Air in cooling water

[Polly's Kettle]

All - I'm after some advice.

I ran my engine up the other day. I had to clear out the seawater strainer a couple of times - full of green. When that seemed to stop happening, I was at first pleased to see no air in the seawater system, but rather suddenly there was loads - I missed the point when it started. There seemed to be some small bubbles in the system after the pump, but before the anti-siphon. However, the first foot after the anti-siphon was mostly air, and obviously lots of air making its way through teh rest of the system.

I think it likely that there is crud etc in the saildrive leg, restricting flow. Otherwise, everything seemed fine.

My first step is to pour some acid via the strainer down into the leg to dissolve off any blockage. (brick acid??).

I'm after thoughts on what other things might be causing the air, and what to try next.....

 

[RichardS]

Crud is not going to cause ingress of air. Are you underway. Is there a source or turbulence near the raw water intake. Do you have an anti-siphon valve after the raw water pump? Could the air be getting in there? I would bypass the valve and see what happens. Is the flow out of the exhaust the normal volume?

Richard

 

[Polly's Kettle]

Thanks Richard - I've just been reading your earlier posts on using brick acid in the inlet/leg - I think that's my plan unless someone comes on and says it destryed their leg.

Boat was stationary, and yes the air is mostly after the anti-siphon. However there are bubbles before it - not much though but enough for me to wonder where they came from.

I wondered if the restricted flow might cause the antisphon to suck air in? That's why I thought the first job was to clear out the leg. Can't quite picture the mechanism though.

Last year's problem was a calcified exchanger, so I'm no longer sure that I know what a normal amount of water in teh exhaust looks like!

 

[vyv_cox]

Hydrochloric acid reacts with aluminium but less so with some of the alloys. I would hope a saildrive leg is a 5xxx alloy, so.more resistant, but I would be very careful about long exposure.

As said, a blockage does not cause an air leak but a restriction can encourage a leak somewhere else where there may be a poor joint.

 

[Polly's Kettle]

Thanks both. I'll try to bypass the anti-siphon and see what happens. Then there really is no way for air to get in. I guess I should check my strainer too.

is it worth me diluting the acid with, say, 50% fresh water, to minimise risk to the leg?

 

[pvb]

All - I'm after some advice.

I ran my engine up the other day. I had to clear out the seawater strainer a couple of times - full of green. When that seemed to stop happening, I was at first pleased to see no air in the seawater system, but rather suddenly there was loads - I missed the point when it started. There seemed to be some small bubbles in the system after the pump, but before the anti-siphon. However, the first foot after the anti-siphon was mostly air, and obviously lots of air making its way through teh rest of the system.

I think it likely that there is crud etc in the saildrive leg, restricting flow. Otherwise, everything seemed fine.

My first step is to pour some acid via the strainer down into the leg to dissolve off any blockage. (brick acid??).

I'm after thoughts on what other things might be causing the air, and what to try next.....

Have you properly replaced the strainer lid? Do you grease the O-ring seal on it? That's the most likely place for air to enter, as it's under vacuum. The anti-syphon is under positive pressure, so air won't leak in there. Don't go chucking acid around without doing the obvious checks first!

 

[Polly's Kettle]

Hmm - you may be right. Under that hypothesis, it comes in at the strainer, and then collects after the antisipon, as it's a high point, thus looking like it came in at the anti-siphon. Food for thought that.

And no, I don't grease the o-ring ... but I will now! What a simple suggestion, that feels rather right. Thank you. (although I don't know why the air would collect after the anti-siphon, rather than immediately after the strainer, where the water is also heading downhill. That is the only caveat in my mind)

As to the acid ... let's just pretend that I'm testing you when I query these "obvious checks" ...

(BTW, I did put Rydlyme down the leg last year - fizzed for ages!)

 

[VicS]

Thanks Richard - I've just been reading your earlier posts on using brick acid in the inlet/leg - I think that's my plan unless someone comes on and says it destryed their leg.

Boat was stationary, and yes the air is mostly after the anti-siphon. However there are bubbles before it - not much though but enough for me to wonder where they came from.

I wondered if the restricted flow might cause the antisphon to suck air in? That's why I thought the first job was to clear out the leg. Can't quite picture the mechanism though.

Last year's problem was a calcified exchanger, so I'm no longer sure that I know what a normal amount of water in teh exhaust looks like!

The fact that there are some bubbles visible before the anti-siphon device does suggest that air is being pulled somewhere , most likely via the filter lid

The greater volume of air bubble visible after the anti-siphon device indicates that air is being drawn in there because the pump is not maintaining a sufficiently high pressure at the top of the loop to prevent it.
Either the pump performance is bad or the inlet via the saildrive is restricted.

A long term solution could be to blank off the connection on the sail-drive and fit a separate inlet seacock.

 

[Polly's Kettle]

Thanks Pete / Rich / Vic

 

[Iliade]

Can you try to blast out the restriction with reversed flow? (i.e Stick a hose in the appropriate bit and blast either air or water down it)

 

[Polly's Kettle]

Can you try to blast out the restriction with reversed flow? (i.e Stick a hose in the appropriate bit and blast either air or water down it)

I intend to try, for sure, but am sceptical as it isn't blocked, just potentially has growth in there, so the air/water will likely just go past whatever's there.

Thanks for contributing.