Question:
 

Why do outboard engines use rubber impellers? They constantly wear out, can cause serious overheating problems and can foul up your entire cooling system if they break apart.

I have 2 British Seagull outboard engines, and they don't use a flexible impeller. They use a solid impeller just like you would expect to find in any car or sump pump. So a solid impeller can be used... So WHY do the outboard makers continue to use rubber impellers?

(And before anyone says anything about pressure or volume, let me remind you that a 1 1/2 hp sump pump running at 1725 rpm has a head of over 20 feet in a 1 inch discharge. That means it can pump a column of water 20 feet straight up.
That's more than enough pressure and flow for any outboard engine).

What they have now is simple. If the driveshaft turns, so does the impeller in the pump, (assuming the rubber vanes are still attached to the hub) and it should make water. Electric pumps are not a good idea on any engine IMO. You simply become more reliant on another electrical device to work. I like mechanical and keep it simple. I get your point about the rubber giving up the ghost.
Corrosion and weight issues I suspect is why they went rubber. That and the fact that they required a pump that will self prime at idle speed of around 400 to 500 RPM, yet run up to 6000+ RPM. Centrifugal pumps will loose efficiency at some point and would need to be slowed down or run the risk of cavitation. IIRC displacement pumps are better suited for the huge range of operating RPM's. Some additional demands are that it must be capable of pumping sand and small debris without running the risk of clogging a vane as is in the case on some centrifugal pumps, must be capable of running dry for brief periods, must fit within the confines of the lower unit so it would need to be small but I don't see why they couldn't work out those issues. So in the end I would say, I don't know. GOOD QUESTION! The answer might be planned obsolescence or the answer may lie in the text above, or be completely unrelated. Interesting that no modern outboards have any "different" designs. The Japanese are some very smart folks. You would think they would have a better mousetrap by now.


On a 2-stroke I would say they don't have a different water pump becuse they can't drive it off the cam.. LOL! IDK!! Grasping at straws.

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.

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

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?

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.

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.

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

There are a LOT of things that don't change over time.

If it works it works.... As for the impeller sure it's a good idea to change yearly but I had a '59 Evinrude 7.5 25 years ago and a friend has it now... To the best of my knowledge it has the original impeller in it as my school teacher who gave it to me said they had never done anything to it. The impeller in it is at least 30 years old now and my friend is still using it.

They aren't THAT bad of a design....

As for the flow of water... The water isn't picked up from the movement of the lower unit through the water... It COULD be if the intake were designed differently but only at high speed... Low speed operation which doesn't necessarily mean low rpm would always require the work of the pump.

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.

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.

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.

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.

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

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?

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

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.

The micro bubbling is more an issue with diesels due to the higher cylinder pressures, but it is present in gas engines as well. Do a search on INTERNATIONAL 7.3's(not the powerstroke, but the IDI version), and you will find plenty of problems with cavitation and cylinder erosion due to the micro bubbling and it's effects. As pressure increases, the effects are normally worsened. Outboards already use automotive grade gaskets or better. Most automotive engines rarely see more than 16-18 PSI whereas in an outboard that is common pressures, and 20-25 can be seen. I understand your thinking, but your over thinking it. I know many think they can build a better mouse trap, but i nthe end the tried and proven design normally wins out. This is one of those situations.

Not looking to build a better mousetrap, just wondering if one has already been built but they want to sell us new impellers instead.

Popcorn... Must remember the popcorn...
My flats boat with the jack plate all the way up puts the cav plate (anti-vent plate) nearly even with the surface of the water. At idle I do think there might be a problem with prime on a centrifugal pump.
Not that it matters much, they make positive displacement pumps with harder vanes than the rubber impeller we see in the modern outboard. Jabsco makes a hard vane puppy pump that I like for fuel transfers. Debris MIGHT be the only spoiler and I know it will move a lot more debris than you would ever want in the water jackets of your outboard.
I am not arguing, The question has no correct or incorrect answer! :booty:

BINGO!!!

Also, in a car, you've got a rubber belt to self destruct if the impeller jams.

Perhaps, but the towing service that comes and hauls your car away for you if that happens doesn't cost you hundreds of dollars, or maybe legally takes your car as salvage. I agree with the bolt removal thing, because it is important
to keep things accessible.

fixed vane impellers don't suck well, just like my pool pump. You have to have a head pressure to get it to pump. Sliding vane impellers work well, but they don't like any kind of contaminant. the one I use for waste oil get stuck up all the time with particulates that get by my 10 micron filter. Gear pumps require too much lubrication and don't make enough volume. roots type pumps move a lot of volumes, but take a lot of horsepower to operate. I squash plate type pump runs too critical clearances adn would cost too much to implement. Enter the floppy vane impeller, moves enough volume, makes enough pressure, is tolerable to running to dry to an extent, able to self prime, very forgiving in tolerances. Cheap to make, easy to repair, works through out a wide variety or rpm ranges. SO why not?

Next we enter the housing aspect. Mercury used to use a small diameter stiff blade impeller in a plastic housing with a stainless sleve. Very good pump, but don't let it sit still for a long time and absolutely don't run it dry, even for 10 seconds. It will ruin everything. OMC large diameter floppy vane, low pressure, high volume. Stainless sleeve in a plastic housing. Run it dry long enough and it will melt the plastic housing. Does OK. Yamaha copied the same design but with a few differences, same issues. Suzuki stepped up with a cast stainless steel housing on the same type of impeller, you can smoke three or four impellers and the housing don't care, but run it in enough sand adn you'll be crying about how much that housing cost. Enter Mercury for round two, large diameter floppy vane with a stamped stainless housing, cheap to make, just as resilient as the zuki,easy to replace. Mercury wins with teh best design water pump assembly. BTW, I probably replace 100-150 impellers a year. I/O and Inboard pumps are a whole nother subject