Raw water strainer filling with air

[MikeBz]

I know what everyone is going to say - “It has to be a leak” - but please read on…

I have a Vetus water strainer type 330, the one with the metal lid approved for fitting below the waterline. It is below the waterline.

At rest it remains full of water.

When motoring or sailing gently in smooth water it remains full of water.

As soon as as things get.a bit lumpy or choppy it gradually fills with air, whether motoring or sailing. The rougher the conditions the more quickly this happens.

With the engine running at 2500rpm if I loosen the lid then the water level rises and air is expelled - so it’s not pump-created low pressure sucking air in.

I can only think that it’s air bubbles under the boat coming up into the strainer.

The strainer is directly above the seacock, some way in front of the engine, whilst the hose to the engine goes down into the bilge and back to the rear end of the engine when whence it rises to the raw water pump input so there is no way for air to make its way back to the strainer from the raw water pump.

The engine still pumps water and its running temperature is rock-steady regardless of how full the strainer is. The water level only seems to go down as far as the bottom of the basket.

The odd thing (I think) about this strainer is that it doesn’t input the water via a raised tube, it’s flush with the bottom of the basket, but I don’t see what that has got to do with it:



The skin fitting is a standard circular type, not a scoop.

It’s as though I need a vent from the top of the strainer to somewhere (well) above the waterline but I’ve not come me across anything like that before.
Has anyone experienced anything like this before or got any suggestions?

 

[RAI]

The pump can only suck. The water can only come up the inlet tube. Air can only get in around the top o-ring seal.
I think you would benefit a new o-ring.

 

[MikeBz]

The pump can only suck. The water can only come up the inlet tube. Air can only get in around the top o-ring seal.
I think you would benefit a new o-ring.

When I create a leak at the top o-ring seal by slackening the lid it expels the air and fills with water, regardless of whether the engine is running.

 

[Mistroma]

The pump can only suck. The water can only come up the inlet tube. Air can only get in around the top o-ring seal.
I think you would benefit a new o-ring.

Makes sense. Loosening a top when strainer is below the waterline will allow the water to rise whether the pump is running or not. The pump must be sucking in water, that's what it does. If the entire engine was below the waterline the pump would push water out. I assume that's not the case and so the pump needs to suck water up onto the engine.

If the pump is sucking water in then there will be -ve pressure at the strainer. Air is being drawn in from a leak at the strainer.

 

[MoodySabre]

A friend has the same issue on his Oceanis 31. He and a mechanic have checked everything for leaks, cracks etc and have put Vaseline round the screw on top. When sailing he shuts the seacock and if he needs to put the engine in he dashes below to open it. Very unsatisfactory. A badly designed set up IMO.

 

[Mistroma]

A friend has the same issue on his Oceanis 31. He and a mechanic have checked everything for leaks, cracks etc and have put Vaseline round the screw on top. When sailing he shuts the seacock and if he needs to put the engine in he dashes below to open it. Very unsatisfactory. A badly designed set up IMO.

I've seen similar when the raw water pump was worn at front and rear faces. It wouldn't self-prime very easily. Leaving the valve open for a long time allowed the pipe to empty. Even worse when heeled as the height difference was greater. Keeping the valve closed kept the pipework full and all was well if it was only opened shortly before starting and closed as soon as it wasn't needed.

Tricky to diagnose as there are no obvious leaks. Air is very slowly sucked in via the exhaust elbow and back down through the system to the worn pump faces.

 

[pete]

Highly unlikely in this case but could your exhaust elbow be restricted causing back pressure?

 

[jbweston]

I confess I'm no expert on this, but if the engine temperature remains steady under load whatever the water level in the strainer, does it matter that there's air in the strainer? It sounds as if the raw water pump is sucking in seawater and circulating it OK.

 

[Refueler]

I have one of those .... the water comes in around the filter and then exits through it down the pipe ... mine also does NOT fill up always - but water is always going through and engine runs fine ... 2GM20....

I can see water passing through it if I look carefully. At rest - the bowl is reasonably full ... but when motoring - I see its lower ..

Never really thought about it as water is spitting out exhaust .. engine running cooled ...

 

[Mistroma]

Highly unlikely in this case but could your exhaust elbow be restricted causing back pressure?

That's a thought. Surely back pressure would increase pressure at the strainer. Wouldn't that result in water trying to leak out of the strainer rather than air being sucked in. Perhaps you were thinking it that exhaust gas was being blown back to the strainer and causing gas bubbles. Surely that would mean a severe flow restriction and overheating.

I'm also putting that down as highly unlikely as you say.

 

[johnalison]

I can’t see how air getting in while sailing can have anything to do with the pump, at least in this instance. I would have thought that something akin to cavitation must be happening at the strainer and microbubbles combining to rise up the hose.

