Keeping Water on the Outside – Siphons

“Nothing good ever comes from water in the bilge.” Jim Merritt, marine surveyor and mentor.

All of the submersion claims we have ever handled have the same cause of loss… too much water on the inside. In order to maintain a boat without water inside we employ valves, loops and siphon breaks, but sometimes this simple concept is misunderstood. The systems that normally require these measures are engines, heads and bilge pumps.

Vented loop anti-siphon valve
First let’s begin with the basic concept of a siphon. Simply put this is the transference of a liquid from a higher point to a lower point through a level higher than the source reservoir’s liquid. In a boat this usually means water entering a through hull and discharging at a point lower than the through hull through a hose run above the waterline.

To prevent siphoning we install siphon breaks, usually in the form of vented loops where the top of the vent is above the maximum healed waterline and the break is a way to allow air into the top of the loop.

Bilge pumps should discharge above the waterline, but those that do not require a siphon break. A vented loop is acceptable per A.B.Y.C. standards but a check valve is not. A check valve can only be used to prevent cycling of the pump, but configuring the pump and float to eliminate the cycling is smarter.

Heads mounted below the waterline require means to prevent flooding. If you leave the intake valve open and the head overflows, this is not siphoning, this is flooding (unless your through hull is higher than your head). In this case a simple loop, unvented, would have prevented the flooding as water seeks its level. A simple trick to know the waterline on the interior of your boat is to have a hose full of water contiguous to the sea, the water level in the hose is the water level in the sea.

Heads that discharge above the level of the top of the head do require vented loops and the vents only work if they allow air into the loop. Vents require maintenance, a clogged vent in a vented loop is just a loop and does not prevent siphoning.

Engines installed at or below the waterline require a loop in the exhaust discharge to minimize the possibility of inflow of raw water, whether the engine is running or not and a siphon break to prevent siphoning through the raw water pump when the engine is stopped. The water lift muffler is below the raw water intake through hull and in certain situations a solid column of water can form between them, eventually filling the muffler and engine with water. A functioning siphon break, usually between the engine and the muffler prevents the flooding. Over cranking this type of engine will also flood the engine with water. If the engine won’t start, shut the through hull while you remedy the problem but remember to open it when the problem is solved.

This is not meant as a “how to keep water out of the bilge” article nor does it address thalassophobia (fear of the sea). There is apparently no name for fear of water in the bilge, perhaps boataquaphobia.