Anyone used Run Dry impeller for Onan

[Simi 60]

Originally Posted by a64pilot

For a main engine you need a variable flow water pump as the engine varies in RPM so an electric pump would be tough to fit, so they may not be a good idea there.
.

I am not 100% convinced on this

I know of at least one trawler with an electric pump on his 855 Cummins
Thinking being that max water flow at low speed is not a problem as its the thermostat that controls temp.

Not sure if that's the correct way of thinking but its been working in that particular vessel for a decade or more

I would be thinking having more water than the exhaust could blow out would be an issue, but the pump used does match the original output and the 855 has a big set of lungs

[a64pilot]

Originally Posted by aucoingc

Question—where is your raw water inlet located? Is it flush with the hull? The answer may be to put a nice sized clamshell over the opening to keep seawater pooled in the vertical while underway and even at anchor. It solved my problem with frequent impellor replacement.

Be very careful doing this if your saying put a scoop strainer on with the scoop facing forward, the reason is when your sailing with the generator off the scoop can push enough water to flood the motor.

[a64pilot]

Originally Posted by Simi 60

I am not 100% convinced on this

I know of at least one trawler with an electric pump on his 855 Cummins
Thinking being that max water flow at low speed is not a problem as its the thermostat that controls temp.

Not sure if that's the correct way of thinking but its been working in that particular vessel for a decade or more

I would be thinking having more water than the exhaust could blow out would be an issue, but the pump used does match the original output and the 855 has a big set of lungs

Temp is not the issue as you say it’s thermostat controlled, and even a regular impeller water pump pumps well in excess of what is needed, that way when your motoring around with a half clogged strainer you don’t burn up your motor.

I believe it’s the second, that you May be pumping in too much water into the exhaust for the engine to blow out, with the engine at low idle, then of course you run the risk of hydro lock and destroying the motor.
The original installation of course is a variable flow rate, and you can’t match that, without a variable pump or regulating its output.

I had thought about and decided a simple ball valve with the lever connected to the throttle cable would work, ball valve opens with throttle and modulates the water flow, be pretty simple to adjust.
But on my engine the impeller pump is slightly below the water line, meaning it’s always full of water so no dry run ever, it’s easily accessed and I now have a strainer to catch impeller bits, so I’m my case an electric pump wouldn’t solve anything.
Don’t fix what ain’t broken.

[Alita49DS]

Not quite on the point but since we are talking about ONAN generators, does anyone amongst you have the manual in PDF for the MDKAU?

I have tried on the Cummins site and cannot find other than the publicity sheet.

If you have a link or can email me I would be very grateful.

[Taipe]

A small stainless steel basket strainer ought to catch more bits of impeller blades, don't you think?

[Paradoxus]

Originally Posted by a64pilot

While not an Onan, I took my raw water pump off after it failed and fitted a March airconditioner pump a few years ago.

Some Catamaran factory installations have an electric pump feeding the impeller pump, to me that’s ignorant, but I can see how they may have to do that to protect the generator warranty.



Anyway if you wire the March pump directly to the electrical output, fused of course, then it won’t begin to pump water for a few seconds until after the generator starts, so the generator will run for a few short seconds with no water, this hurts nothing and you can hear the exhaust note change when the water begins flowing. The water flow also stops well before the generator winds down too, meaning the exhaust system is blown clean of water during shutdown, so even if your sailing in heavy weather the water in the exhaust system can’t flow back into a non running motor.



You cannot overcrank the generator and fill the motor with water, because there is not water flow unless there is electricity.

The March water pumps however must be below water line, they will not self prime at all, however their life span is usually in pretty much decades and tens of thousands of hours. Think of how many hours they run in boats sitting in a marina with the AC on 24/7 for years.



I have used a Globe dry run impeller in my main engine water pump, in my opinion they work fine, but it seems due to the greater hardness of the blades that they caused more water pump wear, that is an impression, I can’t verify it.

For a main engine you need a variable flow water pump as the engine varies in RPM so an electric pump would be tough to fit, so they may not be a good idea there.



But for a fixed RPM generator I can’t see any reason not to fit one, and they put an end to water pump maintenance, once installed you forget about it, there is never any need to change anything, of course carry a spare pump.



