What Is a Waterlock (Silencer) in the Exhaust System?
A water trap in the exhaust line – it looks like a silencer, but its real job is to stop seawater and exhaust water running back into the engine.
A waterlock is the chamber fitted below the engine outlet in a wet exhaust system: when the engine stops, it holds the water in the exhaust hose and prevents it from running back into the cylinders; while running, it also acts as a silencer.
An engine taking on water (hydraulic lock) is one of the most expensive failures on a marine diesel, and the cause is often exhaust system design or poor maintenance. The most critical scenario is repeated long cranking.
Life-saving rule
The long-cranking trap: Never keep cranking an engine that will not start; with every crank the pump sends in water, but with no exhaust gas there is nothing to push it out, the chamber fills, and the overflowing water can run into the cylinder.
If you must persist: close the seawater seacock (and put a note on the key so you do not forget to reopen it once it starts).
Rule of thumb: if the engine has not started within 10-15 seconds, stop, think, and check the cause; stubborn cranking can turn a simple fault into an engine rebuild.
During winter lay-up, emptying the waterlock through its drain plug is standard practice (to prevent freezing). A plastic body can develop fine cracks from vibration; a bilge pool of water with an exhaust smell is the tell-tale sign. Exhaust hose connections should be double-clipped, as this is a hot, vibration-prone line.