Raw water strainer filling with air

[Poignard]

"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 'raised tube' in strainers intended to be installed above the waterline ensures that some water remains in the strainer when the engine is stopped.

Then, when it is next restarted, the pump is primed and does not run dry, and damage the rubber impeller.

It also ensures that the maximum area of the strainer mesh is utilised.

 

[RAI]

Apart from the big O -ring, what is the status of the little O-ring sealing the centre top retaining shaft?

 

[Mistroma]

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.

If it purges air when opened at 2500rpm that tells you that water is flowing in faster than it's being removed. Inward flow is just because the strainer is below the waterline. I'd say that has to indicate that the pump flow rate isn't very high. The engine isn't overheating and the low flow rate seems adequate at present.

I suppose slight back pressure could reduce the flow rate. Still not a problem yet if no overheating.

It is always good to know why things happen as it helps when problems do arise.

Motoring in calm at 2500 rpm doesn't draw air but similar motoring in rough conditions does form a bubble. That pretty much confirms it's being drawn up via the inlet and some air is entering along with water.

 

[Graham376]

We had similar problem with one of our club launches with Vetus filter. Turned out it was the large O ring. Removed it and fitted it back inverted and well greased, problem solved.

 

[MikeBz]

Apart from the big O -ring, what is the status of the little O-ring sealing the centre top retaining shaft?

Sorry, where is that?

 

[MikeBz]

More data today which backs up it being air coming up from under the hull. At the end of the day yesterday I bled the air out by slackening the lid until there was just a small bubble (so any change would be easy to see).

This morning - no change.

Half an hour of gentle motoring (gentle because it was a narrow shallow channel at low tide) - no change.

90 mins of sailing (engine off) in a bit of a lumpy sea - strainer half empty.

I’ll take the pump off this winter and find out how to check for wear.

 

[PabloPicasso]

Have you got a faulty anti-siphon valve? Perhaps that lets air in when it shouldn't?

 

[PabloPicasso]

When did it become an issue?

If its always been like that then the water intake maybe lifting out of the water, or getting cavitation bubbles.

Can you temporarily pipe water from a different thru hull that stays immersed in clear , cavitation free water?

 

[Supertramp]

One way to check this is to measure the amount of cooling water produced from the exhaust. If its the same with no air in the filter as when there is air then no problem.

My system has the strainer above the waterline and has a raised inlet pipe. It is never full of water when I take the lid off, nor does my cooling water flow change, nor does my engine overheat. I suspect these strainers can operate with some air in them  providing the top and rest of the system are sealed.

I'm not convinced you have a problem!

 

[greeny]

Is the fact you just effectively have a hole in the hull as the supply and not a scoop causing the problem. As water passes the hole under certain sea conditions, it draws a partial vacuum which affects the ability of the pump to suck water up. Couple that with maybe a not so good pump then that could be the cause?

 

[PabloPicasso]

When did you last change the impeller?

 

[Refueler]

Seems I wasted time posting about this filter ...

This filter does not run full of water ....... the level drops as the engine speed increases ... in fact I cannot remember last time I saw my filter bowl full.

 

[HunterWanderer]

I'm with refueler here.
The filter outlet is at the side of the bowl so there will normally be air above the water in the filter. It's like this so you can shut it down and clear the bowl without getting water everywhere.
Bubbles will form in the inlet hose as the pressure is reduced as the pump sucks and the air will accumulate at the top of the filter.
The faster the engine the greater the suction and more bubbles form.
Providing you don't get steam from the exhaust at full speed there's no problem.

 

[Channel Sailor]

Is the strainer connected up backwards? A vetus one has arrows on the side to show inlet and outlet.

 

[pete]

I don’t understand how back pressure at the exhaust could cause it to suck air into the strainer

Exhaust at high revs being restricted from escaping via the water lock so passing back down the cooling water pipe to the strainer. Unlikely? Probably, but not impossible

 

[MikeBz]

Exhaust at high revs being restricted from escaping via the water lock so passing back down the cooling water pipe to the strainer. Unlikely? Probably, but not impossible

The hose from the strainer to the raw water pump goes down to the bottom of the bilge for some distance and then back up to the pump inlet quite high on the engine. So air coming back would have to go downhill against the water flow for some distance to get back to the strainer.

When did it become an issue?

If it’s always been like that then the water intake maybe lifting out of the water, or getting cavitation bubbles.

Can you temporarily pipe water from a different thru hull that stays immersed in clear , cavitation free water?

I replaced an old fashioned gauze type strainer last season, I think the issue has been there since then - but with the old strainer there was no transparent lid so no way to know. The thru-hull is midships (fore and aft wise) and pretty much adjacent to the keel. No other options. It would probably be better if it were further aft.

The impeller was new this season.

Is the strainer connected up backwards? A vetus one has arrows on the side to show inlet and outlet.

No it’s definitely correct. Water comes up into the basket through the inlet pipe. Weed collects inside the basket not outside.

Have you got a faulty anti-siphon valve? Perhaps that lets air in when it shouldn't?

There is no anti siphon valve between the thru hull and the raw water pump.


Thinking back to our last boat, it had a standard setup with a plastic strainer well above the waterline which I never had to prime - the raw water pump just sucked all of the air out of it straight away. That was a Volvo MD2020. This is a Perkins Prima M50. Maybe the pump isn’t as good as it should be or maybe it’s just not as powerful.

 

[Poignard]

The hose from the strainer to the raw water pump goes down to the bottom of the bilge for some distance and then back up to the pump inlet quite high on the engine. So air coming back would have to go downhill against the water flow for some distance to get back to the strainer.



I replaced an old fashioned gauze type strainer last season, I think the issue has been there since then - but with the old strainer there was no transparent lid so no way to know. The thru-hull is midships (fore and aft wise) and pretty much adjacent to the keel. No other options. It would probably be better if it were further aft.

The impeller was new this season.

No it’s definitely correct. Water comes up into the basket through the inlet pipe. Weed collects inside the basket not outside.


There is no anti siphon valve between the thru hull and the raw water pump.


Thinking back to our last boat, it had a standard setup with a plastic strainer well above the waterline which I never had to prime - the raw water pump just sucked all of the air out of it straight away. That was a Volvo MD2020. This is a Perkins Prima M50. Maybe the pump isn’t as good as it should be or maybe it’s just not as powerful.

Was the pump cover worn?

 

[MikeBz]

Was the pump cover worn?

Not that I noticed but I will check. Presumably it’s the front cover and the back face which could wear and reduce the performance. I had to squish the blades as usual to get the impeller in so the outer surface can’t be a problem.

 

[Poignard]

Not that I noticed but I will check. Presumably it’s the front cover and the back face which could wear and reduce the performance. I had to squish the blades as usual to get the impeller in so the outer surface can’t be a problem.

There may also be a wear plate at the back.
Also the gasket should be made of very thin material.

 

[MikeBz]

There may also be a wear plate at the back.
Also the gasket should be made of very thin material.

Thanks. The gasket is very thin (standard one supplied with impeller from Parts4Engines).