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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.

White exhaust smoke

 

Possible cause

Solution

Blocked injectors

Service the injectors

Water or air in the fuel

Defective seals, possible leaking head gasket

Check the breather pipe is clear and not obstructed

Remove and clean out

Reduced cooling water flows

Check the raw water system

Blue exhaust smoke

 

Engine oil level too high

Check the oil dipstick. If too high, pump out some of the oil

Worn valve guides and seals

Replace oil seals

Piston ring and bore worn, giving a low compression

Get compression checked by a boatyard pro

Leaking turbocharger seal

If fitted, get the turbocharger checked by a boatyard pro

Black exhaust smoke

 

Blocked air filter element

Inspect and replace

Inadequate ventilation in engine compartment

Check the ventilation isn’t blocked

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

Malfunctioing fuel injection

If the airflow is unobstructed and the engine not overloaded, poor injection could be the problem.

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

Accumulated debris on hull

Inspect and clean if required

 

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