Syphon break problem

[Seagreen]

Here's the problem:
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First, the exhaust is very close to the waterline and doesn't have a swan-neck fitting - its impossible to fit one in the space. Second, the top of the syphon break is only just higher than the exhaust.
Normally, the exhaust and cooling water flows from the engine and out (green arrow), but the syphon break also has a valve at A, to divert the water out via another skin fitting.

My problem?

Does the valve at A have to divert all the water in the direction of the Red arrow, and shut off the rest of the syphon break tube, or does it just provide an alternative exit?

I'm very confused as to what the vale should shut off, if anything. Answers please. TIA

 

[lw395]

I'm of the opinion that the valve is only used to avoid filling the exhaust during cranking/starting or so that the exhaust can be left dry by diverting the water during the last few seconds of running. Also if left in the divert position when the engine is not running, then water which gets past the impeller won't end up in the exhaust. So the syphon breaker (vented loop) is almost superfluous if you use and trust this valve.
Personally I would try very hard to get the vented loop higher, at least a foot above w/l at any angle of heel.
The valve needs to dump water into the exhaust in normal running to cool and silence it.
As a separate issue I would be concerned about following seas shoving water up the exhaust when the engine isn't running, I know waterlocks are supposed to prevent this but a friend of mine lost an engine this way. Maybe just because it was over 24h of quite big weather going to Spain. Perhaps a flap valve on the exhaust outlet would help?
Interested to see what others think.

 

[Bilgediver]

Are you sure you can not make some effort to include a swan neck or at least initially an upward gradient from the stern towards the engine.

The purpose of the valve is to drain the exhaust before stopping the engine. There is a danger if the exhaust is too long of there being too much water in the system for the water trap to cope with, Hence before stopping the engine divert the water direct overside and run for a few seconds at fast tick over so that the exhaust clears the exhaust. the byepass is probably big enough to take full flow..

 

[sailorman]

Re: Siphon break problem

That is completely Wrong
the valve is to break any siphonic action thus stopping water being drawn / sucked into the exhaust valves from either the exhaust hoses or salt water inlet sea cock.
the valve closes when pressure from the exhaust is present. it opens when the pressure ceases (IE the engine stops) letting air into the cooling system to break any vacuum

 

[Seagreen]

Couple of clarifications:

The exhaust port at the stern does have a big shut off valve to stop the "following sea" problem. It would also be very difficult to increase the height of the syphon break tube as this would put it into the space above it which is the VHF and nav electrics locker - which doesn't mix with hot water.

I suppose what I really needed to know was the diverter valve at A must divert all of the water for the last few seconds before shutting off the engine. The point here was the original old valve didn't do this very well, either letting water out of both exits, or just the secondary one. I couldn't see the point in cooling water just coming out of both the main exhaust and the secondary port when the engine was running normally. Surely an "either/or" valve is what's needed?

 

[earlybird]

The anti-syphon device is not a diverter valve, but a device to allow air into the cooling water pipe when the engine is shut down, otherwise water may continue to syphon into the exhaust system through the inlet seacock and sea-water pump. The device can take the form of a spring loaded valve, (eg Vetus ) which requires a very small air pressure to open it, hence Vetus recommend a minimum of 16 inches above the water-line IIRC. This type should not discharge water whilst the engine is running An alternative, which Vetus also offer, is just an orifice which connects to a skin fitting and will almost always discharge whilst the engine is running. When the engine is stopped air just flows inwards to break the syphon. This type is almost fool-proof and shouldn't leak into the boat unless the pipework is faulty, whereas the valve type might. The water flow through the orifice is small compared to the exhaust water flow, which must be maintained at all times to cool the exhaust system, and not diverted.
Edit just re-read your first post, now see diverter valve is separate to anti-syphon, I don't think I'd be keen on that for fear of over-heating the exhaust line if mis-used. A well installed orifice type anti-syphon should be OK I would have thought.

 

[lw395]

Probably no harm in letting so water out of aux exit as a tell-tale when running normally, provided enough goes to cool the exhaust. Could be problem with a changeover valve if it is 'shut before open' pressure may build up during changeover? Probably not an issue? would it give the impeller a hard time?
Is it the heat or the water you are worried about with the radio etc? The air inlet of the syphon breaker can be taken outdoors via a small bore tube. You're right it's not ideal but all boats have compromises. I just like things to be automatic with minimum moving parts where possible.
Cheers.

 

[wotayottie]

I agree - the likely function of the valvce is to let out a stream of water when the engine is running as a teltale - just like with outboards.

 

[sailorman]

No Wrong again
i installed my engine 6 yrs ago ( 650 hrs ) i dont have a valve @ all just a "Piddler" that discharges into the cockpit above a drain, its simple if no water is present with the engine running then no raw water cooling water circulating !!.
this does the same thing as a valve BUT with nothing to go wrong, its fitted via a "Y " fitting after the raw water impellor pump then into the engine. as long as there is a vacuam break it doesnt matter where its fitted /forums/images/graemlins/ooo.gif

 

[VicS]

What is shown in your diagram is totally wrong! <ul type="square">[*]A syphon break is needed when the exhaust injection point is below or less than 6" above the water line. [*]The siphon break valve should be at least 12 inches above the water line, preferably 16".[*]There are two types:
One simply has a valve in the top which opens to allow air to enter.
The other has no valve but is connected to a small diameter overboard discharge. [/list]
These diagrams show the two alternatives.

