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Old 10-17-2001
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Anti-Siphoning Exhaust

Experts like Nigel Calder and Don Casey have recommended a small vent from an inboard diesel exhaust to the cockpit to avoid back-flooding of the engine. On our boat this resulted in hot water being splattered at the feet of those in the cockpit when the engine was running. Can this be avoided? We could put a one-way valve on the cockpit vent, but wouldn’t that defeat the purpose of avoiding the risk of a clogged valve when it is needed?

Sue & Larry respond:
We think pulling the anti-siphon valve outlet out of the engine room and leading it to another location makes good sense. As you know, if you use a traditional siphon break that employs a one-way valve, the valve can clog with salt or other debris. If this happens and the valve is mounted in the engine room, two things can occur: the valve could cease to function as a siphon break and/or salt water could be sprayed all over your diesel engine. Obviously, this is not a good thing.

By the way that you’ve described your siphon-break installation, it sounds to us that the water pressure from your raw-water cooling is great enough to lift the water enough to force it out your vent into the cockpit instead of it all flowing aft through your exhaust. As you mention, you could place a one-way valve into the siphon break. Then, you’d only burn your feet when the valve clogged. Another option may be to raise the outlet point of the siphon break hose, but you may still run into problems if it leads to the cockpit.

Our recommendation to you is to lead the end of the vent hose to one of two locations. Either run the hose aft along with your exhaust to the transom, or run it outboard to your port side through your lazarette locker. When we installed our new engine on Serengetti, we used a Vetus Air Vent model H. This siphon break comes with a hose and a fitting for the transom. When the motor is on, there is a very small stream of water that flows out of the fitting.

As a general resource, you may want to contact Vetus and request their catalogues, www.vetus.com. There you’ll find detailed drawings of many onboard mechanical systems, which we’ve found to be a great help when laying out the engine system in our own boat.

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