Siphons can lead to serious problems, including down-flooding and even sinking. Raw-water siphons are most common in engines and generators, heads, and bilges. For engines and generators, siphons are a danger when they are installed below the waterline and can occur when water drawn into the engine for cooling continues to flow after the engine is shut down (like filling the aforementioned garden hose and then lowering it to the ground). If this happens, seawater will first flow into the exhaust the same way it does while the boat’s running; however, and depending on the exhaust system design, without exhaust gases to expel the water, it will eventually fill the muffler, the hoses, and then the engine itself. Under the right circumstances, water may then leak out of the engine’s air intake and into the boat, causing it to flood and possibly sink. I’ve seen this transpire over the course of just a day or two.

Heads and bilge pumps can suffer the same fate. If the overboard discharge is lower than the inlet, and most are, then seawater may continue to flow after a head has been flushed. The insidious aspect of this problem is that it may not happen for years. A siphon is often not possible if the rubber flapper valves in the head are in good condition—also true for an engine’s impeller or (diaphragm) bilge pump—or simply if the right, or wrong, conditions of wind, waves, angle of heel, etc. don’t manifest themselves.

For bilge pumps, the problem is exceedingly simple. Siphons begin when the overboard discharge is submerged, in spite of the fact that the hose rises at some point above the heeled waterline. (If it doesn’t rise above the waterline, then it’s simple down-flooding rather than siphoning, and an antisiphon valve would not prevent it.) The water running back into the bilge initiates the self-sustaining siphon.