This is the strainer I installed to catch impeller vanes on my main engine, I once tried an after market budget impeller, and yes it soon came apart, so after having to disassemble the heat exchanger in a deserted anchorage, I fitted the strainer.

I loved the idea of bypassing the impeller pump and have just installed a March pump as a64pilot described. Works like a charm, makes you wonder why this is not standard on generators.

[s/v Jedi]

Most impeller eating gensets and engines suffer from an installation problem that leaves the impeller dry after stopping the motor. To always keep it wet, make sure that the feed hose from the sea strainer comes to the impeller pump from above, so that water doesn’t easily drain back into it.

We also have a small strainer between pump and heat exchanger and the clear glass allows us to see if there’s impeller bits, as well as confirm water flow after starting.

We had changed to a March pump but went back to impeller when the genset started to overheat at high load. We also changed out the thermostat so we don’t know which one was causing the problem.

[YANDINA]

While stuck in the mash after hurricane Hugo, I dug a hole that was underwater at high tide for about 3 hours twice a day. I plumbed a 120 volt sump pump to the generator raw water intake and was able to keep the inverter batteries charged while living aboard waiting for help.

Full story at Hurricane Hugo

[YANDINA]

Many years ago they came out with a "run dry" impeller. I don't know if it is still sold.
While it was true they could run dry, they failed due to an unusual problem. After turning off the genset, the hot water sitting in the block would slowly leak back to the pump and the high temperature destroyed the run-dry impeller. Happened twice until I figured out what was happening because you didn't discover it until the next start and blamed the failure of the run-dry feature.

[a64pilot]

Originally Posted by Paradoxus

I loved the idea of bypassing the impeller pump and have just installed a March pump as a64pilot described. Works like a charm, makes you wonder why this is not standard on generators.

I have a theory on that, manufacturers want a “package” that they can install quickly, if everything is in one box then installation is easier for the boat manufacturer, plus of course the impeller pump does work.
What’s odd to me is at least one Catamaran manufacturer recognizes that they have a problem, and the problem is exacerbated by the genset being so far above the waterline, so they have wired in an electric pump to feed water to the existing impeller pump, it apparently helps but the impeller will of course eventually come apart.
On my main engine, the impeller is slightly below the waterline, so of course it stays completely filled with water, and I assume because of this it lasts years, same for an outboard, the pump is below the water, and they also last for years.

[a64pilot]

Originally Posted by YANDINA

Many years ago they came out with a "run dry" impeller. I don't know if it is still sold.
While it was true they could run dry, they failed due to an unusual problem. After turning off the genset, the hot water sitting in the block would slowly leak back to the pump and the high temperature destroyed the run-dry impeller. Happened twice until I figured out what was happening because you didn't discover it until the next start and blamed the failure of the run-dry feature.

I used a Globe dry run impeller once, worked fine, but I didn’t have a problem just went with the dry run because I thought it might be a better product.
It seemed to me that the impeller material was harder, and it seemed to cause more wear on the pump metal. Now I didn’t measure anything, it was just an impression

[a64pilot]

Originally Posted by s/v Jedi

We also have a small strainer between pump and heat exchanger and the clear glass allows us to see if there’s impeller bits, as well as confirm water flow after starting.

I tried a cheap aftermarket impeller once, to only have to disassemble the heat exchanger in an anchorage at an uninhabited island to get the bits out.
This is the strainer I installed immediately after the pump on the main engine.
Type or position of a pump to supply raw water is irrelevant to an engine, just as long as it gets enough, the march I put on my generator is in excess of the size it calls for, but as with any pump you lose capacity with lift and hose run, I think it’s best to oversize a bit.

[s/v Jedi]

Originally Posted by a64pilot

I tried a cheap aftermarket impeller once, to only have to disassemble the heat exchanger in an anchorage at an uninhabited island to get the bits out.
This is the strainer I installed immediately after the pump on the main engine.
Type or position of a pump to supply raw water is irrelevant to an engine, just as long as it gets enough, the march I put on my generator is in excess of the size it calls for, but as with any pump you lose capacity with lift and hose run, I think it’s best to oversize a bit.

I have it a bit different now but check out my custom screen... based on diameter of heat exchanger channels