Note the whole of the water flow leaving the engine is diverted to the antisiphon valve. In the second type just a small telltale flow exits overboard, like the "pee stream" from an outboard.

There are better diagrams on the Vetus website . Click thse thumbnails to see what is on the vetus site

<span style="color:white"> ................ </span> <span style="color:white"> ,,,,,,,,, </span>

 

[VicS]

[ QUOTE ]
its fitted via a "Y " fitting after the raw water impeller pump

[/ QUOTE ] But that, presumably inverted, Y fitting should be well above the water line. Vetus used to recommend 12" with a discharge rising to 16" but now they say 16" for the whole shooting match.

The objection I would raise to putting it between pump discharge and engine is that you are wasting water that should be cooling the engine. Akin to having a poorly performing pump!

 

[VicS]

Earlybird you are spot on.
Pity you did not copy the diagrams from the Vetus catalogue. It would have saved me doing it!

 

[sailorman]

It only " Piddles" engine temps ok so far /forums/images/graemlins/wink.gif

 

[Bilgediver]

Surely an "either/or" valve is what's needed?
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Yes the best solution is a 3 way ball valve fitted if in the sea water line to the exhaust at the position you show.

In one position normal cooling.....and in the other position the sea water diverted to the direct overboard to alow the exhaust to drain before stopping and closing the exit valve

Please not that some posters have not read your drawing correctlyhence the wrong solutions.

/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[charles_reed]

The valve should close under pressure, when the water is being injected into the exhaust and open when the engine stops running, so as to allow air into the gooseneck and prevent a siphon effect drawing seawater into the exhaust manifold and thus into the engine.

 

[Seagreen]

Perhaps. VicS says the system is totally wrong, but that's what has kept the boat running for the last 10 years.

As it was and is set up, there is very little height difference between the top of the exhaust inlet, the exhaust outlet, and the top of the syphon pipe. The exhaust out is about 2' higher than the waterlock, and the Antisyphon pipe is perhaps just 12" above the start of the exhaust.

I was told that the secondary pipe was to pump the exhaust dry in the moments before switching off the engine to prevent back-flooding the engine. There may be more water in the exhaust pipe, especially in a following sea, than the waterlock could absorb, hence the valve. I was just wondering how the exactly which way the valve had to work, and whether the one I've got needs replacing. As the rubber pipe is perished , it all needs replacing anyway.

I think that the valve in the original diagram at the top ought to be on the downstream side of the syphon break valve.

 

[earlybird]

You are dealing with two separate issues here. If there is a risk of water from a following sea flooding the exhaust when the engine is stopped, then the only prevention is to close the exhaust outlet seacock, hardly convenient, but better than a ruined engine. Clearing the exhaust with the diverter valve might delay, but not prevent this flooding. The anti-syphon device is neccesary to prevent flooding via the cooling water inlet. If the water-lock has adequate volume, and the anti-syphon is effective then the diverter valve is an unneccesary complication. If, at times, you really want to clear as much water as possible from the exhaust, why not just close the cooling water seacock a few seconds before stopping the engine.

 

[Tranona]

The diverter valve to "drain the system" is unecessary if you have an anti syphon valve of either sort installed correctly. The valve can be in the accomodation as normally there is no leakage, and if you have the type with a bleed to skin fitting, no chance of leakage at all.

Water coming up the exhaust because there is no gooseneck at the outlet is a different issue. Are you sure there is no room? All you need is height above the water line and about 250-300mm space fore and aft in front of the transom. A loop of the flexible exhaust hose connected to a 90 degree bend attached to the skin fitting does the job. Alternately Vetus do a neat moulded plastic vertical gooseneck - see page 148 of catalogue. If there is not space immediately in front of the outlet, the loop can be anywhere downstream of the waterlock.

 

[VicS]

Beginning to understand your system a little better now

BUT

EDIT: On reflection "totally ineffective" is exaggerating a bit but only provided the bit ringed in pink is well above the waterline.


The valve should send all the water over the siphon loop when running and into the exhaust. The valve on the antisiphon valve should automatically close.

The water trap should be big enough to hold all the water lying in the exhaust system when the engine is stopped. If it is there is no need for a diversion system.

However when operated I would expect valve "A" to divert all the water overboard but the engine must be stopped fairly promptly as there will then be no water cooling of the exhaust gases leaving the engine. For it to work like this valve "A" must be a three port valve.

I Don't like it!
Do away with it.
Fit an adequately sized water trap.
Position the antisiphon valve at least 12" above the water line.
Regularly check the operation of the valve on the antisiphon valve or better fit a device that discharges a telltale stream overboard or into a cockpit drain.

 

[Bilgediver]

I think that the valve in the original diagram at the top ought to be on the downstream side of the syphon break valve.

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NO......it is exactly where it should be as the height of the syphon break is needed to divert the flow.

The only improvement is to change the valve to a three way valve which will divert all the flow while drying however you must be sure to put it in the run position once started.

This system isn t ideal for strangers but the best of a bad job for your installation. You are not alone with this set up as a few others have mentioned similar arrangements in the past where the constraints of the boat did not allow for the correct set up of swan neck..large enough water trap etc.