'll go along with what you say as far as the re-engineering of various gaskets to take a higher pressure. So you'd have to use an automotive type of gasket. (Or some kind of pressure regulator elsewhere in the engine) But the cavitation and micro bubbling you speak of I have my doubts about. After all, you would have the same effects in a car, and I don't recall ever reading about it in and of my shop manuals.

You are spot on about the seagulls.. they are low speed engines for sailboats and other non-planing hulls. Still not convinced about the rubber impeller though.