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Old 21-05-2022, 05:23   #1
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Why no service on fresh water pump?

We are all used to replace the impellor in the water pump">raw water pump on the diesel, but I've never heard of any service required on the other water pump (fresh water/coolant).

Just curious, how does it operate, and why don't we have to replace impellors in it?
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Old 21-05-2022, 05:37   #2
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Re: Why no service on fresh water pump?

A lot of pumps are sealed if you are discussing cabin water pumps.. I’m in fresh water and on dinghies in close contact with sand and silk it’s an annual event. I have a spar impeller for the Dinghy in a locker.
The Yanmar impeller seems to get very little wear and discarded before its due. Poor winter storage can damage water systems.
Most bilge pumps are sealed. The interior of the pump expires with the impeller. The water pumps which take replacement impellers eventually wear out.
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Old 21-05-2022, 05:43   #3
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Re: Why no service on fresh water pump?

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A lot of pumps are sealed if you are discussing cabin water pumps.. Iím in fresh water and on dinghies in close contact with sand and silk itís an annual event. I have a spar impeller for the Dinghy in a locker.
The Yanmar impeller seems to get very little wear and discarded before its due. Poor winter storage can damage water systems.
Most bilge pumps are sealed. The interior of the pump expires with the impeller. The water pumps which take replacement impellers eventually wear out.
No, I was thinking of the pump on a diesel, that pumps the water/coolant from the heat exchanger through the engine.
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Old 21-05-2022, 05:43   #4
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Re: Why no service on fresh water pump?

Most house water pumps are a diaphragm type pump and they are serviceable. The three most common repairs I am familiar with are replacing the pressure switch, replacing the diaphragm and seals, and replacing the whole pump head. I think you don't hear about repairs to house water pumps like you do with raw water pumps as the service regime is pretty different. They may break as often or even more often on a per hour of use basis, but are used much less time in a year of cruising than an engine raw water pump. Also they are not subjected to sea water so corrosion is much less an issue. Finally, if the problem lies with the electric motor most cruisers will likely just replace the pump rather than trying to rebuild the motor.
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Old 21-05-2022, 05:44   #5
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Re: Why no service on fresh water pump?

Raw water pumps use a flexible impeller to make them self-priming. Rubbing and bending the impeller every rotation wears it out. Coolant (fresh water) pumps are centrifugal and use a rigid impeller (often metal). Since the blades don't touch and don't flex, they almost never wear out. If the engine is run without coolant or significantly overheated, the seal will need replaced.
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Old 21-05-2022, 05:47   #6
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Re: Why no service on fresh water pump?

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Raw water pumps use a flexible impeller to make them self-priming. Rubbing and bending the impeller every rotation wears it out. Coolant (fresh water) pumps are centrifugal and use a rigid impeller (often metal). Since the blades don't touch and don't flex, they almost never wear out. If the engine is run without coolant or significantly overheated, the seal will need replaced.
Indeed.
And, they're pumping "clean" water/fluid.
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Old 21-05-2022, 06:07   #7
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Re: Why no service on fresh water pump?

If you are referring to the fresh water pump bolted to the front of your engine, that circulates a water/ anti-freeze mix, It is a centrifugal pump. this type of pump has a (usually) cast iron impeller, immersed in a liquid, and has no contact with the sides of the pump, so no wear. Really old versions of this pump had a grease fitting for the shaft support bearings, but alas this practice has been discontinued.
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Old 21-05-2022, 06:17   #8
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Re: Why no service on fresh water pump?

Thanks for the good answers, I was just curious about this.
So the reason that we can't have a service-free raw-water pump is, that there will be impurities, sand and other debris in the water we take in, so that pump has to be made differently?
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Old 21-05-2022, 06:22   #9
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Re: Why no service on fresh water pump?

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Originally Posted by SaylorMade View Post
Thanks for the good answers, I was just curious about this.
So the reason that we can't have a service-free raw-water pump is, that there will be impurities, sand and other debris in the water we take in, so that pump has to be made differently?

No, it's because the raw water pump needs to be self-priming.


It's not entirely true that fresh water pumps are maintenance-free. They have a very long life if you keep belt tension right, but they do lose their seals and/or ruin their bearings from time to time. I carry complete spare fresh water pumps for both main engine and genset.
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Old 21-05-2022, 06:30   #10
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Re: Why no service on fresh water pump?

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No, it's because the raw water pump needs to be self-priming.


It's not entirely true that fresh water pumps are maintenance-free. They have a very long life if you keep belt tension right, but they do lose their seals and/or ruin their bearings from time to time. I carry complete spare fresh water pumps for both main engine and genset.
Aaah OK - got it - thanks
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Old 21-05-2022, 07:22   #11
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Re: Why no service on fresh water pump?

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Originally Posted by SaylorMade View Post
We are all used to replace the impellor in the raw water pump on the diesel, but I've never heard of any service required on the other water pump (fresh water/coolant).

Just curious, how does it operate, and why don't we have to replace impellors in it?

Do your service the coolant pump on your car? Same reasons.
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Old 21-05-2022, 07:41   #12
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Re: Why no service on fresh water pump?

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Do your service the coolant pump on your car? Same reasons.
I don't service anything on my car myself, but I know that every time the timing belt is replaced, they replace the water pump at the same time, since they have it all torn apart anyway. This is common practice here for all car makes.
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Old 21-05-2022, 08:24   #13
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Re: Why no service on fresh water pump?

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I don't service anything on my car myself, but I know that every time the timing belt is replaced, they replace the water pump at the same time, since they have it all torn apart anyway. This is common practice here for all car makes.
That sounds like "It's not broke but I'll fix it anyway" thinking.
An old work truck that I had went 170,000 miles never having needed a new water pump, several belts were changed during that time frame.
Does your boat use a high-tension timing belt to drive the pump?
Failure of coolant pumps, (the bearings(s) having eccentric loading,) is primarily due to excessive belt tension.
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Old 21-05-2022, 08:41   #14
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Re: Why no service on fresh water pump?

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That sounds like "It's not broke but I'll fix it anyway" thinking.
An old work truck that I had went 170,000 miles never having needed a new water pump, several belts were changed during that time frame.

Bowdrie,
A modern timing belt driven overhead cam engine is a very different animal from your old work truck. The water pump on many (most) modern engines is driven by the timing belt. A water pump failure will result in a timing belt failure, with catastrophic results. in order to meet emission and CAFE standards most engines will be interference types. The pistons will collide with the valves, resulting in bent valves, possibly damaged pistons, occasionally cylinder head or cylinder damage. Weigh the risk/reward.
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Old 21-05-2022, 08:51   #15
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Re: Why no service on fresh water pump?

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Originally Posted by SaylorMade View Post
No, I was thinking of the pump on a diesel, that pumps the water/coolant from the heat exchanger through the engine.
It is generally a centrifugal type pump and because it works in clean, solids free fluid there is no wear on the impeller. If they fail at all it is generally because the mechanical seal on the shaft starts to leak.
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