Question:

[THEFERMANATOR]

The main reason that I can think of is that rubber impellers are tolerant of MANY different types of operating enviroments. A metal impeller like found in an automotive water pump does not tolerate sand and grit very well going through whereas a rubber impeller riding inside of a stainless housing could care less. Also rubber impellers are self priming by design, so you don't have to worry about it sucking air and losing prime, and then stopping pumping. Also rubber impellers are flexible and lay over at RPM's which allows them to get to a certain pressure, and just sort of self limit the pressure output without the need for a popoff valve. There are MANY good reasons to run a rubber impeller over a metal impeller IMHO, and the biggest one to me is it has proven itself to work reliably IF maintained.

[smokeonthewater]

I would say the rpm range is the biggest reason... The rubber pump is a self regulating variable displacement pump.

[Destroyer]

Ok, by way of further information. The two British Seagull outboards I have both have identical impellers. They run off the drive shaft just like the rubber ones do, and in fact are mounted in the same place. The difference being that they are in a centrifugal pump housing instead of a positive displacement type like the rubber ones are. The pump body is stainless steel and the impeller is made of some kind of hard plastic. I would hazard a guess that it's nylon. Point being that they work the same way off the drive shaft. This is what one looks like: http://www.ebay.com/itm/British-Seag...item4183e342a7

I understand that at high revs the rubber impeller collapses , but I would think that with the proper ducting of incoming water there really wouldn't be any cavitation problems with a centrifugal impeller. As to the self prime feature, if the pump is designed to be submerged when the engine is in the normal operating position I don't think that would be an issue. I fully understand that they have been making outboard engines with rubber pumps since Ole Evinrude invented the outboard in 1907, but that doesn't mean there aren't other pumps that can do the job.

So, except for the fact that they sell a new pump to everyone once a year thereby guaranteeing themselves sales... I say again... What is a factual reason for the use of a rubber impeller?

[bradford]

I'll bet a dollar its the constant maintainance/yearly change out that's the big reason. Can't build something bullet proof, nobody would need to buy another one.

[THEFERMANATOR]

Lets say your running along and get some grass on your lower unit, not enough to cause you to come off plane, but just enough to allow the water pickup to suck air in. With a rubber impeller no big deal as it will continue to pull in the aerated water mixture, BUT your centrifigal pump could stop pumping all together as soon as some air hits it. I understand your way of thinking, but I still feel there is a reason for sticking with a tried and proven design. And it comes down if it aint broke, don't try and fix it. I'm willing to bet others have tried running centrifigal pumps on outboards, and I would bet they found out real quick they didn't work well on engines that can ingest an air water mixture at speed.

[Destroyer]

Originally Posted by THEFERMANATOR

Lets say your running along and get some grass on your lower unit, not enough to cause you to come off plane, but just enough to allow the water pickup to suck air in. With a rubber impeller no big deal as it will continue to pull in the aerated water mixture, BUT your centrifigal pump could stop pumping all together as soon as some air hits it. I understand your way of thinking, but I still feel there is a reason for sticking with a tried and proven design. And it comes down if it aint broke, don't try and fix it. I'm willing to bet others have tried running centrifigal pumps on outboards, and I would bet they found out real quick they didn't work well on engines that can ingest an air water mixture at speed.

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.

[phatdaddy]

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

[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.