Seawater cooling failure due to airlock?

[Sunjammer]

Beneteau First305 with Volvo MD202 engine. After 6 hours sailing into a Force 4 headwind, needed the engine to help me out of a jibsail problem - no seawater cooling. Checked filter, water inlet - all ok. Tried priming through the filter but didn't work. Stripped pump to check impeller, it was only 3 months old and fine. Also checked impeller was spinning with engine. In calmer waters I closed seacock, filled the filter with water and fitted lid, started engine, opened seacock - worked OK and has been fine since. I've had the boat for 5 years and never experienced this problem before. Could it be because I was sailing fast and close-hauled causing water to be sucked out of feed pipe and form an airlock? But the feedpipe inlet is close to hull centreline - I was definitely not heeled over that much!!

 

[DownWest]

Unlikely to form an airlock on a raw water system. I assume your pump is under the water line? Had a problem with a fresh water system, the only thing we could think of, was a plastic bag had been sucked onto the raw inlet, then dropped off after it had overheated and been turned off.

 

[Mistroma]

Our old BUKH had priming issues after many years use. It would be fine in calm water but would refuse to pump after sailing in lumpy conditions. It could be primed by pouring water into the filter. The impeller was fine but there was wear inside the pump. I think the gap behind and in front of the impeller had become large enough to make pumping less efficient.

Not 100% certain how air was getting in but worn seals seemed likely. The BUKH has 2 seals on the pump shaft with a gap in between. It either leaks oil or water and that drips out of the Judas holes, as long as they aren't covered in paint. I think the outer seal had just enough wear to allow air in but not let water out.

 

[jwfrary]

How high is your strainer fitted? If you loose prime which can happen when sailing hard then sometimes the pump can struggle to reprime over a high loop.

Previous posts are also very valid the back and cover plate wear and you can loose efficiency, try turning the cover plate the other way around.

Dont forget to check your exhaust as running dry can often damage it.

 

[Plum]

Beneteau First305 with Volvo MD202 engine. After 6 hours sailing into a Force 4 headwind, needed the engine to help me out of a jibsail problem - no seawater cooling. Checked filter, water inlet - all ok. Tried priming through the filter but didn't work. Stripped pump to check impeller, it was only 3 months old and fine. Also checked impeller was spinning with engine. In calmer waters I closed seacock, filled the filter with water and fitted lid, started engine, opened seacock - worked OK and has been fine since. I've had the boat for 5 years and never experienced this problem before. Could it be because I was sailing fast and close-hauled causing water to be sucked out of feed pipe and form an airlock? But the feedpipe inlet is close to hull centreline - I was definitely not heeled over that much!!

Different boat and different VP engine but I have had the same symptoms and also restored the water flow by pouring water into the water strainer with the seacock closed. The cause was an air leak on the seal of the lid of the strainer which meant there was insufficient suction to lift the seawater. It is surprising how small an air leak you need before the pump to stop the pump from priming.

www.solocoastalsailing.co.uk

 

[Graham376]

If you have a Vetus plastic water strainer, the O ring seal is a likely problem, invert it and grease well.

 

[graham]

Try priming down through the strainer ,replace the top ,start with sea cock closed then open it the second the engine fires.

Has worked for me before to vlear a seawater airlock.

PS just noticed that "Plum " allready suggested that.

 

[Sunjammer]

How high is your strainer fitted? If you loose prime which can happen when sailing hard then sometimes the pump can struggle to reprime over a high loop.

Previous posts are also very valid the back and cover plate wear and you can loose efficiency, try turning the cover plate the other way around.

Dont forget to check your exhaust as running dry can often damage it.

The strainer isn’t very high but there is a high loop after the pump, presumably to maintain some volume of water in the pump. All the boat movement may have compromised this function?

 

[Tranona]

The high loop probably has a syphon break in it. Quite normal and has no effect on water coming in. Like others my money would be on an air leak either the cap on the filter or slightly loose hose clamps.

 

[VicS]

Oops

 

[pvb]

The strainer isn’t very high but there is a high loop after the pump, presumably to maintain some volume of water in the pump. All the boat movement may have compromised this function?

It would be worth double-checking that the strainer is correctly plumbed. Many strainers have the inlet pipe extending up almost to the top of the strainer. This means that when the engine is switched off, the strainer remains more or less full of water, helping to prime the pump next time the engine is started. If the pipe connections are swapped, that won't happen.

Here's a little Vetus diagram showing what I described...

 

[Sunjammer]

It would be worth double-checking that the strainer is correctly plumbed. Many strainers have the inlet pipe extending up almost to the top of the strainer. This means that when the engine is switched off, the strainer remains more or less full of water, helping to prime the pump next time the engine is started. If the pipe connections are swapped, that won't happen.

Here's a little Vetus diagram showing what I described...

View attachment 121884

Thanks pvb, yes, the strainer is all plumbed OK. As part of my attempts at problem solving, I inserted a tube down the inlet section to blow and ensure the seawater inlet wasn't blocked

 

[Sunjammer]

Thanks everyone for your input. Of course, now I know about the condition and how to treat it, the problem diminishes. But hopefully this thread raises awareness for someone who experiences a similar event and remembers how to solve it!