Most diesel-powered sailboats have a wet exhaust system. Seawater comes in through a raw-water seacock below the waterline. The raw water pump’s impeller draws it through a strainer and into the heat exchanger. It does not circulate inside the engine, which has a separate cooling system filled with antifreeze coolant that also flows through the heat exchanger where heat passes from the hot coolant into the raw seawater.
After leaving the heat exchanger, seawater enters the exhaust at the mixing elbow where it mixes with hot exhaust gas. The mixing elbow is high enough above the waterline to keep water from running backward into the engine. It is subject to corrosion buildup on some engines and must be removed for cleaning—or replaced—from time to time.
The cooled mixture of exhaust and seawater falls into a waterlift muffler located low in the boat. The muffler reduces exhaust noise, of course, but also collects water when the engine stops. It uses exhaust pressure to push batches or “gulps” of water upward and overboard. The muffler must be large enough to hold all the water that drains back from the exhaust hose after shutdown. If it is undersized, water can back up into the engine.
After leaving the muffler, the exhaust hose usually rises in a loop near the stern before dropping to the hull outlet. The top of this loop should be well above the vessel’s loaded and heeled waterline. This helps prevent following seas, stern waves, or prolonged heeling from forcing seawater back into the engine.
The outlet stream passes through the transom above the waterline, although pitching or heeling might submerge the outlet momentarily. A flapper sometimes adds protection but is no substitute for a properly designed high loop. Water entering through the exhaust can cause hydraulic lock, bent connecting rods, or complete engine failure. Common causes to be avoided include:
- Excessive cranking without the engine starting.
- Clogged mixing elbow.
- Blocked or missing anti-siphon valve.
- Exhaust loop too low.
- Undersized waterlift muffler.
- Submerged exhaust outlet.
- Heavy following seas.
- Incorrect hose routing.
- A failed exhaust manifold or elbow.
If the engine does not start right away, close the raw-water seacock so the pump does not fill the muffler while there is no exhaust to expel the water. As soon as the engine starts, shut it off immediately and open the raw water seacock before starting it again. Running with that seacock closed will ruin the impeller in seconds.
A healthy wet exhaust normally discharges intermittent surges of water. White steam, low rate of water discharge, an unusually hot exhaust hose, or a change in the exhaust sound can indicate insufficient cooling water flow. After starting the engine, always next look aft at the exhaust discharge.