View Full Version : Anti-Siphon loop on raw water exhaust.

03-21-2008, 03:15 PM
Setting up a new motor and using Calders book as a reference. This is in a 21' doubler ender, sail, and the motor will be completely below the water line.

I am hoping someone can explain to me the whys and hows of the anti siphon loop in the raw water exhaust. I am planning a loop about 18" above water line. Will this impede the flow of the exhaust water into the exhaust manifold exit tube. Any pics of how others have set this up would be worth 10,000 words.

Also I found a short thread from last year speaking to the back pressure created by the exhaust loop being too long after the waterlock. What advice say you............


03-21-2008, 03:22 PM
Feel free to come over and check out Talisman's wet exhaust. It'd be tough to get a picture in there.


03-21-2008, 05:36 PM
Are you streaming all your cooling water into the exhaust or does some bypass?

03-21-2008, 08:24 PM
Gareth, This is a heat exchanger situation and all raw water is going thru the exchanger and into the outlet exhaust elbow. Calders writing suggest I should severe the rubber tube raw water outlet and raise it 6" prefereable 12" above water line at all angles of heel, and situate a(n) relief valve or tube at the apex of that loop to create a anti- siphon loop. This makes sense, though I fail to see how this works. Certainly it does, though I suppose I need simplified explanation to understand the mechanics.

In addition I will loop the aft end of the waterlock section of exhaust tube into a "gooseneck" configuration to help prevent the hydrostatic siphoning along the hull over the opening at the thru hull.

I visited two yards today to look at installations in sailboats .. I looked at 7 different boats and only one would I say was anything close to being correct, according to Calders descriptions. I was astounded and enlightened to see that often this is done in a manner that, in rough conditions, the integrity of the motor is definitely compromised.

I saw a couple of the rubber flap devises and noticed that they were indeed, as Calder warns, encrusted with salt and not fully "stopped" down. Rendering them less than effective.

From what I learned from yards is that a good many motors are flooded and some wrecked due to these poor installations.


03-21-2008, 08:35 PM
Much as i hate to agree with Nigel, yes the flaps are useless on a sailing boat, but do O.K. on a powerboat, where they serve a different purpose. The anti syphon valve allows the water in the tube to drain by letting air in at the top. Does he talk about having a thin tube instead of a valve, the valves in this application take a lot of abuse "hot salt water".

I did realise it was a heat exchanger, but some applications have only some of the cooling water being mixed with the exhaust.

I trust Calder also tells you to make sure that the sea cock is readily available. With this set up, if your engine does not start, it will also back up with sea water, the solution is, if you are having trouble starting, turn off the sea cock, turn it on (quickly) when it starts.

03-21-2008, 08:40 PM
Yes NC does suggest that the best set up is a thin tube serving as the vent as opposed to a mechanic (spring) vent. Lead this high enough that it should never see weeping or water levels.

All the raw water coolant is going to the exhaust elbow, none is diverted for other applications.

He does mention a seacock. Where is this installed in the loop ? This is to prevent raw water from coming to the engine on the inlet side, correct ? So the system doesn't get filled with raw water and no exhaust yet present (due to not starting)to lift it.

I think he also mentions a seacock at the exhaust thru hull to prevent strong following seas from working, but I was unclear about that part.