Raw water strainer emptying whilst sailing

[vyv_cox]

Ah that could be the problem, I should have done a bit more research before believing the salty old blokes in the boatyard.

This is not quite the model I have but it's pretty similar, the installation diagram is at the bottom:

http://www.vetus-shop.com/vetus-raw-water-strainer-type-140-inlet-dia-19mm-p-1019.html

That looks like the one I have. When I run the engine in the yard using a hose pipe to top up the strainer it definitely drains out of the hole at the bottom going to the pump.

But even plumbing it according to the instructions won't stop air getting in, if it's getting in via the through-hull intake.

But there will be a strainer full of water, as opposed to a short length of tubing, which should be sufficient to prime the pump if it is reasonably unworn.

 

[TonyS]

I had exactly this problem 3 years ago and on talking to many skippers it is a not uncommon problem. I comes up on these forums about every 2 months. The pumps on small engines up to 30HP are small and wear quickly or are poorly machined in the first place. The water is pulled out when sailing by Bernoulli's principle (water rushing past the inlet creates low pressure - suction). The item that allows this to happen is the anti syphon valve which lets air in when the pump is not in use. I have clear hoses and you can see the water level dropping through the pump. The solution is to close the inlet when sailing. The other simpler and very cheap option is to fit a non return valve just below the anti syphon valve on the pump side. I chose a SS 3/4" BSP with a light spring loaded ball. Cost £5 from RS components. The advantage is that I never need to change impellers and hopefully the pump seal will also last a lot more than the normal 800 hrs - I will tell you in a few years. Rubber components are designed to last huge number of hours when constantly lubricated by water but fail quite quickly when run dry.

 

[Ruffles]

The item that allows this to happen is the anti syphon valve which lets air in when the pump is not in use.

Hang on though. The anti syphon valve is the OTHER side (down stream) of the pump otherwise it will suck air when running. And an impeller pump is a seal when it's not turning. Unless it's worn of course.

PS: I see that Vetus are challenging Volvo Penta on the price of spares. £13.35 for some 'O rings!

 

[Tranona]

No, the same on my Bavaria. In the line from the strainer to the pump. Don't ask me why, but has been like that for 2500 hours and never had a problem.

 

[charles_reed]

I have a raw water strainer fitted to my Albin Vega with a clear lid so you can check the water level/fouling etc. When sailing after an hour or so it is completely dry, on my initial sail on the boat this was a disaster resulting in melting the water lock and an unplanned 27 hour non-stop trip under sail from Portsmouth to Plymouth instead of a coastal hopping jolly.

There doesn't seem to be any air leaks on the system allowing the water to simply drain out. I think that it could be air bubbles rising up the inlet pipe and gradually draining the bowl.

It isn't a problem when running the engine although I do keep a very careful eye on the water level. When I shut down I now routinely isolate the water inlet valve for sailing, but this means the engine is not immediately ready in event of an emergency as the companionway steps need to be removed to access the valve. When I open the valve after sailing a small bubble always comes up the pipe into the strainer supporting the theory that it is bubbles collecting in the inlet, gradually rising up and emptying the strainer.

Does anyone have any suggestions or alternatives?

having suffered similar problems, it's almost certainly a leaking screw-on lid.

A new O-ring usually sorts out the problem.

A small air-leak gives you a double whammy, the water runs back out into the sea whilst you're sailing and the reduction of the suction effect means the raw-water pump can't prime itself.

I finally replaced the old Vetus unit with the much more compact new model - not so much lid circumference to spring a leak.

 

[TNT]

water strainer

Nice one TonyS, I was just going to suggest that but would it not make it easier to fit it onto the stop vlv at the skin. There are several different types available with very little resistance and will maintain free flow while stopping the return water back out, not sure if they are available in bronze but even if itsplastic or brass it can be replaced every couple of years if it cures the problem and is only a few pounds.

 

[TonyS]

There is a typo in your message but I assume you are asking if it is easier/better at the skin fitting. The answer is that it makes no difference. If you did not have an anti syphon valve the water would remain. So anywhere between the anti syphon and the skin fitting is OK. It is better above the water line than below as it is another point that could potentially leak and sink the boat and therefore would need double clips and frequent testing to make sure they are tight. Mine is next to the anti syphon and works perfectly.

