Strainer empty, is this normal?

[Jokani]

My 1976 vintage Centaur has the original Volvo MD2B engine.

The water strainer is fitted a fraction above the water level, such that (with the top removed) it fills to within 1cm of the top when the seacock is opened.

Pipe fitted is 3/4" ID sanitary pipe

When the engine is running the strainer is constantly empty, I put some floating material in just to be sure. Water flows out of the inlet and straight to the bottom, and never fills.

There is plenty of water existing the exhaust.

Is this normal or should I be concerned?

 

[30boat]

My 1976 vintage Centaur has the original Volvo MD2B engine.

The water strainer is fitted a fraction above the water level, such that (with the top removed) it fills to within 1cm of the top when the seacock is opened.

Pipe fitted is 3/4" ID sanitary pipe

When the engine is running the strainer is constantly empty, I put some floating material in just to be sure. Water flows out of the inlet and straight to the bottom, and never fills.

There is plenty of water existing the exhaust.

Is this normal or should I be concerned?

It should fill up.Are you sure you don't have an air leak somewhere?

 

[RichardS]

My 1976 vintage Centaur has the original Volvo MD2B engine.

The water strainer is fitted a fraction above the water level, such that (with the top removed) it fills to within 1cm of the top when the seacock is opened.

Pipe fitted is 3/4" ID sanitary pipe

When the engine is running the strainer is constantly empty, I put some floating material in just to be sure. Water flows out of the inlet and straight to the bottom, and never fills.

There is plenty of water existing the exhaust.

Is this normal or should I be concerned?

When you say 1 cm from the top do you mean 1 cm from the top of the pipe which pokes up in the middle of the filter? If so, that sounds good.

What happens when you pour water into the outer mesh part of he strainer? Does this vanish immediately so that you can't actually ever establish a level of water in the mesh section? If so it sounds as if the water is flowing easily past the impeller and presumably out through the elbow which must be lower than the strainer and into the exhaust water trap. Don't pour is too much in case you will up the whole system and water finds it's way back into the exhaust manifold.

You could try pouring say a litre of hot water into the strainer and the holding your hand on the exhaust elbow to feel for warmth (engine cold and not running of course)

If the water is running easily past the impeller this would explain why the strainer never fills up. However, if when you start the engine the pump self-primes properly and water starts to come out of the exhaust within the first few seconds, the seal on the top of the strainer must be OK and the impeller, although a bit leaky, is still doing its job and keeping the engine cool.

The pump can only suck in as much water into the strainer as it is sucking out so if the strainer starts off empty it is probably going to stay empty although it is still doing it's job. A new impeller and/or pump faceplate might increase the level of water in the strainer.

Richard

 

[Jokani]

It should fill up.Are you sure you don't have an air leak somewhere?

I don't think so, when the engine is not running the strainer fills with water. I can close the seacock and the water stays there even when the boat drys out.

It seems as though the pump pulls the water out faster than it can fill

 

[30boat]

I don't think so, when the engine is not running the strainer fills with water. I can close the seacock and the water stays there even when the boat drys out.

It seems as though the pump pulls the water out faster than it can fill

Test the temperature of the exhaust water ,if it feels too hot you should lower the level of the filter.That happened to me when I relocated my filter a bit higher and the exhaust water was coming out a little hotter than normal but not enough for the engine to overheat,The filter wasn't filling up just like yours.I lowered it a few centimeters and everything returned to normal.

 

[Jokani]

When you say 1 cm from the top do you mean 1 cm from the top of the pipe which pokes up in the middle of the filter? If so, that sounds good.

1cm from the very top, is is just at the top of the pipe in the middle, and the water seems to just dribble over the edge to fill the strainer.

The pump can only suck in as much water into the strainer as it is sucking out so if the strainer starts off empty it is probably going to stay empty although it is still doing it's job. A new impeller and/or pump faceplate might increase the level of water in the strainer.

It starts full, but empties when the engine starts. When I stop the engine and remove the lid, the strainer slowly fills again

 

[VicS]

I don't think so, when the engine is not running the strainer fills with water. I can close the seacock and the water stays there even when the boat drys out.

