vetus syphon vent for engine

[colvic987]

i have just replaced the exhaust elbow, and as normal each season, wondered why i dont have a syphon vent, and was wondering wether to put one in, i have never had any back syphoning happen, during the last 10 years, have been looking at the vetus syphons and they are marked up as type h and type v, i realise that the type h has a hose to let the water out at a suitable place at drain or over the side, but the type v is an automatic pressure valve.

The question is does the automatic, push water out all the time or does it only let air in when it needs to and does not need a hose to get rid of excess water..

 

[VicS]

The automatic type is just that. the valve opens automatically to allow air to enter in oder to break the syphon when the engine is stopped otherwise it is shut and there is no flow.

The H type produces a continuous stream and there are no fiddly bits of valve to get stuck or bunged up with salt.

The recommendation in the Vetus catalogue is that an antisyphon valve should be fitted if the exhaust injection point is less than 15cm above the water line and that the antisyphon valve itself should be located 40 cm above the wl

 

[donncha]

You only need a anti siphon if the engine is under the waterline. Like VicS says, the exhaust elbow in particular, however i believe that the intake comes into play as well.

I have such a vent and if you look at my other posts, painting over the output caused the engine to siphon and hydrolock. Thankfully the engine survived, and i believe these to be good vents. I would agree that the type with continuous stream to be much better, no parts to get clogged, providing of course you don't paint over the outlet. I'd imagine that 15cm is to take into account the boat rolling in a big sea.

For my engine these are the two different installations, one requiring a as device one not.

 

[ancientsailor]

Is it needed?

We had a Westerly Griffin surveyed. The exhaust outlet was on the transom about 2 ft above the water line. The surveyor said that as there was no evidence of a problem over the previous 15 years, there was no point in fitting one.


On our 20+ year old Moody the exhaust outlet was on the waterline on the port quarter. This was immersed as soon as we heeled to port. There was a substantial anti-siphon loop in the cockpit locker which always remained above the waterline even when heeled. We didn’t bother with a device then either.


If I were to fit one, it would be “no moving parts”.

 

[colvic987]

thanks to both of you, very informative posts, i will decide which one, easiest is the auto, as no holes to drill in hull, but the one to get clogged up easiest and without any telltale signs initially...

 

[VicS]

however i believe that the intake comes into play as well.

No that wont make any difference.

I see your diag shows the A/S valve between pump and engine. Can also go ( maybe more usual ) between engine and exhaust injection point.
Probably makes no difference but I always feel is better not to waste the cooling water before its had a chance to cool the engine in the case of the continuously peeing type.

The important factor is the height of the injection point into the exhaust system.

I don't see that the height of the water pump has any relevance and the height of the exhaust outlet does not affect the requirement or other wise for the A/S valve. Unless perhaps if it is actually underwater

 

[donncha]

Any chance someone could please explain to me exactly what happens when an engine siphons water? I presumed it was due to a pressure difference between the intake and the exhaust? But if the level of the intake doesn't matter, I trust VicS on this, then my presumption must be incorrect.

 

[colvic987]

i have a md7a, and i presume that the syphon would go from the exit of the thermostat up to the syphon vent and then back to the exhaust water outlet. which at the moment are joined by a straight hose, of around 300mm or so.

 

[VicS]

Any chance someone could please explain to me exactly what happens when an engine siphons water?

You are familiar with the idea of syphoning from one container to another. Eg fuel from a tank to a can.

Once the syphon tube is full the liquid will flow from the higher level to the lower level.

In the case of the boat the "syphon tube" is the whole cooling system from the inlet to the injection point in the exhaust system. The level of liquid in the "higher container" is the water level outside. Running the engine fills the syphon but when the engine stops the syphon will continue to flow if the outlet end, ie the injection point, is below the waterline. In practise a 6" safety margin is added. That will fill the exhaust syutem possibly flooding up to the exhaust valves and into the cylinders

With the A/S located at the highest point, and well above the waterline, once the pump stops there will be a slightly negative pressure at this point and the valve will open ato allow air in. Once there is air in the top of the loop the syphon is broken.

 

[donncha]

Thanks VicS. Sorry for hijacking the thread, having recently experienced water in the cylinders i just wanted to know exactly what happened.

 

[doug748]

I fitted one with my new engine, though I never had any problem without. I used the valve type, the Vetus one is expensive but I got a similar thing for £15 from one of the discount houses (sorry I can't remember which one).
It works ok, though it does dribble a bit now and again (I know how it feels). I have never had a problem with salt jamming it up. I have a length of tube attached so that I can blow down it now and again to check it is working ok - it always is so far. The end of the tube normally lives in a plastic milk bottle in the engine tray, which is why I know it leaks a bit now and again. Someone said that this tube could itself form a syphon and flood the boat but I didn't really believe them and kept it. So far, so good.

