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For a goodly number of years I was under the impression that outboard cooling pumps were centrifugal pumps with alloy impellers.
Here is our Johnson 15's impeller and it definitely is NOT aluminum nor is it from a centrifugal pump. This impeller was in the engine since new and the engine never failed to pump enough water to have a steady stream from the indicator hole. How this engine survived on this impeller for so many years (2007 to today) of hard daily use w/o any over heating problems, I do not know!
We LOVE this engine and will do everything in our power to see it has a long and happy life.
They don't make 'em like they used to!
I've never seen an outboard that didn't use a typical rubber impeller. Our 1998 15hp Honda OB was on the original impeller when we sold it in 2014. I don't think long impeller life in small outboards is too unusual if they are not run in sand a lot. Of the many OB's I've owned, water pump problems have always been very rare. In fact, I don't think I have ever had one - only replaced a couple a few times just because I was ashamed that I hadn't…
I've never seen an outboard that didn't use a typical rubber impeller. Our 1998 15hp Honda OB was on the original impeller when we sold it in 2014. I don't think long impeller life in small outboards is too unusual if they are not run in sand a lot. Of the many OB's I've owned, water pump problems have always been very rare. In fact, I don't think I have ever had one - only replaced a couple a few times just because I was ashamed that I hadn't…Mark
Perhaps you're right, but this OB cooling system has never once been rinsed with fresh water and has always been used in salt water. That pretty much goes against every care and feeding instruction in the manual.
It also got it's fair share of sand with numerous beach landings each week.
As far as I know, a centrifugal pump moves water by throwing it outwards from the centre, this is just what the rubber vanes are doing. For those of us with good working OMC Saildrive motors, we are very familiar with the longevity of the impellers...10 years or more is not unusual. I think 2 things that do work in our favour, we normally don't sail in sand, and because our boats are always in the drink, the impeller never dries out. I changed my impeller after 15 years just because I thought that it was the right thing to do...the one I took out looked fine; the replacement works well after about 12 years. I think that the manufacturers suggesting replacement every 2 or 3 years is bogus, but they also know it is not a big job on a smallish outboard. Replacing the impeller on my saildrive is a pretty big task, and OMC never suggested in their manuals that it should be done for no reason.
No. The typical flexible vane pump is positive displacement, i.e. small quantities of water are trapped between the vanes and forced out the discharge. Most of the water can't recirculate back to the suction due to the shape of the pump chamber. Centrifugal pumps work by increasing the velocity of the water in the impeller and then slowing the water down efficiently in the outer housing, converting the velocity to pressure. Positive displacement pumps tend to put out almost the same GPM regardless of pressure, while the GPM of centrifugal pumps falls off dramatically as the pressure increases.
The bigger problem is when the bits and pieces get pumped up and further into the unit. then things can get ugly... But I agree I replaced mine in my antique 1983 Yachtwin... it had been in there for probably 8 years of sitting... dry rotted.
My brandy new Merc 3.5 will get a new impeller every other year... because it's cheap and easy insurance for a $1500 motor.
By the way, that motor has already impressed me since I screwed up while it was running, it popped off the motor mount, and went under water while running. Water in the carb, cylider, etc. It took us about 2 hours, but we managed to get it running again, and thankfully its been run for several hours since. Tough little motors.
If you REALLY examine the pump housing and impeller as you rotate it in a cut-away model...
The outboard pump's housing is not centered on the shaft. The vanes are compressed and relaxed, changing the shape of the volume formed between vanes as the impeller rotates.
This is NOT centrifugal operation.
In a typical centrifugal pump the ends of the impeller vanes "aim" the water into a "volute", or a sort of curved cone and the impeller throws the water (or other fluid) into the volute.
The typical rubber impeller pump used in outboards are positive displacement pumps, forcing the water into a port at an end of the impeller.
IMHO the engineer who designed outboards so that to replace the impeller involves disconnecting the shift linkage which is always buried someplace, then dropping the lower unit, then taking the impeller off the main drive shaft, then trying to push the splines back into the head on the driveshaft in the right orientation whilst standing on your head then hook back up and align and set the shift linkage......
This guy should be punished for this unserviceable design.
Cruel and unusual I say. No wonder people don't replace them till they fail! Who can blame them!
I should have known that a simple response would not go unchallenged. To quote from the OMC repair manual...
"The pump inlet port is open to the blades when the space between them is increasing. The pump outlet port is open to the blades when the space between them is decreasing. Thus at low speeds, the impeller works as a displacement pump. At higher speeds, water resistance keeps the blades from contact with the housing, and the pump acts as a centrifugal pump".
This is how the engineers apparently see it.
Regarding Capta's comments. To change the OMC Saildrive impeller, the engine block must first be removed from the boat. Once the pump is removed, there is nothing to keep the ocean out, thus, the operation is best done with the ship on the hard...can be quite time consuming, and expensive.
Regarding Capta's comments. To change the OMC Saildrive impeller, the engine block must first be removed from the boat. Once the pump is removed, there is nothing to keep the ocean out, thus, the operation is best done with the ship on the hard...can be quite time consuming, and expensive.
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