HOW IT WORKS - CLEANING AN ANTI-SYPHON VALVE

On sailing boats the exhaust pipe is swept up into a swan neck to stop following waves flooding the exhaust when the engine isn't running.

- Pat Manley

Exhaust gas normally drives water up over the swan neck and out of the system, but of course, if the engine isn't running and seawater is entering the system, water will accumulate in the lowest section of the exhaust pipe. Eventuall,y there will be enough water to enter the exhaust manifold and run into any cylinder that has its exhaust valve open. Because water is incompressible it can shatter the piston or bend a connecting rod if you attempt to start the engine with water in the cylinder. Seawater entering working parts can also contaminate the oil and cause serious corrosion.