 

[pete]

That's a thought. Surely back pressure would increase pressure at the strainer. Wouldn't that result in water trying to leak out of the strainer rather than air being sucked in. Perhaps you were thinking it that exhaust gas was being blown back to the strainer and causing gas bubbles. Surely that would mean a severe flow restriction and overheating.

I'm also putting that down as highly unlikely as you say.

We have just sorted out a 115hp outboard engine that pumped cooling water around fine until you gave it some throttle then the water almost stopped (which wasn't that obvious when going fast). It turned out to be exhaust gasses entering the pump inlet via a warped rubber gasket in the skeg between the water inlet and the exhaust outlet below the pump housing which was almost stopping the flow. This is a bit of a different problem, it took us ages to get to the bottom of it.

 

[Halo]

It seems likely the air is coming in with the water.
I sometimes crew on a Challenge 72 and this has a burp valve fitted at the inlet strainer to vent air that can enter the system when she is sailed healed well over. So it’s not unheard of for air to come in via the raw water pipe work The symptoms you describe also seem to support this.
What happens if you run he engine on high idle ? ( Max revs in neutral - best done with engine warm). Does the air get purged out of the strainer , does any air come in with the boat running in flat marina conditions?
On my own boat the strainer seal needs changing often and needs plenty of Vaseline.

 

[Mistroma]

We have just sorted out a 115hp outboard engine that pumped cooling water around fine until you gave it some throttle then the water almost stopped (which wasn't that obvious when going fast). It turned out to be exhaust gasses entering the pump inlet via a warped rubber gasket in the skeg between the water inlet and the exhaust outlet below the pump housing which was almost stopping the flow. This is a bit of a different problem, it took us ages to get to the bottom of it.

I had a problem with a BUKH DV20 many years ago. It would start, run perfectly in neutral and rev. as expected. However, it only ran at tickover in gear and stopped when throttle opened.

The exhaust hose had delaminated in one spot and was acting like a flap valve. Fine at tickover and with no load. Back pressure stopped the engine when it was being asked to do any useful work.

Not easy to diagnose. I had a lie down for about half an hour trying to think of something that explained the symptoms. Then I removed the fairly short exhaust hose and had a poke around inside. Stuck out like a sore thumb at that point.

 

[Refueler]

Blimey ... I post about the same strainer and it does it ... and still people talking about xxxxxxxx

The only time I had a problem was on launching the boat ... I had to slack the top thumbscrew to release the air and let water in ... once that was done all fine ...
The strainer - when engine is at cruise rpm - does have a low water content ... and it fills back up when throttle is closed.

 

[Mistroma]

It seems likely the air is coming in with the water.
I sometimes crew on a Challenge 72 and this has a burp valve fitted at the inlet strainer to vent air that can enter the system when she is sailed healed well over. So it’s not unheard of for air to come in via the raw water pipe work The symptoms you describe also seem to support this.
What happens if you run he engine on high idle ? ( Max revs in neutral - best done with engine warm). Does the air get purged out of the strainer , does any air come in with the boat running in flat marina conditions?
On my own boat the strainer seal needs changing often and needs plenty of Vaseline.

OP could clarify a point. I read"When motoring or sailing gently in smooth water it remains full of water." as motoring or sailing both being in at low speed. i.e. Engine only running at low revs.

It just dawned on me that I might have picked it up wrongly.


Does the problem happen in calm conditions when running the engine at high revs? If not, then it is likely to be air coming in when it's rough or well heeled.

 

[jbweston]

I think I must be missing something.

In his original post MikeBz says:
The engine still pumps water and its running temperature is rock-steady regardless of how full the strainer is. The water level only seems to go down as far as the bottom of the basket.

So what's the problem? The cooling system is working fine. It might be interesting to satisfy the curious as to why it works like that. But work it does.

 

[MikeBz]

Thanks for all the input!

I haven’t tried motoring at high revs in smooth water, at least not > 2400, but the same or lower revs in rough water result in air coming in. I do know that at 2500 it still purges air when I crack the lid.

The strainer is 1 year old and the lid & seal well greased, I have no doubt that it is well sealed just by the behaviour when I slacken it to refill.

I don’t understand how back pressure at the exhaust could cause it to suck air into the strainer when deliberately creating a tiny leak by cracking the lid purges the air? I will check the elbow this winter anyway.

I will see if I can get some sort of vacuum pump onto it over the winter just to check.

I think it has to be air coming up from the skin fitting, but don’t understand why this isn’t a more common problem. Maybe the scoop type skin fitting is less prone to it.

Maybe it is fine to run it with the basket almost empty.

 

[Poignard]

Being at the highest point in the system, any air sucked in in rough weather will collect in the strainer, where it will remain if the pump suction is insufficient to draw it out.

The first thing I would do is overhaul, or renew, the pump.

If that doesn't work, I would move the skin fitting.

 

[MikeBz]

Maybe the pump is worn - another thing to check this winter. New pump £500 from Parts4Engines!!