Originally Posted by O C Diver
Why do you need a valve? Was considering adding an anti-siphon valve to my engine when I repowered. Then thought if I did, I would route a piece of 3/8" hose from the top of the loop to the galley sink drain pipe after the P trap which is 4' above my engine. Have seen numerous pics of engine rooms where there is or was some spray from the valve that has corroded stuff. Thought routing the vent to an overboard discharge was a cleaner solution. Any reason that wouldn't work?
I recently reworked some of my exhaust plumbing and installed a Groco vented loop vacuum break which had been omitted from a prior re-power. I chose the pricey Groco unit because if its large vacuum breaker diaphragm and reputation for being reliable. After I finished the installation, I ran across this article on Designing a Marine Exhaust System by Tony Athens at Seaboard Marine
. Sections 5 and 8 discuss the subject of anti-syphon bypass.
While everything was still accessible, I installed an active bypass in the uphill cooling water discharge line before it enters the vented loop and goes over the top into the muffler. This bypass discharges overboard via a through-hull above the waterline. It provides a redundant, positive vacuum break upon engine shut-down, and a visual indication of cooling water flow when the engine is running. Actually, it is similar in concept to the water that flows from the cooling water tell-tale (pee hole) on an outboard motor.
Being cautious, I also installed an in-line ball valve so I could control the bypass flow or close it completely in the event of a weak or failing raw water pump. Tony also mentioned that the raw cooling water flow was greater than what was actually needed to cool the exhaust system, and that reducing the flow also reduces exhaust back pressure. I found this to be true as I am bypassing about 1/3 of the cooling water and the muffler still runs under 100F at every engine speed and load. One additional safety feature is that the risk of filling a water-lift muffler due to prolonged cranking is reduced or even eliminated.
Finally, this arrangement seems to be common on commercial vessels with wet exhaust for safety, and some sportfishing boats for performance.