 

[nigelm]

i have just replaced the exhaust elbow, and as normal each season, wondered why i dont have a syphon vent, and was wondering wether to put one in, i have never had any back syphoning happen, during the last 10 years, have been looking at the vetus syphons and they are marked up as type h and type v, i realise that the type h has a hose to let the water out at a suitable place at drain or over the side, but the type v is an automatic pressure valve.

The question is does the automatic, push water out all the time or does it only let air in when it needs to and does not need a hose to get rid of excess water..

Hi fellow colvic owner , as you know we are doing the same repair on the same engine , whilst removing my elbow i found that the water hose going to elbow dissapeared up the side of the engine compartment , i traced this back to a anti syphon valve tucked away behind a piece of ply lining the side of quarter berth , the other end of hose from the valve went back to the front end of exhaust manifold , just wanted to say what i found and mention it out of interest really .

ps , i have a volvo installation manual and that shows the anti-syphon valve in the diagram .

 

[VicS]

Someone said that this tube could itself form a syphon and flood the boat but I didn't really believe them and kept it. So far, so good.

If that tube goes below the water line then it could indeed create a syphon and fill the bilges.
(depends on the injection point position ... if both are below the water line then you could fill the exhaust system and flood the engine while slowly filling the boat until it sinks!)

Lead your tube to a bottle by all means but fix it well above the waterline!

 

[Leighb]

i have a md7a, and i presume that the syphon would go from the exit of the thermostat up to the syphon vent and then back to the exhaust water outlet. which at the moment are joined by a straight hose, of around 300mm or so.

That was exactly the arrangement on my MD7A, this is currently being replaced by a Beta, but I expect something similar will be needed.

 

[Burnham Bob]

I had one of the simple automatic vetus ones on my old boat for years. No problems. Just every year I used to clean it out which is a simple job.

 

[alahol2]

Someone said that this tube could itself form a syphon and flood the boat but I didn't really believe them and kept it. So far, so good.

If that tube goes below the water line then it could indeed create a syphon and fill the bilges.
(depends on the injection point position ... if both are below the water line then you could fill the exhaust system and flood the engine while slowly filling the boat until it sinks!)

Lead your tube to a bottle by all means but fix it well above the waterline!

I very nearly replied with the same message but, thinking about it, I can't think of any mode of failure that would create this situation. The exhaust pipe would have to be full of water and the anti-siphon valve open.

Our own installation uses the 'simple' vetus without the valve.

 

[anniebray]

My boat is fitted with a valve type vacuum breaker (Vetus I think) which also has an 8mm dia nozzle. A Yanmar mechanic who recently did some engine work for me said that if I removed the valve internals & fitted a tube to the nozzle, I could convert it to the "water bleed" type. I plan to do this because my exhaust outlet is situated where it can't be seen from the cockpit & I so will place the vacuum bleed line outlet where I can check for cooling water flow. I had cooling water problems last season which resulted in a melted waterlock ! Incidentally I have also fitted an RS Components temperature switch to the exhaust elbow linked to an alarm as advised by a previous thread on this forum.

 

[VicS]

I very nearly replied with the same message but, thinking about it, I can't think of any mode of failure that would create this situation

You are probably right. The valve should not open to allow it to happen but what if it stuck open (The exhaust injection point would also have to be below the waterline or it would itself break the syphon.)


However if the injection point is below the water line and the vent tube was also extended to below the water line and was full of water because the valve was dribbling then the anti syphon effect would be lost and the exhaust system could fill.

A bad idea to extend the vent pipe into the bilges anyway. Into a reservoir well above the water line is fine but into the bilges is a bad idea.

 

[disko troop]

I noticed with horror that there was water coming out of a tiny hull fitting on my Kelt9m, staining the hull, this evening. After tracing the pipe through the cockpit locker, under the battery through the heads and into the engine compartment where the Yanmar 3ym20 lives I found the offending item which from this forum I have discovered is a vetus anti-syphon valve. I can only assume that as I have never seen water squirting out before that it has been blocked since I bought the boat at the end of last season. Is brain freeze and early dementia a result of buying a boat or is it a necessary pre-requisite?

 

[LeonF]

Can someone (Vic S ? ) please explain what the Vetus anti syphon fitting that is in the raw water hose after the impeller pump does ? It's situated high up in the starboard locker about three feet above the waterline. It's like an upside down 'U', and I've seen them mentioned in magazine articles, but have never been sure what it's for and whether I should be servicing it. The exhaust itself has nothing, but sits in the port locker like a many coiled python before exiting just above the waterline.