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Old 20-10-2014, 08:47   #1
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Air lock in raw water pump

My water pump">raw water pump often air-locks. Apparently when the boat heels while sailing the water intake is exposed to air and later when the motor is started the air is sucked into the pump and water flow stops. I can avoid the problem by closing the motor's thru-hull while sailing. When the pump was new it didn't air-lock but after 300 hrs of use the problem was first noticed. I changed the impeller several times- didn't help. Several years later I replaced the pump due to leaks at the shaft and the new pump worked for a year and then acted the same as the original pump. This is a Johnson pump, the motor is Yanmar 2-cylinder, the boat is a Pacific Seacraft Dana (24 ft).

I recently sailed on a friend's Catalina 36 (1985) and found that he is having the same problem. He closes the thru-hull but if it is left open and air gets in then he must disconnect a hose to let the trapped air out- then water flow is restored. So I see two different boats, quite different, but both with a similar problem. It's hard for me to believe that the typical boat owner puts up with a boat where motor water flow is uncertain each time you start the motor. I installed an air bleed system (accessible from outside the motor compartment) and this is a crude-but-effective fix. Any comments from all you forum members?
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Old 20-10-2014, 09:54   #2
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Re: Air lock in raw water pump

I had that problem for a while, very frustrating. I too thought the pump was working properly. However a new cam and another new impeller seems to have fixed it.
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Old 20-10-2014, 10:03   #3
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Re: Air lock in raw water pump

Relocate the seacock to a position that is always submerged. Make certain there are no traps in the intake line. Overhaul the pump.
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Old 25-10-2014, 17:14   #4
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Re: Air lock in raw water pump

something is rotten in Denmark.
A self priming pump should easily lift water 3' to the engine. (The theoretical max lift is around 25') If you have an air leak (loose clamp?) on the hose from the thru-hull to the pump, nearer the pump the air intake will prohibit water reaching the pump. Or the thru-hull, screen or strainer has something clogging it. But easiest, and first thing was the annual impeller change out...
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Old 25-10-2014, 17:26   #5
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Re: Air lock in raw water pump

Is the engine raw (sea) water cooled? Or does the raw water circulate thru a heat exchanger; thereby cooling the engine coolant soluation - FRESH WATER COOLED ENGINE? Important to know. It is really unlikely that you're heeling to the point of exposing the raw water inlet to air (if it is properly placed, this would require >45 degrees heel). But, if so, a 30 second gap to reprime the raw water pump ON AN ENGINE THAT IS FRESH WATER COOLED, should do no damage.

There are different synthetic rubber compositions for impellers. Check that your impeller remains as flexible at water temp of 32 F (freezing point of water - ice cubes in water) as it is in 70 F water (room temp). If you detect a difference, this is a problem. Next carefully inspect the bronze surface of the pump body in which the impeller turns. Any gouges or pits? Have you used the engine to get off a grounding? Is there any growths that are impeding sea water flow into the seacock? Is the seacock a seacock, ball valve OR gate valve? Is it fully opened or is there marine growth restricting flow?

Something is wrong and it is unlikely to be the thru-hull placement else the problemn would have been with you from day 1 you owned the boat.
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Old 25-10-2014, 19:41   #6
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Re: Air lock in raw water pump

Thank you Cal 246 and JKK. Good suggestions. The motor is fresh water cooled so running for a short time with no raw water probably isn't too harmful. It seems that this model pump when brand new with zero wear will self prime. I think for my first year of sailing I didn't get heeled to large angles very often- that's the only reason I can think of as to why the problem didn't start immediately. In addition to heeling if there are three foot waves this helps get air under the boat where it can enter the intake line. The intake is as low on the hull as it can be, right next to the keel. I have seen other boats expose their keels when heeling so I know it is possible to expose the intake.

I changed the impeller several times with no change to the problem but when I had to buy a new pump the problem went away for awhile. The pump doesn't look to be worn but the cam must be slightly worn or there is leakage on the sides of the impeller.

I have found two solutions but neither is desireable. I can close the water intake when sailing (it's a ball valve and in good condition). Or I can open the top of the water strainer and let water overflow to get rid of the air. These two methods always result in proper raw water flow. Now I have added an air release line and valve to the top of the water strainer so I can release the air without going inside the motor compartment. When I open the valve I can hear air escape.

