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Old 13-12-2019, 06:41   #1
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Loss of raw water pump prime

I have a problem with losing the prime on my cooling system water pump">raw water pump. In the past, it loses its prime when I haul out. No big deal. Lost once before after sitting for a long time. Should be part of my "back into service" plan.

However, recently, when out sailing in stronger winds and chop, it lost its prime. This is more concerning as that is exactly when I might need engine assistance most.

Engine is Westerbeke W30, AKA 4-91. Recently went from 3/4" to 1" seacock, added watermaker which tees off with its own valve which remains closed. It was doing it prior to adding the watermaker. Seacock is near centerline and deep underwater. 3/4" wire reinforced hose to strainer which is below waterline, then 3/4" hose to pump, which is also below the waterline. There are no leaks or drips in the system, so I cannot identify anywhere air might be entering. The entire system is below the waterline, no siphon break or anything.

When I have started engine and notice no cooling water in exhaust, I open strainer and get water pouring out immediately which does not restore prime. In order to get it working again, I have to pull the outflow hose off the pump which fixes it immediately, but is a bit of an inconvenience.

I am thinking the water is somehow draining from the hose to the strainer, then the pump loses its prime and cannot reprime. Impeller was good when checked a few months ago.

I will recheck the impeller, but am thinking about adding a check valve in the system. A bit worried about adding a restriction as when I switched from 3/4" seacock to 1", it fixed an overheating problem I had a prolonged higher RPM's.

Any input?
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Old 13-12-2019, 06:47   #2
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Loss of raw water pump prime

Impeller pumps are self priming, you have an air leak somewhere so that the pump will pull air which is of course easier than water, or you have a blocked water line.
Iíd suspect the cover on the water pump being excessively scored or loose or something, try flipping it over, most you can use either side.
If you changed an impeller just before started then that may somehow be it, maybe a wrong impeller? Reaching for straws now, but maybe?

Under no circumstance would I add a check valve, they stick eventually and always at the wrong time.
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Old 13-12-2019, 07:12   #3
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Re: Loss of raw water pump prime

Iím with A64pilot on this one across the board.

Check valve is probably not a good idea.

The one time I had an issue with my raw water, it was the o-ring on the raw water strainer. Looked perfectly fine, but was apparently not; A bit of Super Lube took care of it until I was able to replace it.

Just a thought, hope it helps.
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Old 13-12-2019, 07:17   #4
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Re: Loss of raw water pump prime

Have a look at the inside of the cover plate, it might be scored/worn, this will cause the pump to have problems priming.
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Old 13-12-2019, 07:19   #5
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Re: Loss of raw water pump prime

"3/4" hose to pump, which is also below the waterline."

With the pump under the waterline, you must be sucking air somewhere between the seacock and the discharge side of the pump. Air past the pump would not back flow against ambient water pressure coming from being below the waterline.

a64Pilot has good suggestions. Let me add looking for the "air in," and sealing it. You are sucking air, and it has to be getting into the system somewhere.

This is a priority repair, because your raw water pump spinning air could (and may have already) burn out the impeller, and lack of cooling water is detimental to the health and welfare of internal combustion engines.
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Old 13-12-2019, 07:45   #6
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Re: Loss of raw water pump prime

I will check the impeller. Coverplate has been resurfaced and should be OK.


Why not sucking air all the time? After hauling, I understand water draining out, I am assuming a similar mechanism. Why only when it was rough going? Angle of heel might be a factor?
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Old 13-12-2019, 08:56   #7
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Re: Loss of raw water pump prime

Hmmm. If you burped some air in rough water (must have been a ride to remember if you did that!) the impeller pump should have been able to suck it through and keep going. It's a displacement pump. I'm still inclined toward an air leak.

Thought: Is there any evidence of a water leak into the bilge between the through hull and the discharge side of the pump with the seacock open and the boat sitting? Not finding a leak out, given that it's all below the waterline, would suggest (but not prove) that I am wrong.
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Old 13-12-2019, 09:13   #8
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Re: Loss of raw water pump prime

Just a thought and maybe not doable at all. Is there anyway you can turn off the through hull and attach a small hand held vacuum pump at the end of the hose, where it inters the impeller housing? I know you would have to do some reducing on the fittings, but at least you could rule out the sucking air theory.
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Old 13-12-2019, 09:14   #9
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Re: Loss of raw water pump prime

I had a similar problem and it turned out to be pieces of shell blocking the flow just before the valve.
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Old 13-12-2019, 09:39   #10
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Re: Loss of raw water pump prime

I would also check the downstream of the pump to make sure there is nothing to impede the water flow
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Old 13-12-2019, 12:57   #11
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Re: Loss of raw water pump prime

The fact that it looses prime after getting primed and filling with water tells you that there is an air leak. If it was an impeller or wear plate issue it would not get water glow and fill the hoses in the first place. Loosing water back out the thru hull meals you have a leak at or above the pump.
If your hoses are not new - replace them. Cracks or cuts are often problems. Fittings not properly bedded can be issue also. Face plate on pump not fully sealed air tight as well. If your pump uses O-ring replace it. If lip or ceramic seal between impeller and bearings replace it. If heat exchanger end caps are not fully sealed ( donít forget rubber or copper washer) air can get in. Raw water from thru hull to heat exchanger should be a fully sealed (hold a vacuum) system.
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Old 13-12-2019, 13:02   #12
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Loss of raw water pump prime

Hauling doesnít matter, an impeller pump is self priming, it will pump air until it sucks water. Pretty sure that is why they are used even though the require contestant maintenance. You should never have to prime your raw water pump, if you do you have a problem.
A centrifugal pump, like your fresh water doesnít require maintenance and regular replacement of anything, but they donít self prime, so Iím pretty sure that is why they are not used for a raw water pump.
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Old 13-12-2019, 13:45   #13
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Re: Loss of raw water pump prime

As mentioned, it could be an air leak between the thru hull and the pump, and one way to check that is to install a short section of clear plastic hose just before the pump. If there is air getting into the system you will see the bubbles flowing in the clear hose.

But, for my money you have wear of some sort in the pump, and when it looses its water prime there isn't enough suction to reprime. The pump has to be very tight to reprime when pumping air.

Even though the thru hull fitting is located very low in the hull, when sailing fast and heeling over in a rough sea, the water flow along the hull dips in the center of the boat and can expose the hull/keel junction and water will drain from the thru hull and hose. You see this all the time when racing......done it myself and fried impellors.

Check the pump cover plate and gasket, rear wear plate, cam, cam securing screw, seal and impellor.

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Old 13-12-2019, 15:44   #14
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Re: Loss of raw water pump prime

A temporary work around is to shut off the cooling water seacock, and fill up the water pump, the tubing and strainer with fresh or sea water (by undoing e.g. the cap of the strainer if that is high enough and then re-sealing it). Now start the engine. As soon as the engine starts, open the sea cock.
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Old 13-12-2019, 15:51   #15
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Re: Loss of raw water pump prime

I had the same darn thing haopen on a w30

The Impeller LOOKED fine taking off the cover, but a number of vanes wede cracked from the hub, not completely, but 90 percent from rear to front. Pull the impeller and inspect.

All the above advice re scoring, etc, holds true as well.

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