A wet exhaust carries the engine's cooling water out with the exhaust gas. When it fails it is dangerous both ways: water can run back into the engine, and gas can leak into the boat. The survey follows the whole line from elbow to outlet.
The parts of the line
- Mixing elbow
- Where cooling water joins the exhaust gas. It narrows inside with salt and carbon and eventually perforates.
- Exhaust hose
- Should be hose made and marked for wet exhaust. Look for hardening, cracks and heat marks.
- Waterlock (muffler)
- Holds the water in the line when the engine stops. Checked for cracks and leaking joints.
- Gooseneck and outlet
- The loop rising above the waterline and the hull outlet; they stop the sea coming back in.
- Siphon break
- Where the elbow is near or below the waterline, it stops seawater siphoning back into the engine. A blocked siphon break looks fine but does nothing.
What the survey looks at
- Salt crystals, rust and weeping on the outside of the elbow.
- Hose type, age and double clamps at every joint.
- Hose touching hot surfaces, and heat-discoloured sections.
- Elbow and hose temperatures on sea trial, with an infrared thermometer and thermal camera.
- Soot or exhaust smell in the engine room; a CO detector in the sleeping areas.
What we often see
- Corroded exhaust elbow: Salt crystals and rust on the outside of the mixing elbow; high elbow temperature on sea trial.
How siphoning happens
When the engine stops, water remains in the line from the raw-water pump to the exhaust elbow. If the elbow is near or below the waterline, that water tries to level with the sea and can run from the elbow into the exhaust manifold and on to the cylinders. It happens most when the engine stands for long or the boat lies over in a sea. An engine with water in a cylinder can be badly damaged when someone tries to start it. That is why a siphon break, where the elbow's position calls for one, is a small but vital part.
Warning signs
- Hard starting, or white smoke on the first start.
- Milky oil on the dipstick, or a rising oil level.
- Less water than usual from the exhaust outlet.
- Soot or exhaust smell in the engine room, or heat discolouration on the hose.
Life of the exhaust elbow
The mixing elbow, where hot exhaust gas meets seawater, is one of the fastest-wearing parts aboard. Salt and carbon build up inside, the passage narrows and eventually the metal perforates. Perforation is not always visible from outside, so the elbow is removed at intervals, checked inside and replaced when needed. On survey, salt marks and rust outside and the temperature read on the sea trial show whether that check is due.
The generator's exhaust
Generators also usually run with a wet exhaust and carry the same risks. On boats where the generator runs often, especially overnight, the exhaust line's tightness and a CO detector in the sleeping areas matter all the more.
The exhaust at lay-up
At lay-up, the engine's cooling circuit is drained or protected against frost, and the waterlock is drained too. During a long lay-up, the exhaust outlet is kept from blocking or taking in water. At the first start of the season, check straight away that water comes steadily from the exhaust.