Alert ! This could sink your boat!

[Peter123]

My 36i has an anti-syphon valve to prevent raw cooling water from syphoning into the exhaust and then back into the engine ... but the drain pipe from the top of the valve was so long that it ran right down to below the level of the exhaust and well below sea-level.
If the anti-syphon valve gets blocked open by salt (mine did) it can (I think) syphon water from the raw water intake straight into the bilges... until the boat sinks.
I think the pipe should be just long enough to take any water drips away from the top of the engine but should not run to a point below sea-level. Please correct me if I'm wrong.
If mine was plumbed in with too long a tube.... perhaps yours is too.
I've re-routed and shortened mine.

[krawall]

Peter,

thanks for the heads up. Where exactly is that drain pipe located? In the engine compartment?

Cheers,

Tom

[j]

Mar 17, 2012 2:14:09 GMT krawall said:

Peter,
Where exactly is that drain pipe located? In the engine compartment?

It is mounted above the engine (beside the coolant overflow tank). The hose comes out of the top of it. If you open the stairs and look aft you'll be staring straight at it.

The anti siphon valve is this one:
www.vetus-shop.com/vetus-anti-syphon-airvent-with-valve-13-32mm-p-517.html

The manual is here (Type V) : www.vetusweb.com/manuals/files/Exhaust_systems/030405.03%20r01%2001-07.pdf

Our 2009 36i has the drain pipe running down under the engine, the valve is due for a clean, and we'll consider changing the drain pipe then.

Our old boat had the valves drain pipe running out into the cockpit.

[Peter123]

Here is the anti-syphon valve

[Peter123]

re-routed drain pipe from valve

[Peter123]

thru to "shed" behind shower

[Peter123]

feeding into wider pipe and then downhill into new skin fitting

[ianqv]

Hi Peter,

Thanks for posting.....

I will check mine out at the w/end

Many thanks

Ian

[sitara]

It seems to me that an easy solution to this problem would be to shorten the tube from the anti-syphon valve to well above waterline and lead it into a much large diameter tube that could then end below waterline. The large tube would act like a funnel and all could be held in place with cable ties.

[rxc]

I've been thinking about this for a few days, and am not sure that the problem described is possible. The air vent is designed to let air into the loop when the engine is stopped, so that water is not syphoned back from the muffler into the exhaust manifold. For the air to flow into the loop, the pressure in the loop would have to be lower than the pressure outside the loop (i.e., atmospheric pressure inside the engine compartment).

If the vent fails closed, water will come back up into the loop, and over the top into the exhaust manifold. The pressure inside the loop is still lower than atmospheric, so there is no way for that water to enter the boat. If the water dissolves the salt plug, it will open, and air will flow back into the loop, breaking the syphon.

If the vent is failed open, then when the engine stops, air enters, breaks the syphon, and all is good. However, while the engine is running, the pressure inside the loop is higher than atmospheric - it has to be to push the water out the exhaust port. If the vent is failed open, then it is possible that water will flow out the vent valve into the interior of the boat. There is a very small hole in the vent valve insert to pass the air, so the amounts of water we are talking about are extremely small. I guess that if you put the valve back together without the flow restricting insert, it might give you a large flow, but not enough to sink the boat.

I think that the hose from the valve top is designed to direct a possibly small dribble of water to the outside of the boat. It is installed to satisfy people who like their bilges pristine. Probably some former nuclear submariner or engineer off the Long Beach(CGN-9).

Personally, I don't like to cut holes in my hull unless I absolutely have to, so I would prefer to run this line to the bilge, where I could monitor it, and see if there are any leaks from the vent valve.

Just my 2 cents.

[Peter123]

Sitara .. thank you. Yes, I agree, your solution is simple and effective. I set up my system so that any water that does come through, goes overboard, but perhaps it is overkill.

rxc .. thank you .. I was getting 100-200 mls of water in the bilges every time I ran the engine (because the valve had failed open).. I was thinking of taking the spring valve out so that it couldnt fail closed .. another reason for my re-route overboard. I think if it failed open and dripped for a few weeks while moored, quite a lot of water might get in. Of course, assuming you remember to close the engine sea-cock, this wouldnt happen.

thanks for you comments.

I'm not an engineer or a submariner but I do like the bilges to be dry if possible.

[dnickj]

I would be more worried about creating an air lock in the long run of tubing and flooding the engine
with the motor off the valve should be open this will allow air into the system and break the siphon(as it is designed to do)
there is a chance that there would be leakage into the bilge with the motor running but only if the valve was stuck open - I have had this happen once or twice but at the most 20 drips a minute - as our engine bilge has a hose connection to the bilge pump sump this is not a problem.
when we are off the boat all the sea-cocks are closed anyway

[rxc]

One last thought. I believe that it is important that the end of the hose from the vacuum breaker is never under water. If it is, then when the engine stops it will suck water into the loop, and defeat its purpose. Therefore, it is important that the end of this hose end somewhere other than in a bilge sump, or elsewhere in the bilge that is possible to be filled with water.

On a steam-powered ship, you see drains like this from the steam traps, to the drains system. The steam traps drain condensed water out of the steam lines, to ensure that the water doesn't get into the turbine (bad, bad, bad), but the drains have to be designed to make sure that they don't suck water back out of the drain system when the steam pipes are cold. So, the traps drain to a covered funnel, which provides the syphon break.

Also, I believe that all of the vacuum breakers are installed (or should be installed) above the water line, so that there is always a pressure drop into the the loop thru the vacuum breaker. The lines for head flush water are supposed to be installed with vacuum breakers above the water line for exactly this reason, and they CAN sink your boat if they are installed improperly and fail shut.