 

[TonyS]

Hang on though. The anti syphon valve is the OTHER side (down stream) of the pump otherwise it will suck air when running. And an impeller pump is a seal when it's not turning. Unless it's worn of course.QUOTE]

I have already said quite clearly that the impeller pump is not a seal. You can see the water flow through it. It was the same when nearly new. Some multi vane pumps on larger engines do seal but not the smaller ones.

The anti syphon is down stream of the pump!! Therefore it cannot suck air on its outlet side. The pressure on the outlet or downstream side of the pump closes the valve in the antisyphon or you would get an engine room full of water.

With the NDV fitted and the engine turned off, the water level can now be seen dropping down the tube into the heat exchanger but not to the pump. It has been working perfectly like this for 3 years. I have discussed it with Volvo and they say it is a good idea but won't fit it as it costs money and they would also lose money on all the impellers, pumps and o rings (service items) that would no longer be required!

 

[alan_d]

I have already said quite clearly that the impeller pump is not a seal. You can see the water flow through it. It was the same when nearly new. Some multi vane pumps on larger engines do seal but not the smaller ones.

I would concur. I have a directly cooled Bukh and when I close the seacock after shutting off the engine, it and the adjacent pipe are hot because of back-flow of cooling water. Doesn't cause any problems, though.

 

[KenMcCulloch]

Vyv is absolutely correct. The intake hose goes to the pipe with the upstand and any debris is then caught in the basket, easy to clean out.

 

[sam_uk]

I also have this problem

On a recent night sail I started the engine to charge the batteries whilst motorsailing and got a overheat alarm pretty quickly. I shut down the engine.

After priming the strainer with water the engine ran happily again.

However sat on the comfort of the pontoon the pump does seem to have enough power to prime the 'seacock side pipe' so long as there is water in the 'pump side pipe'

What I don't understand is why it wouldn't do this in the middle of the night offshore! Is it just sods law?

I guess maybe I was just unlucky and the through hull popped out of the water just as the pump was using the last of the 'pump side' water, thus sucking more air in and putting it in a position it couldn't recover from?

Any other thoughts? I have checked my pipework and clamps..

Thanks

Sam

 

[forelle541]

My water pump was doing this, I think when a bit old they suck air past the seals on the shaft. When pumping water the pressure forces the seal against the shaft so no leak. I got a new one from ASAP and problem gone. I did not bother to fit new seals as the shaft could have been worn.

 

[tolhurstorganic]

Cooling problems are clearly one of the hottest topics on this forum, why is this so when the engine is surrounded by a perfect medium for cooling-seawater in copius amounts? Its about time that marine engine suppliers/builders were able to come up with a simple and reliable way to cool an engine that is superior to the hotch potch of dodgy rubber and plastic that can fail catostrophicaly at the slightest problem.
Until such time it would help most boat owners to install an easily accessible and reliable method of turning on/off the inlet valve. So many times we hear of how difficult they are to get to.I have rigged a bowden cable and a simple lever in the cockpit locker that allows instant access. If you have to climb down inside the boat and crawl into some dark dirty place to switch the water on/off its not going to happen as often as it should. So it gets done when it needs to. Also a removeable notice on the ignition switch as a reminder, to make sure you really do turn it on, don't ask how I came up with that one...

 

[catalac08]

I had a similar problem on a boat and it was that when heeling to starboard the water inlet on the port side came clear of the water and the pump became unprimed and once full of air the impeller was not suffuciently efficient to start the flow again. I used to open the filter and fill it with water from a kettle. Annoying and I never managed to stop the problem,just learned to live with it.

 

[nedmin]

Looks like its coming out due to venturi effect altho. as mentioned it could be an air leak.Why not get a piece of plastic pipe that is a tight fit in the inlet pipe,leave about an inch showing and cut it off at about 45 deg so that the cut bit faces forward so that when you get forward motion it pushes water up the pipe.Worth a try.

 

[Halo]

having suffered similar problems, it's almost certainly a leaking screw-on lid.

A new O-ring usually sorts out the problem.

A small air-leak gives you a double whammy, the water runs back out into the sea whilst you're sailing and the reduction of the suction effect means the raw-water pump can't prime itself.

I finally replaced the old Vetus unit with the much more compact new model - not so much lid circumference to spring a leak.

+1 Best thing is to put some vaseline on the seal and tighten hard as you dare (this has sorted it for me) - I bet you have a slight air leak on the seal which gives you the problem. Good idea to unscrew from time to time so it does not jam solid.