Anti syphon valve?

[Quandary]

The diesel engines on every boat I have owned regardless of type of cooling, has been fitted with a an anti syphon valve above it, usually a VP inverted Y fitting that needs a tiny rubber disc with a hole in it replaced every year and for most of the time, until the revs. rise a bit, it dribbles into a plastic sweety jar or similar placed below it. ( I know some folk use big plastic Vetus versions and sometimes like to watch them dribble through a pipe in to the cockpit).
This boat, a Moody with a indirect cooled VP2020 and saildrive, does not have one, (or if it is there I have not found it), the VP 2020 manual states that 'some engines will have one', implying that it is not always essential. I presume this has something to do with heights or angles of pipework but this plumbing looks not much different from the others?
Can some one explain the criteria defining when one is needed, I don't want to introduce the bothersome task of looking after something that I can do without and the boat has managed twenty years without one, but I still would like to understand why she does not need it when most boats have them?

 

[RichardS]

I don't have an anti-syphon on my engines cooling systems. I'm not sure where any seawater would syphon to as it's a closed system ...... inlet port with seacock at one end and exhaust outlet at the other end. I suppose that if your exhaust elbow was below sea level then a syphon into an open exhaust port could arise but only if the seacock was left open.

Richard

 

[knuterikt]

If the mixing elbow for exhaust&seawater is below sea level on the outside, heeled or floating there is a risk of sea water siphoning into the exhaust system and filling engine through exhaust port.
You might be lucky for a while..
Theoretically you could run down and close the seacock immediately after you shut of the engine - but what if you need to start it quickly?

 

[macd]

Quandary: if you go to this page: https://www.vetus.com/en/exhaust-sy...irvent-with-hose-for-13-19-25-32-mm-hose.html
you'll find a link entitled 'Support and manuals'. Click on that and a pdf will appear telling you what you want to know.
Slightly more specific information should be available in the installation manual for your engine, should you possess one. Even the generic info in any installation manual should suffice: plenty of those available free on the Beta website.

 

[Quandary]

Sounds like I do not need it then, but if Moody knew this twenty years ago I am puzzled why Finngulf with similar indirect cooling and exhaust location and a not very different D1 30 engine was still installing one ten years later?

 

[Tranona]

There is no reason why the valve should dribble. there are, however two types. The one you are familiar with that has a flap valve and the other which is open and intended to pee overboard or into the cockpit.

I have had both Volvo and Vetus valves and neither has dribbled. No need to change the little flap every year, just occasionally check that it is not stuck. On a saildrive installation the usual way to plumb is inlet to above waterline filter, down to pump, up through valve as per Vetus schematic and over to heat exchanger

 

[macd]

Sounds like I do not need it then, but if Moody knew this twenty years ago I am puzzled why Finngulf with similar indirect cooling and exhaust location and a not very different D1 30 engine was still installing one ten years later?

The issue isn't exhaust location (as in where it exits overboard). It's position of the water-injection point relative to sea level. (Apologies if this was already your understanding). It's not specific to any engine-type, since the details of their installation will differ from vessel to vessel.

 

[Quandary]

It is a fairly primitive device but despite that they do not dribble when you fit a new flap (if you are really careful about cleanliness) but after about 6 months you get a wee drop of water perhaps about 100ml. per month, dismantling and cleaning sometimes cured it but it was worth fitting a new flap when going to the trouble. That is why I used a collector as the discharge was right over the engine, which I thought might encourage it to rust. It was important to ensure that the collector never filled up to the discharge pipe as then it itself would syphon. I experimented to see when it happened and it dripped only at tickover, if I set the tickover a bit higher it rarely happened, I presume because there was more pressure to close the flap.
I never succeeded in installing a flap that permanently did not drip at some stage and heaven knows I tried hard enough.

 

[ColinR]

i wouldn't take any chances. I had a Yanmar fill with water several times because the engineer had teed a feed to lubricate the shaft seal into the wrong side of the anti syphon valve. In prolonged sailing well heeled, in a strong wind on the starboard tack, water flowed back into the engine and filled it. The first time I discovered this was in the dark, in a full gale, off the rocky entrance to a Spanish Ria. Luckily it was an old Yanmar with de-compression levers so I could decompress the engine, turn it over to blow out the water and it started. It was a scary situation but because it was only a problem with this exact sequence of events it took ages to find the cause. I don't know what I'd do now I've got a modern diesel without de-compression levers. The Yanmar was fine after several oil and filter changes.

 

[Tranona]

Sounds like I do not need it then, but if Moody knew this twenty years ago I am puzzled why Finngulf with similar indirect cooling and exhaust location and a not very different D1 30 engine was still installing one ten years later?

It is the other way round. Valves were not always fitted in earlier times because the issue of syphoning was not fully understood. Finngulf (like most other builders) adopted the sensible policy of always fitting them.

 

[macd]

i wouldn't take any chances.

Nor would I, given that the fix is so simple. When we bought our present boat it had gone 20 years without an anti-syphon, and without (to the best of our knowledge) inhaling water, either. So it just went on a longish list of things to do. Then we had a serious family emergency in Turkey, which required taking the boat to a marina and high-tailing it to an airport. In all the fuss the intake seacock was closed perhaps an hour after berthing. When we returned three weeks later the engine was a write-off.

Just because it's never gone pear-shaped doesn't make it immune.

