Sporadic loss of water/airlock in strainer

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DHV90

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Hi guys,

Looking for a bit of guidance regarding a raw water issue. The engine is a yanmar 3gm30 raw, coolant arrangement is simple, seacock - strainer - water pump. Currently everything is below the water line, the strainer only just so but when I remove the lid water does trickle out.

Every so often I am losing water, and needing to reprime. Removing the hose off the inlet side of the water pump and sucking water through, then reconnecting, solves the problem every time but I want the system to self prime. As far as I can see this suggests the problem area is between the seacock and the inlet to the water pump, but not after.

For example, on our return home today the lock was very turbulent so there were lots of bubbles in the air and the cooling water stopped around this time. We had been motoring for about 20 minutes after a 4 hour sail, with no problems, but after 15 mins sat in the lock the water stopped. I imagine it was air pockets in the water which caused the airlock, I don't believe it is correlating with anything else like water draining out whilst sailing, after sailing the cooling water was spraying straight away and didn't stop.


What can I do to encourage the engine water to self prime, and is there anything you think I can check?

thanks in advance!
 
You need to check the seal on the lid of the strainer for air leaks. Is it tight? Is there an O-ring? Is it in good condition? Have you tried lubricating a good O-ring with silicone grease?
 
pvb said:
You need to check the seal on the lid of the strainer for air leaks. Is it tight? Is there an O-ring? Is it in good condition? Have you tried lubricating a good O-ring with silicone grease?
Click to expand...

+1

And also check the condition of the impeller and the pump faceplate as these pumps sometimes struggle to self-prime if they are getting on a bit and there is some air in the system. A good way to check is to slather the impeller and the pump faceplate with silicone grease or, less ideally, vaseline. Let some air into the strainer and start the engine and if the pump suddenly seems to have developed a miraculous ability to pump the air through then you have a clue as to what's going on.

Richard
 
Yes to all the above, I would have thought that seal problems would show up problems on start up after sailing, rather than during running but im not sure? It all looks good and tight but i cant be sure, best test I did was taking the water pump inlet pipe off and blowing into it with the sea cock closed to see if I can hear any leakage from the strainer seal but not very scientific!
Any good way to check?
 
A quick check on the seacock and inlet, that it is not blocked with weed or barnacles might also be a good step. If partly blocked, the water pump will induct air from e.g the strainer rim at higher revs.

It's worth having a flexible steel wire to prod down the strainer inlet to the seacock to dislodge visitors.

BTW, is the pump belt nice and tight ? Any belt dust near the pulleys might indicate slippage - again at higher revs .
 
Good shout on letting air into the strainer to simulate the problem, ill go give that a try now.

Worth noting that this has been happening every so often since going back in the water in spring, and I replaced the impeller at the time so I will do a test to make sure I can simulate the air lock, then check the impeller/grease and reassemble then see if that sorts it.

Belt is nice and tight, seacock clear etc.
 
Check all hose clamps and the hose underneath the clamp for splits. I once had a clamp that seemed to be tight but in fact the screw was worn and it was not clamping tightly enough.

Air seems to be getting in somewhere - you just have to find where.
 
searj said:
Good shout on letting air into the strainer to simulate the problem, ill go give that a try now.

Worth noting that this has been happening every so often since going back in the water in spring, and I replaced the impeller at the time so I will do a test to make sure I can simulate the air lock, then check the impeller/grease and reassemble then see if that sorts it.

Belt is nice and tight, seacock clear etc.
Click to expand...

I, personally, would not recommend nice and tight but prefer nice and loose .... but that's not related to the airlock issue. :)

Richard
 
well, belt is the correct tension!

Quick update - I just spent an hour trying to get the strainer to airlock but, even if I drained the entire lot, as soon as I opened the seacock water would poor out of the hose that feeds the pump and nothing I did would stop it. I tried loosening the cap on the strainer/deliberately mis-seating it but no luck. Im starting to think it might be the pump itself? Its the only thing I've really changed (impeller) since last year when I had no problems. Could the pump itself just occasionally airlock so badly that it won't get going again? Is that normal or a problem?
 
Hmmm, interesting. It seems you were right and that the strainer and hoses are all below the waterline so the whole system is essentially self-priming. I am therefore very surprised that the pump can ever fail to self-prime and start pumping even when a lot of air has been drawn in through the seawater intake.

