Most marine exhausts are water cooled, with water from the engine’s raw water cooling system used to cool the exhaust gas. The water and exhaust gas mix together in the injection bend, where it turns into steam before cooling and condensing back into water which then makes its way out of the hull via the exhaust system.
Exhaust system inspection
With the boat ashore, it is wise to inspect the exhaust system for corrosion damage, especially around the injection bend:
- Injection bend – this is where the raw water cooling system meets up with the exhaust gases to in order to cool the gases enough to prevent the flexible rubber sections of the exhaust from melting. Check the condition carefully, looking for any signs of corrosion.
- Exhaust piping – look for external signs of deterioration in the flexible rubber piping, connections and clips, checking the condition of the double clips all along the system. Any signs of white crystals indicate there have been sea water leaks.
Smoking exhaust
If you have noticed the engine exhaust smoking a lot during the sailing season this can indicate a number of potential problems with the engine that may need to be rectified. It is normal for an engine exhaust to smoke until it is warmed up so you only need be concerned if the exhaust is smoking after the engine has fully warmed up.
The colour of the smoke can be a guide to what might be wrong. White, blue or black smoke from a diesel exhaust indicates there is a problem. White smoke has two general causes: overcooling where proper combustion is not taking place or worn piston rings. Blue smoke comes from burning oil. It can be caused by worn valves or piston rings. Black smoke typically indicates partially burned fuel, worn injectors or clogged air filters. The chart below looks at possible causes and solutions. To help solve the mystery you may need advice from your boatyard mechanic.
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White exhaust smoke |
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Possible cause |
Solution |
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Blocked injectors |
Service the injectors |
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Water or air in the fuel |
Defective seals, possible leaking head gasket |
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Check the breather pipe is clear and not obstructed |
Remove and clean out |
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Reduced cooling water flows |
Check the raw water system |
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Blue exhaust smoke |
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Engine oil level too high |
Check the oil dipstick. If too high, pump out some of the oil |
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Worn valve guides and seals |
Replace oil seals |
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Piston ring and bore worn, giving a low compression |
Get compression checked by a boatyard pro |
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Leaking turbocharger seal |
If fitted, get the turbocharger checked by a boatyard pro |
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Black exhaust smoke |
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Blocked air filter element |
Inspect and replace |
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Inadequate ventilation in engine compartment |
Check the ventilation isn’t blocked |
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Blocked or damaged exhaust hose |
Check the exhaust hose isn’t blocked or restricted in any way; internal inspection may be required as inner layers of exhaust flexible hose can collapse |
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Malfunctioing fuel injection |
If the airflow is unobstructed and the engine not overloaded, poor injection could be the problem. |
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Over pitched propeller – engine will not reach its full rpm |
Check the engine will reach full rpm in neutral; if it does not get the propeller checked / re-pitched if necessary |
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Accumulated debris on hull |
Inspect and clean if required |
