The Offshore Voyaging Reference Site

Check The Siphon Break

The engine on most sailboats is installed at least partially below the waterline, consequently, with most exhaust systems, the only thing preventing the engine from flooding with water after it stops is the siphon break installed at the highest point in the raw-water cooling system.

That’s bad enough, but the other problem with siphon breaks is that they are usually installed in some inaccessible place, so on a lot of boats they don’t get any love from one year to the next.

This is also made worse because I have never seen an engine manual that calls for regular disassembly and cleaning of the siphon break.

Maybe the engine manufacturers want it to stick closed so our engines will flood and they can sell us new ones…not really, but I do sometimes wonder…particularly when I’m thinking about saildrives, the existence of which clearly proves how much engine manufacturers actually hate us owners.

Sorry, rant over.

Anyway, the photo above shows the state of the siphon break on our J/109 when we got her. Looks to me like no one had looked at it since the builder buried it high in the engine space 18 years before.

I figure that the only reason the engine had not flooded was because the J/109 has a shallow hull form, so the engine is higher in relation to the waterline than on most cruising boats.

Needless to say, I replaced it.

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Eric Klem

Hi John,

I am not a fan of valves in this application at all. My preference is to remove the valve and run a hose to a thru hull high up where it will be submerged only extremely rarely. The reason I like this is that you don’t have a valve that can get stuck and either leak exhaust down below or allow a siphon. Also, you can see the stream of water and for boats where you can’t hear the exhaust, it is a great tell-tale. The downside of doing this is that you get a small exhaust stain on the topsides. Not all boats will be able to find a good way to implement one of these. In our case, the exit is just below the toe rail a few feet forward of the transom. We have a diverter cap on top of the thru hull, I put a little surge tube inside, and the diameter is quite small so I think it is very unlikely we could ever flood the exhaust with this. I know of some people who put the exit into a cockpit drain and that seems like a good solution too.

I really like the idea of your water separation exhaust from Halyard but I don’t think I have ever seen one in person. I have been on several boats with dry exhausts and am generally not a fan. Also several jacketed exhausts and they terrify me, I know of way too many failures ending in water in the cylinders. Unfortunately, large boats with the engines many feet below the waterline have backpressure problems if you put a waterlift by the engine so you often have to run dry for a large portion of the exhaust.


Wilson Fitt

Hi John

After a near disaster with water siphoning into the engine many years ago, I got rid of the anti-siphon valve on the top of the loop and replaced it with a barb fitting and a small tube into the cockpit foot well, down low. When the engine runs, a little dribble of water disappears under the floor grating and into the cockpit drain.


William Murdoch

I got rid of the anti-siphon valve on the top of the loop and replaced it with a barb fitting and a small tube…”

Some care is needed with small tubes. Water tubes of a 1/4″ or less in inside diameter are small enough that when vertical, open at their lower end, closed at their upper end, and full of water; the water will not run out as air will not run in from the lower end to replace it. The water will just hang there like water in a soda straw with a finger on its top end. A vacuum at the top end will have to suck the water up the tube, and the vacuum will persist until the water is sucked out of the tube. During that time the purpose of the vacuum breaker is defeated, and its effective height is the height of the bottom of the water column standing in the small tube.

I ran my small tube from a barb fitting on the siphon break to the bilge where the lower end of the tube was high enough not to be submerged. Luckily, my tubing was transparent and after shutting down the engine I saw the water still standing in the small tubing. Ouch. Now my small and short tubing from the barb runs into the open top of larger tubing at the elevation of the siphon break and the larger tubing runs to the bilge. The water stream snakes down the inside of the larger tubing when the engine is running.

Dick Stevenson

Hi John,
And do not forget the other siphon breaks that are usually ”out of sight, out of mind”: on Alchemy this includes (from memory) those on the toilet, one of the bilge pumps, and an unusual sink pump-out.
I also like the tell-tale stream on my engine.
When I purchase an anti-siphon loop, I try to purchase and extra “vent” as I found, in salt water, they got gummed up and were hard to remove to clean without damage. A rag soaked with hot water and let sit for a while helped.

My best, Dick Stevenson, s/v Alchemy

Neil McCubbin

Like other posters, I mush prefer a visible, continuous “pisser” over a siphon break valve for the engine
We twice filled an engine with water before going that route
On our head, the user has to put his finger on a hole connected to the high point on the water inlet line

Ben Logsdon

I don’t quite understand the purpose of the siphon break. My Sabre 34 has an engine below the waterline, but there’s not a siphon break, nor was there ever one installed—it’s absent on the drawings. Is this something that I should consider retrofitting?