Both my pumps are prone to failing to prime if the boat has been out of the water and the inlet hoses have drained down. I can always correct this by re-greasing the impellers which restores their efficiency but I just tend to give them a quick burst of revs if no water appears out of the exhaust at idle speed. A few seconds at 1500 revs and the exhaust water will appear, after which they will pump fine even at idle.

I should add that I have fitted Speedseal Lifes to both engines so running them dry is not going to do any harm but, even with ordinary impellers, a burst of high revs will not cause a problem.

Richard
 
Can air accumulate in the top chamber of the strainer?
My strainer fills from the bottom, but the take off for the engine is near the top about 10mm below the lid. Potentially a void that could fill with air with nowhere to go maybe?

edit - to clarify, i would expect a self priming system to need a continuous rise, whereas this system has the void at the top of the strainer as a high spot, the system rises to there and then descends to the pump, should a healthy pump be able to suck through trapped air in the top of a strainer?

Im going to buy a new strainer anyway to be on the safe side, and ill dismantle the pump to check and grease and go from there.
 
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pvb said:
It's incorrectly installed. The inlet should go to the higher point.
Click to expand...

excellent, thanks for that. I will rectify it. But, curiously, the strainer has always been that way round and hadn't been a problem in several years so I feel like something else is amiss as well.
 
PVB, in my strainer the sea inlet leads to the strainer inlet located in the middle of the bottom face. When running, the water fountains up though the centre of the basket, thence though the perforated side walls to an outlet towards the top , thence to the water pump. That's the way weedy bits are trapped in the basket.
 
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sarabande said:
PVB, in my strainer the sea inlet leads to the strainer inlet located in the middle of the bottom face. When running, the water fountains up though the centre of the basket, thence though the perforated side walls to an outlet towards the top , thence to the water pump. That's the way weedy bits are trapped in the basket.
Click to expand...

Yes, I'm sure it does, but many strainers (including the OP's apparently) have the inlet pipe going high up inside the strainer body. The purpose of this is so that all the water in the strainer doesn't drain back into the sea when the lid is removed, thereby helping to prime the pump when the engine is restarted. For example...

ftr140_dwg-399x399.jpg
 
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sarabande said:
PVB, in my strainer the sea inlet leads to the strainer inlet located in the middle of the bottom face. When running, the water fountains up though the centre of the basket, thence though the perforated side walls to an outlet towards the top , thence to the water pump. That's the way weedy bits are trapped in the basket.
Click to expand...

pvb said:
Yes, I'm sure it does, but many strainers (including the OP's apparently) have the inlet pipe going high up inside the strainer body. The purpose of this is so that all the water in the strainer doesn't drain back into the sea when the lid is removed, thereby helping to prime the pump when the engine is restarted. For example...
Click to expand...

Maybe I'm mistaken but I think you're both agreeing with each other and talking about a strainer the same as my Vetus one where the inlet is the highest point in the strainer, just a cm below the clear lid, and the water then falls back through the strainer mesh to the outlet which is below the mesh and thereby well below the inlet aperture.

The OP's is indeed wrongly installed. If you remove the clear lid the seawater should come in through the high pipe in the middle of the strainer if it's below sea level. In my case with a higher mounting, I can pour water down the central pipe and it goes straight into the sea.

Richard
 
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Graham376 said:
We have the type pictured above which has never given a problem but our club launches are fitted with another type with the lid held down by a wing nut and we have had the same problems as the OP and only a new O ring seal cured it - https://www.ebay.co.uk/itm/Raw-Wate...110785?hash=item3ac12dda01:g:R60AAOSw8RJXCLaC
Click to expand...

Thats the type I have, where did they source an O ring? do generic seals go up that big or was it somewhere specific (if you remember!) Thats really useful though, thanks!
 
The pump is self priming even from completely dry, although greasing will help to seal it. Another source of air that can prevent this from happening, other than the face plate already mentioned, is wear of the shaft seals. Our pump would almost always fail to prime after sailing in boisterous conditions, which appears to indicate that air is entering from the top of the column formed by the suction hose. The seal did not leak water for several years while this was happening but when it did finally fail and was replaced the problem went away and has never returned.
 
searj said:
Thats the type I have, where did they source an O ring? do generic seals go up that big or was it somewhere specific (if you remember!) Thats really useful though, thanks!
Click to expand...

I don't know where new O rings were sourced (it's a few years ago) and I'm not in UK to ask anyone at the moment. IIRC, temporary fix was to take the ring out, smear some grease into the groove and then re-insert the seal upside down. Some motor factors will make up a new ring if you take the old one in as a sample. I'll PM a phone number of a guy who may be able to help.
 
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