The idea about the impeller being inflexible when cold is possible- our water is 54 degrees all year long. It could be the cause but I don't see a fix. I just started this thread to see if this was a problem common to many boats and maybe get some ideas on how others with the problem had improved things. Based on responses so far it seems that most pumps aren't bothered by a little air unless they are worn out and need a rebuild.
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Old 25-10-2014, 23:16   #7
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Re: Air lock in raw water pump

I wonder if the location or orientation of the strainer has anything to do with it? Are there any similarities in your setup and your friend's? I can hear the howls of protest already, but have you considered a check valve of some sort?

A flexible vane impeller pumps' efficiency decreases somewhat as time goes by, but it shouldn't decrease by the amount you describe. I'd think there is some simple fundamental error in the way the plumbing is set up...
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Old 26-10-2014, 01:03   #8
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Re: Air lock in raw water pump

I would try calling Johnson and see if they are aware of this problem and have already figured out a fix.
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Old 26-10-2014, 05:52   #9
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Re: Air lock in raw water pump

Another option is to change pumps. Go with an Oberdorfer. You might have to do some custom work to fit it but it could be worth it.


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Old 26-10-2014, 06:11   #10
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Re: Air lock in raw water pump

What level is the strainer in relation to the pump? If its a little higher than the pump then water should be drawn from it and less air. If it is a lot higher that could be a disadvantage. Maybe a larger strainer will help. If the impeller and its mating surfaces are not worn they will pull enough water and/or air despite waves and heeling. Greasing the impeller once a year may be one of those "factory recommendations". Air will always get in because bubbles are always running under the hull.
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Old 26-10-2014, 08:44   #11
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Re: Air lock in raw water pump

Are you taking the boat >45 degrees of heel? Surely it would require such a steep angle to expose the keel.

The water temp of 54F may be the key. I would run the experiment of ice water bath effect on the impeller. Also compare flexibility of older impeller to new impeller. We experienced much the same in the Pacific Northwest with water temps in the upper 40's to low 50's. The impellers were failing; it turned out the rubber was not suitable for such temperatures.
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Old 26-10-2014, 11:47   #12
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Re: Air lock in raw water pump

The sea strainer plumbing must have a continual rise. If there is a trap in it, it the pump will suck air.
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Old 27-10-2014, 15:58   #13
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Re: Air lock in raw water pump

Cal246- I haven't had any impellers fail (break) but they could be too stiff when cold and therefore not seal adequately.
Terra Nova- You are right about trapping air. My pump is slightly lower than the thru-hull but the strainer is one foot higher than the pump and at the top of the trap. There isn't a good place to mount the strainer lower but at least the strainer location makes it easy to get the air out. I just wish the pump would prime as it should when it gets a gulp of air. A more flexible impeller might be the answer.
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Old 27-10-2014, 16:54   #14
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Re: Air lock in raw water pump

Quote:
Originally Posted by EveningTide View Post
...My pump is slightly lower than the thru-hull but the strainer is one foot higher than the pump and at the top of the trap...
There's your problem--very poorly laid out intake.
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Old 15-01-2015, 17:05   #15
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Re: Air lock in raw water pump

I have exactly the same problem with my hallbergrassy 40 and volvo md55 engine.

After heeling a lot or in rough waves my engine will run dry.

Last time even when my engine was running in rough water. I was lucky that I heard the change of the sound and stopped the engine before it ran hot.

Than its not enough to fill the strainer with the seacock closed and opened again when the strainer is filled with water.
even with the system full of water the pump does not start sucking.
I have to open the impellor cover ( 4 screws) and drain the system by gravity. Than it will start pumping again.

I tried new impellors, later changed the water hoses becauses the old had flattened on one place due to the vacuum.

I still have the same problem....

The engine is quite new, only ~500 hours.

Next what I will try when I go back to my boat in April is to lower the Vetus cooling water strainer which is maybe mounted to high.
But as it is a hallberg-rassy I think they should know how high over the waterline a strainer has to be mounted ?!

..and I will have a look for terra novas tip: "The sea strainer plumbing must have a continual rise. If there is a trap in it, it the pump will suck air. "

as far as I remember the cooling water passes the gearbox on its way to the water pump, maybe there is a trap...

or the heat exchanger clogged?

When this will not help I try a brandnew volvo waterpump ....


all my former boats were able to fill the empty watersystem just by starting the engine !
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