Raw Water Anti Siphon Vented Loop

[Sailor_Grant]

What are people's thoughts about anti-siphon valves on the raw water system of a diesel engine? The exhaust of my boat is halfway up the transom, so I do not believe it is possible to siphon water into my boat.

In addition, I don't want to start this debate, but it concerns opening and closing the raw water seacock. If the engine seacock is open, will it flood the engine with raw water, or will it be held back by the height of the mixing elbow which is above the waterline, and would an anti-siphon valve solve this problem or provide better peace of mind?

[wingssail]

Originally Posted by Sailor_Grant

What are people's thoughts about anti-siphon valves on the raw water system of a diesel engine? The exhaust of my boat is halfway up the transom, so I do not believe it is possible to siphon water into my boat.

In addition, I don't want to start this debate, but it concerns opening and closing the raw water seacock. If the engine seacock is open, will it flood the engine with raw water, or will it be held back by the height of the mixing elbow which is above the waterline, and would an anti-siphon valve solve this problem or provide better peace of mind?

The issue is not seawater syphoning in from the exhaust outlet at the transom.

The problem is that raw water in the cooling system can run from the heat exchanger outlet into the mixing elbow (if that is below the water line). Then the exhaust system fills up until the water level backs up into the exhaust manifold and then in to any open exhaust valve, and into the cylinder. This can occur if the top of engine is below the waterline. Closing the seacock prevents this. The other thing which saves the engine is that the exhaust pressure when the engine is running blows water out the exhaust and prevents it from filling the exhaust manifold.

The anti-syphon break is installed between the heat exchanger outlet and the exhaust mixing elbow. Often all that is needed is a long loop in the hose with a "T" at the highest point opening into some draining point in the boat (galley sink or cockpit drain, for example). When the engine is running a small amount of water is ejected out the "T" but when the engine is turned off air enters that "T" and prevents raw water from being syphoned over the loop and into the exhaust system. For this reason an electric raw water pump which runs when the engine is not operating is dangerous, because it can fill the exhaust system.

Now, if you have a exhaust outlet below the waterline it could be possible for seawater to syphon back to the engine through the exhaust hose, so it is a good idea to have a high loop in the exhaust hose, and use a large hose, so that raw water runs out of the hose and the syphoning effect cannot work.

Finally, there is some concern that a following sea will strike the exhaust outlet and force water backwards into the engine, running or not. Some owners install a valve in the exhaust hose to prevent that while the engine is not running, however it cannot be closed when the engine is operating. I have this valve but have never closed it. My exhaust outlet is actually very close to the water's surface at the transom, but that problem has just never occurred.

A flap over the exhaust outlet could prevent a wave from entering without restricting the escape of exhaust water and gases.

[barnakiel]

The break breaks the intake water, not the exhaust water.


All boats I have sailed on had one.



barnakiel

[Sailor_Grant]

So if the mixing elbow intake is above the waterline by 6 inches a vented loop is not necessary?

[Manateeman]

Hi. There are two ways to drown the engine or generator. First, exhaust.
Power yachts often have issues with following seas. There is an exhaust builder in Miami who has some excellent pages on the subject. Since you have a smaller engine, you might consider a water / exhaust gas separator. Add a drain valve to your water lift muffler so cranking your engine will not fill it too much.
I’m tired of going into the engine room to open and close my raw water valve so I’m adding a motorized full flow valve with auto off and a light and switch next to my generator start panel. Hope you are not this old and a bit more nimble. A valve and a wood reminder stick on your engine key will work fine.
Happy trails.
Mark a hobbling manatee

[wingssail]

Originally Posted by Sailor_Grant

So if the mixing elbow intake is above the waterline by 6 inches a vented loop is not necessary?

No, what matters is the height of the injection point, (water hose going into the mixing elbow). If that injection point is below the waterline the raw water in the heat exchanger will flood the exhaust system unless a syphon break is present. It may syphon over quite high loops (including exhaust mixing elbows) and into the exhaust manifold.

If any portion of the exhaust manifold is below the waterline, even with a higher mixing elbow, a wave or heeling may allow the mixing elbow to fill and then syphoning can occur there also. The syphon break in the hose prior to the injection point prevents the exhaust system from filling.

[S/V Illusion]

Originally Posted by Sailor_Grant

So if the mixing elbow intake is above the waterline by 6 inches a vented loop is not necessary?

Depends on wave height as well as how much heel or stern squat occurs.

[Cheechako]

Water does come in from the exhaust. Not uncommon at all. I had one boat it happened because the boat was sitting at an outside dock that got some wind chop action on the stern. Evidently it pumped water into the rear cylinder one splash at the exhaust outlet at a time, eventually filling the exhaust muffler and hose until it reached that cylinder.

[Scrimshaw4]

Doesn't your raw water pump's impellers prevent siphoning thru your raw water sea-cock?

[Cpt Mark]

As a safety measure to protect your engine, why wouldn't you put in a siphon break? There was mention of not needing one because the mixing elbow is 6" above the water line. Is that true if you're heeled over? Perkins engines were notorious for back siphoning. Mine did it twice at the dock.

[Stu Jackson]

Diagram might help

[Cheechako]

Originally Posted by Scrimshaw4

Doesn't your raw water pump's impellers prevent siphoning thru your raw water sea-cock?

Apparently they generally do. But it only takes that one time. Strangely, most of my boats never had the siphon break. The only boat I had an issue with had a siphon break , but it was an exhaust ingress issue.

[wingssail]

Originally Posted by Scrimshaw4

Doesn't your raw water pump's impellers prevent siphoning thru your raw water sea-cock?

Not generally, the impellor, when stopped, will permit water to flow past the blades. This is not syphoning. The raw water pump is often below water level and the water flows freely past the impellor.

[waterman46]

Originally Posted by Sailor_Grant

So if the mixing elbow intake is above the waterline by 6 inches a vented loop is not necessary?

Strictly speaking, yes, but what's the resistance to spending a few bucks and a couple hours work to protect your engine from a possibility of flooding, no matter how remote?

Exhaust elbow only six inches above waterline would not make me feel comfortable about not having a vented loop.

[HeinSdL]

Checking the proper operation of the anti syphon valve yearly is thus to be recommended (extremely simple, it's just a spring loaded device allowing air into the system when the pressure inside drops). The small valve can get stuck in place leading to problems.