It seems as though the pump pulls the water out faster than it can fill

So the "space" in the strainer above the water is either a vacuum or air. It's not likely to be a vacuum so it must be air.

How does the air get in there ? Find out how the air gets in and prevent that happening and you "problem" will be solved.

Is this normal or should I be concerned?

Yes you should be concerned because if the air ingress increases you may reach the point where an inadequate water flow through the engine and exhaust system is the result.

 

[Jokani]

So the "space" in the strainer above the water is either a vacuum or air. It's not likely to be a vacuum so it must be air.

How does the air get in there ? Find out how the air gets in and prevent that happening and you "problem" will be solved.



Yes you should be concerned because if the air ingress increases you may reach the point where an inadequate water flow through the engine and exhaust system is the result.

VicS, I love your power of logic "How does the air get there?" that was a EUREKA moment. My original thoughts were along the lines that the pump was just working faster than filter would allow the water in, but now I can see that does not make any sense at all.

All the pipes are relatively new and each has two jubilee clips that I have confirmed are tight. But, here is a nick on the the face of the water pump, I thought it had a good seal as there have not been any water leaks/drips and the bilge is dry, but it possible that when the engine is under running it the pump seal allows air to enter, but does not leak water?

Anyway, I'm taking the gamble that this is the problem, and I have ordered a new water pump, if that fixes the issue I can get the old one repaired and keep it as a spare.

Thanks for you help I'll let you know how I get on.

 

[VicS]

VicS, I love your power of logic "How does the air get there?" that was a EUREKA moment. My original thoughts were along the lines that the pump was just working faster than filter would allow the water in, but now I can see that does not make any sense at all.

All the pipes are relatively new and each has two jubilee clips that I have confirmed are tight. But, here is a nick on the the face of the water pump, I thought it had a good seal as there have not been any water leaks/drips and the bilge is dry, but it possible that when the engine is under running it the pump seal allows air to enter, but does not leak water?

Anyway, I'm taking the gamble that this is the problem, and I have ordered a new water pump, if that fixes the issue I can get the old one repaired and keep it as a spare.

Thanks for you help I'll let you know how I get on.

Surely the pump is after the strainer ???

Any pump defect that allows air to enter might affect the pump performance but the air will just be swept though the rest of the cooling system , not find its way back to the strainer.. Except when you stop the engine of course when air will enter and allow the strainer to empty, but only down to the top of the inlet pipe

Favourite sources of air in leakages would be the strainer cap. But you must avoid over tightening worm drive hose clamps, esp on to plastic fittings or you can create leaks rather that prevent them. Ideally the hoses should fit tightly enough to make water tight joints. The clamps should only be necessary to prevent the hoses coming off again.


Under way ( meaning making way ) air bubbles might find their way under the hull and be drawn into the water inlet. That's another explanation perhaps.

 

[RichardS]

VicS, I love your power of logic "How does the air get there?" that was a EUREKA moment. My original thoughts were along the lines that the pump was just working faster than filter would allow the water in, but now I can see that does not make any sense at all.

All the pipes are relatively new and each has two jubilee clips that I have confirmed are tight. But, here is a nick on the the face of the water pump, I thought it had a good seal as there have not been any water leaks/drips and the bilge is dry, but it possible that when the engine is under running it the pump seal allows air to enter, but does not leak water?

Anyway, I'm taking the gamble that this is the problem, and I have ordered a new water pump, if that fixes the issue I can get the old one repaired and keep it as a spare.

Thanks for you help I'll let you know how I get on.

We seem to be narrowing this down now. So both sides of the strainer are full until you start the engine at which point the strainer / output side drops down. As Vic says in can't be a vacuum in the space as that would be instantly filled by water being sucked in through the seacock, so it must be air getting in somewhere.

However, I would be very surprised if the air ingress could be anywhere as far downstream as the seawater pump, simply because when the engine is running there appears to be a good flow of water out of the exhaust so the pump is pumping and I would have expected that any air getting at the pump would be swept downstream with the flow. I can't see it going back up towards the strainer until the engine stops but that is not when the strainer drains out.