 

[Quandary]

Did a search on the Moody Owners forum for anti syphon (and siphon) valve, nothing came up and it is a big info. rescource so the absence can not have caused too many problems yet. If neither Moody or Marine Projects deemed it necessary unlikely to be a problem, Marine Projects did fit them as standard on Sigmas so they did know about them. Moody did place all the S31 seacocks in one convenient location so no excuse not to use them.
If I was as lucky as Tranona and could get one to work indefinitely without ever spitting salty water over the engine I might fit one but I am not.

 

[A1Sailor]

My Moody336, built in 1989, had an anti-syphon valve in the cockpit locker. The engine had just been replaced with a 35HP Betamarine when I bought her in 2003 but the tubing and "Y-valve" didn't look new. So I think the boat had one when constructed by Marine Projects then fitted out by Moody.

Edit:
This will surely answer the question, but I no longer have a copy:

Both my Moody336, and my Sadler25 with a Vetus engine fitted around 2002, had anti-syphon valves...

 

[Pagetslady]

I have always had a anti siphon but mine have just been a take off at the top of the loop I have taken the pipe to the cockpit side and it shows me that there is water circulating, it simply drains through the cockpit drains, very reassuring. My boat is centre cockpit so the pipe rises all the way to the outlet, this may be difficult on an aft cockpit.
Mike

 

[Ludd]

Moody 33 had a mixer box above (well above) waterline level,as does my Nab35. Dry exhaust to box,injection elbow at manifold to box, with leak off pipe to stern gland. Both metal,bothe required new pipe inside box after 40 years. I have seen and heard about engines ruined by backfilling ,wouldn't risk being without one. The plastic boxes fitted at the LOWEST point and designed to contain however much syphons back to that point I wouldn,'t trust for a minute!

 

[A1Sailor]

Did a search on the Moody Owners forum for anti syphon (and siphon) valve, nothing came up and it is a big info. rescource so the absence can not have caused too many problems yet. If neither Moody or Marine Projects deemed it necessary unlikely to be a problem, Marine Projects did fit them as standard on Sigmas so they did know about them. Moody did place all the S31 seacocks in one convenient location so no excuse not to use them.
If I was as lucky as Tranona and could get one to work indefinitely without ever spitting salty water over the engine I might fit one but I am not.

What does this link:
http://moodyowners.info/threads/anti-syphon-loops-and-splashbacks.12881/
say? It is entitled:

Moody S31 - Anti Syphon Loops And Splashbacks

I'm not a member so can't access the thread. If you searched for "syphon" surely you should have found it...
Maybe it is about toilets!!!!

I simply Googled "moody 31 anti syphon".

 

[Quandary]

What does this link:
http://moodyowners.info/threads/anti-syphon-loops-and-splashbacks.12881/
say? It is entitled:

I'm not a member so can't access the thread. If you searched for "syphon" surely you should have found it...
Maybe it is about toilets!!!!

I simply Googled "moody 31 anti syphon".

Their search seems to work a bit like YBWs, I tried tech info and forum using your title - zilch. It did find 'Moody 31 toilet loops' and I think that is what it may refer to.
The S31 lacks any convenient void space above the engine which is in a box in the middle of the aft cabin so installation at a suitable height would involve an extended and untidy route out and up to the coaming void, unless I was prepared to sleep with the thing. Four previous owners managed as she is, maybe I can do the same?
But I do appreciate that you are all trying to look after me. I am comforted by the Vetus dimension of mixing point min. 150mm. above water level, and baffled by Macd's experience. I will not knock NC and I have an old copy but there is stuff in it which might be questioned today. Volvo Penta, who I regard as conservative, make a passing reference in the instruction book that 'some engines have a vacuum valve installed in the sea water piping'. I also checked the Moody S31 'Yacht Owners Handbook', it describes the cooling system in some detail without any reference to an anti syphon valve .
So I asked a question, got a clear consensus on the answer then chose after consideration, at this point in time, to ignore it, how churlish is that?
You must think that I deserve to have a flooded engine but I still feel that the discussion has been very useful, thank you for your advice.

 

[RichardS]

Their search seems to work a bit like YBWs, I tried tech info and forum using your title - zilch. It did find 'Moody 31 toilet loops' and I think that is what it may refer to.
The S31 lacks any convenient void space above the engine which is in a box in the middle of the aft cabin so installation at a suitable height would involve an extended and untidy route out and up to the coaming void, unless I was prepared to sleep with the thing. Four previous owners managed as she is, maybe I can do the same?
But I do appreciate that you are all trying to look after me. I am comforted by the Vetus dimension of mixing point min. 150mm. above water level, and baffled by Macd's experience. I will not knock NC and I have an old copy but there is stuff in it which might be questioned today. Volvo Penta, who I regard as conservative, make a passing reference in the instruction book that 'some engines have a vacuum valve installed in the sea water piping'. I also checked the Moody S31 'Yacht Owners Handbook', it describes the cooling system in some detail without any reference to an anti syphon valve .
So I asked a question, got a clear consensus on the answer then chose after consideration, at this point in time, to ignore it, how churlish is that?
You must think that I deserve to have a flooded engine but I still feel that the discussion has been very useful, thank you for your advice.

If your seawater pump is working well with plenty of water coming out of the exhaust then the impeller is presumably in reasonable condition and sealing well ..... in which case I would be amazed if a syphon would ever generate enough head of pressure to push water past the impeller. It would be easy to test by pulling the hose off the pump outlet spigot to see if a syphon actually starts. If it doesn't start with that water level difference it will be even less likely to start at the higher exhaust elbow.

Richard