Question:

Destroyer · #1 04-20-2015, 02:04 PM

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).

RidgeRunner · #2 04-20-2015, 03:00 PM

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.

THEFERMANATOR · #3 04-20-2015, 03:11 PM

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 · #4 04-20-2015, 03:41 PM

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

Destroyer · #5 04-20-2015, 04:48 PM

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 · #6 04-20-2015, 04:56 PM

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 · #7 04-20-2015, 05:41 PM

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 · #8 04-21-2015, 11:28 PM

Quote:

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 · #9 04-22-2015, 05:23 AM

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

smokeonthewater · #10 04-22-2015, 07:42 AM

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.