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Old 28-06-2021, 00:18   #1
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Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

So, after having the water temp alarm go and assuming a munched impeller on our Yanmar 2QM20H we pulled the pump apart today to investigate, sure enough, the impeller had a couple broken lobes, pump itself and bearings look good, but theres something a bit odd here. This boat is new to us and It seems to have a second water pump mounted on the front of the main water pump, I havn't seen this before. The front pump has it's own impeller driven off the end of the main water pump shaft, but heres the thing, the intake and output of the outboard pump are blocked and dont go to anything. I'm wondering if this was for an old accessory that needed raw water circulation and was removed. Could you pull the front impeller to lessen the drag on the main water pump? It just seems a bit redundant to have that there. Any advice appreciated.

Cheers

Ryan
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Old 28-06-2021, 01:14   #2
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

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Originally Posted by rkarakai21 View Post
So, after having the water temp alarm go and assuming a munched impeller on our Yanmar 2QM20H we pulled the pump apart today to investigate, sure enough, the impeller had a couple broken lobes, pump itself and bearings look good, but theres something a bit odd here. This boat is new to us and It seems to have a second water pump mounted on the front of the main water pump, I havn't seen this before. The front pump has it's own impeller driven off the end of the main water pump shaft, but heres the thing, the intake and output of the outboard pump are blocked and dont go to anything. I'm wondering if this was for an old accessory that needed raw water circulation and was removed. Could you pull the front impeller to lessen the drag on the main water pump? It just seems a bit redundant to have that there. Any advice appreciated.

Cheers

Ryan
The 2QM20H has an option to have a bilge pump driven by the same shaft as the raw water pump. Your engine has that option fitted. Not normally required for your average sailboat but a good idea on a old wooden planked workboat.

Presumably you could leave the "optional" impeller out although I have never tried it.

From the manual...
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Old 28-06-2021, 03:24   #3
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

Not unusual for marine engines to have two water pumps. The original bolted onto the front of the block is used to circulate cooling fluid through the engine and heat exchanger and the other to supply cool sea water to the heat exchanger.
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Old 28-06-2021, 04:36   #4
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

^^ however the OP' engine (2QM20H) is a simple raw water cooled engine without any fresh water/coolant pump....
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Old 28-06-2021, 11:11   #5
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

While I have no direct experience, I don't see why you couldn't remove the unused impeller. I'm actually surprised that the impeller with the plugged lines isn't in poor condition. I would have thought the plugged lines would have caused excessively high and low pressures resulting in damage.



Another option is to plumb the unused impeller hoses to each other to make a loop and let it cycle water. You can then think about adding T's and valves for an emergency de-watering pump. It is also a quick backup to a failed primary pump assuming you can just swap the hoses.
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Old 28-06-2021, 11:29   #6
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

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Another option is to plumb the unused impeller hoses to each other to make a loop and let it cycle water. You can then think about adding T's and valves for an emergency de-watering pump. It is also a quick backup to a failed primary pump assuming you can just swap the hoses.

Good thinking! I like this idea.
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Old 28-06-2021, 11:31   #7
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

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^^ however the OP' engine (2QM20H) is a simple raw water cooled engine without any fresh water/coolant pump....
Yes but for the manufacturer or marinizer there's at least two reasons to just leave the block mounted pump in place and plug the outlet. The first is that if one removed it a blanking plate would have to be fabricated ind fitted. And. The alternator belt drive would require re-engineering.
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Old 28-06-2021, 17:44   #8
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

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Yes but for the manufacturer or marinizer there's at least two reasons to just leave the block mounted pump in place and plug the outlet. The first is that if one removed it a blanking plate would have to be fabricated ind fitted. And. The alternator belt drive would require re-engineering.
A closer inspection of the above posts will give greater insight and also show the pump in question is gear driven.
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Old 28-06-2021, 21:22   #9
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

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A closer inspection of the above posts will give greater insight and also show the pump in question is gear driven.
Many industrial engines come with the capability to have extra drives off the front. For example the Kubotu V2203 I marinized for my last boat had both an integral water pump and the provision to drive other pumps from the front of the camshaft. Since I raw water cooled the engine I removed the water pump housing and put a blanking plate in place of it and machined an adapter to mount an impeller pump to provide sea water cooling. Being a centrifugal pump the integral cooling water pump was too high above the water line and centrifugal pumps on engines wont self prime.
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Old 28-06-2021, 22:15   #10
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Re: Twin water pumps/impellers on Yanmar 2QM20H, Can I get rid of this?

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Many industrial engines come with the capability to have extra drives off the front. For example the Kubotu V2203 I marinized for my last boat had both an integral water pump and the provision to drive other pumps from the front of the camshaft. Since I raw water cooled the engine I removed the water pump housing and put a blanking plate in place of it and machined an adapter to mount an impeller pump to provide sea water cooling. Being a centrifugal pump the integral cooling water pump was too high above the water line and centrifugal pumps on engines wont self prime.
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