I think the air must be getting in somewhere close to the strainer. The most likely place is the lid but I would re-check all these upstream hose fittings. You need to make sure that the strainer lid and its rubber seal is complete without any cracks or gaps and then get a good dollop of vaseline / silicone grease / rubber grease on there. If that doesn't work try stretching a piece of cling film ot thin polythene (flimsy shopping bag) across the strainer before screwing the lid back. This will rule out any small cracks in the lid which are not that unusual.

Richard

Sorry. I appear to be reading VicS mind and stealing his ideas!

 

[ghostlymoron]

I fitted a Vetus strainer which has the inlet connected to the central pipe. The anulus around the pipe remains full when the engine is turned off unless the water pump impellor is shot. The level in the pipe should remain full also whilst afloat unless the cap is not fitted properly (or there's a crack in the casing), but will slowly empty if you dry out.
NB the raw water pump is between the strainer and the engine.

 

[30boat]

There is a very easy way of checking the air tightness of the system.Close the seackock and disconnect the pipe going into the raw water pump.Then suck the air out of the system with you mouth and block the pipe with your tongue,You'll imediately feel a decrease in suction if there's an air leak.

 

[vyv_cox]

I don't think so, when the engine is not running the strainer fills with water. I can close the seacock and the water stays there even when the boat drys out.

In that case the air leak has to be at the lid of the strainer, provided I am understanding what you are saying.

 

[nortonmotors]

Is it that the strainer is connected the wrong way?
Water should enter at the bottom, and be sucked out near the top - but the cover has to be in place to allow a continuous flow.
With water taken from the bottom, and the cover removed so you can see what's happening, then flow into the housing is the result of the difference in level between water outside and the inlet pipe. That won't be enough to meet the demands of the pump. With the cover in place, you have a closed circuit and water flow will equal engine demand.

 

[vyv_cox]

Is it that the strainer is connected the wrong way?
Water should enter at the bottom, and be sucked out near the top - but the cover has to be in place to allow a continuous flow.
With water taken from the bottom, and the cover removed so you can see what's happening, then flow into the housing is the result of the difference in level between water outside and the inlet pipe. That won't be enough to meet the demands of the pump. With the cover in place, you have a closed circuit and water flow will equal engine demand.

Not with my Vetus strainer, just the opposite. It would be a bit pointless having a strainer full of water that the engine could not use. After we lift out I flush the engine heat exchanger and seawater circuit by tipping fresh water into the strainer while the engine runs briefly. I keep the strainer topped up until my water bottle is empty, then turn the engine off as the level reaches the bottom.

 

[Rhyddid]

Didn't Dylan of this parish suffer something similar? I could be wrong ( and often are), but the strainer lid/screwed was top sucking air there...... Vaseline was the cure or something like that.

 

[maby]

There has been a batch of strainers around with a production fault that made the lid tend to crack. It did on our Beneteau and when I went to the chandlery to get a replacement they remarked "Oh, not another one!"

 

[Artic Warrior]

Hi,,,, I had the same problem last year,
I have the vetus strainer, and it was letting so much air it made the impellor fail.
Ive checked to plastic for cracks etc and the seal and its not hard,
but I need to tighten the lid right down to seal the problem

 

[Plum]

Surely the pump is after the strainer ???.

I recall reading that some Volvo Penta engines have a raw water filter AFTER the pump. The logic was that should the rubber impeller get damaged as a result of running dry any pieces of impeller blade get caught in the basket instead of a costly task of retrieving them from within the heat exchanger.

Colin. Www.solocoastalsailing.co.uk

 

[30boat]

I recall reading that some Volvo Penta engines have a raw water filter AFTER the pump. The logic was that should the rubber impeller get damaged as a result of running dry any pieces of impeller blade get caught in the basket instead of a costly task of retrieving them from within the heat exchanger.

Colin. Www.solocoastalsailing.co.uk

Better to fit two strainers then, the one after pump being smaller.