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Old 05-03-2014, 13:30   #46
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Thumbs up Re: ONAN Genset Raw Water Impeller Failure

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Originally Posted by St. Elsewhere View Post
Can you supply more information? Does Sherwood/Onan manufacture these impellers? What design features enable them to run dry? How is friction and wear eliminated?

Globe Marine's Run-dry Impellers
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Old 05-03-2014, 13:40   #47
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Re: ONAN Genset Raw Water Impeller Failure

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I had this problem for quite a while with my onan. I was getting an average of 50 hours per impeller. The pump in my boat is about 3 feet above the waterline. The installation manual says this is ok. The truth is that it's OK for about 50 hours per impeller. I solved the problem by putting a check valve in the raw water line. The water no longer drains to sea between runs. I've replaced only one impeller since putting in the check valve and that was with about 600 hrs on it. I only have about 300 hours on the current impeller so I don't have an average life yet, but it's a lot more than 50.
That's what I was thinking too. This can be prevented by doing the installation right: the hose that comes from the seacock/strainer, must first go above the impeller-pump, then come down to- and connect to it. Now, when the engine is stopped a water column will keep the impeller flooded for a lubricated start next time.

The mistake is that normally the connection for the hose is on the bottom of the pump, which makes installers route the hose from below up to the pump, which makes the water run out the pump when the engine stops, resulting in a dry start and broken impeller.

The check valve is a solution that creates it's own troubles as it fouls easily, resulting in it leaking the water out again. An extra foot or two of hose is better. IMO of-course
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Old 06-03-2014, 13:49   #48
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Re: ONAN Genset Raw Water Impeller Failure

Quote:
Originally Posted by s/v Jedi View Post
That's what I was thinking too. This can be prevented by doing the installation right: the hose that comes from the seacock/strainer, must first go above the impeller-pump, then come down to- and connect to it. Now, when the engine is stopped a water column will keep the impeller flooded for a lubricated start next time.



The mistake is that normally the connection for the hose is on the bottom of the pump, which makes installers route the hose from below up to the pump, which makes the water run out the pump when the engine stops, resulting in a dry start and broken impeller.



The check valve is a solution that creates it's own troubles as it fouls easily, resulting in it leaking the water out again. An extra foot or two of hose is better. IMO of-course

Sadly I have to say we did this and it did not make much difference. The Onan people in SA have stated they are aware these impellers are 'playing up'. I also tried the 'check valve' trick, and yes, added to another set of problems.
Captain Bill seems to have found an answer to his impeller issues which were not the same for me My water pump sits substantially higher than his does though!
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Old 06-03-2014, 20:37   #49
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Re: ONAN Genset Raw Water Impeller Failure

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Sadly I have to say we did this and it did not make much difference. The Onan people in SA have stated they are aware these impellers are 'playing up'. I also tried the 'check valve' trick, and yes, added to another set of problems.
Captain Bill seems to have found an answer to his impeller issues which were not the same for me My water pump sits substantially higher than his does though!
Would have thought better from Onan! Anyway, the best option then is one of the red/white epoxy March pumps connected to the genset AC output. These pumps cost $$$ but last forever with zero maintenance.
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Old 07-03-2014, 02:36   #50
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Re: ONAN Genset Raw Water Impeller Failure

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Originally Posted by s/v Jedi View Post
Would have thought better from Onan! Anyway, the best option then is one of the red/white epoxy March pumps connected to the genset AC output. These pumps cost $$$ but last forever with zero maintenance.


Yes, this has been successful and we have not had issues since. I now need to place a forward facing scoop on the inlet against the outside of the hull. When running the genset in lumpy seas under way we have had the occasional air lock and bleeding the system is a pain. When I met with Onan at the boat show in Miami, they agreed this was the best option!
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Old 07-03-2014, 07:00   #51
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Re: ONAN Genset Raw Water Impeller Failure

Showing my ignorance. What pump are you referring? Link?
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Old 07-03-2014, 14:45   #52
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Re: ONAN Genset Raw Water Impeller Failure

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Showing my ignorance. What pump are you referring? Link?

Bradley - not at Impi right now as it is hauled in the pacific, but if memory serves me correctly it is a March LC-3CP-MD pump as used on air conditioners. The important thing is to match the water flow rate. It really works well!
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Old 09-06-2014, 09:51   #53
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Re: ONAN Genset Raw Water Impeller Failure

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Originally Posted by Albro359 View Post
Sadly I have to report that we only got 173 hours from the so called run-dry impeller in the new Sherwood pump.

So the issue is not solved
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Old 15-06-2014, 11:49   #54
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Re: ONAN Genset Raw Water Impeller Failure

Hi. I had the same ongoing impeller failure on a 4kva Onan. Solved the problem by by-passing the Sherwood pump, and installing a 220v March pump. Wired to the genset 220v output. Works 100%. Also enlarged the heat exchanger delivery opening from 5mm to 8mm. Now more flow through cooling system, and less pressure inside heat exhanger.(no more leaks at end cap!) Onan paid for the replacement pump!!

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Old 15-06-2014, 15:35   #55
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Re: ONAN Genset Raw Water Impeller Failure

Thanks for that.sounds like a good idea.
Which model March pump did you use ???
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Old 15-06-2014, 17:39   #56
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Re: ONAN Genset Raw Water Impeller Failure

Sorry , coming in late to this conversation . I did exactly what Impi did with the a/c pump . Worked perfect for me . 1400 hrs or so since replacement and still going strong . I went a step further and created a seachest and manifold of 3 pumps . One for the a/c system , one for the genset ,one spare . A good sidenote is that now I can carry less inventory of spare parts , and if my main eng waterpump fails , I can crank the genny and supply the main as well with the spare pump . I strongly recomend going this route , Sailors exchange in St Aug had a surplus of the a/c pumps new for about $150 each .
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Old 22-08-2016, 11:36   #57
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Re: ONAN Genset Raw Water Impeller Failure

Any new solutions to the annoying Onan impeller problem in the 2 years since the last post in this thread?

I just changed my impeller again. 130 hours. Only three vanes were still attached. Still cooling well enough to not overheat but I was clearly on borrowed time. Also changed my Yanmar impeller after 600 hours and it looked good for anther 600.

Unlike most of the previous posts, my impeller in not dry. It (and the entire hose run) stays about 4" below the waterline. I can't feel or see any rough areas in pump assembly that would cause unusual wear.

I've only used the Onan impellers. It amazes me that Onan doesn't address this. Anyone with more experience with another brand? Globe Rundry?

I'm considering the air conditioner pump solution several mentioned. For those who did it, where exactly did you make the wiring connection? What capacity pump did you install?

Thanks
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Old 22-08-2016, 12:31   #58
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Re: ONAN Genset Raw Water Impeller Failure

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I'm considering the air conditioner pump solution several mentioned. For those who did it, where exactly did you make the wiring connection? What capacity pump did you install?

Thanks

I have a Baby Gen, the Nexgen 3.5 KW.
I used the same March pump my AC uses so one pump spare covers both, it is a 500 GPH pump I believe, in excess of what the generator needs, more water coming from the exhaust now but that doesn't seem to hurt anything.
I wired in directly to the generator output, if the generator is making power, the pump is running, generator shuts off, pump stops, as simple as that. A good thing is you can't fill the exhaust from over cranking as there is no water flow until engine gets up to RPM. If you lost your generator though, the water flow would stop is the only problem, but the over temp Murphy switch would shut it down.
I have yet to see a downside, only thing is the pump must be below waterline.
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