White, black or blue exhaust smoke: what does the colour mean?

What comes out of a boat engine’s exhaust is a good clue to its health, provided you know how to read it. A marine diesel discharges its gases mixed with the seawater used to cool it: a little steam is normal, and you need to learn to tell it apart from real smoke. Then the colour points the way: white, black or blue, it does not indicate the same causes. It only points the way: it is a clue, not a verdict, and your engine manufacturer’s manual always has the last word.

1. What you observe

• A light white cloud on a cold start, which disappears within a few minutes as the engine warms up: condensation and fuel burning poorly while the engine is cold. This is normal.
• A constant white vapour in cool or damp weather, which disperses within a few metres and has no smell: water vapour, also normal.
• Thick, persistent white smoke with the engine warm, lingering over the water, smelling of diesel or with a sweetish smell: this is no longer steam.
• Black smoke, when accelerating or continuously under load, often with soot marks on the hull around the exhaust outlet.
• Blue or bluish-grey smoke, with a smell of hot oil.

To tell steam from smoke: steam disperses quickly and leaves neither smell nor trace; smoke lingers, smells, and leaves an oily film or soot on the water and on the hull.

2. Safety first

• Smoke in the engine compartment, rather than at the exhaust, is a different problem: exhaust leak, a hose burning, a short circuit. Stop the engine, do not throw the compartment wide open if you suspect a fire (air feeds it), keep the extinguisher ready.
• White smoke or steam with the temperature alarm: the engine is short of cooling water. Throttle back and read the guide on the temperature alarm.
• Thick black smoke and falling revs: the engine is struggling. Throttle back, do not force it.
• Exhaust gases contain carbon monoxide, invisible and odourless. A smell of exhaust in the cabin is never harmless: ventilate, and fit the boat with a carbon monoxide detector.

3. Likely causes, colour by colour

White smoke, engine warm

• Poorly burnt diesel — whitish-grey smoke that stings the eyes and smells of diesel: an engine idling for a long time without warming up, a dripping injector, incorrect injection timing, faulty preheating.
• Water in the fuel — it passes into the combustion and comes out as steam; the water separator shows it.
• Little seawater at the exhaust — the gases come out too hot and make steam; see little or no water at the exhaust.
• Water getting into the cylinders — sweetish smell, coolant level dropping with no visible leak: head gasket, or a perforated exhaust elbow letting seawater into the manifold. This is the most serious case of white smoke.

Black smoke

• Overloaded engine — fouled hull, dirty or unsuitable propeller, heavily laden boat: the engine no longer reaches its maximum revs and receives more fuel than it burns.
• Lack of air — clogged air filter, blocked air intakes to the engine compartment.
• Worn injectors that atomise poorly.
• Partly blocked exhaust — elbow clogged with carbon: the gases cannot get out properly.
• Turbocharger past its best, on engines that have one.

A small black puff on sudden acceleration is common on a diesel with some hours on it; continuous black smoke at cruising revs is not.

Blue smoke

• Too much oil — a level above the maximum after an oil change: the excess burns.
• Engine often idling without load — a diesel that runs for long periods with no load, for example to charge the batteries at the pontoon, eventually stops controlling its oil properly.
• Internal wear — piston rings, valve guides: oil gets into the combustion. The engine uses oil between oil changes.
• Turbocharger whose seals let oil through.

How serious, in short

• Nothing serious: steam on a cold start that disappears, light vapour in damp weather.
• To watch and correct: black smoke linked to the hull, the propeller or the air filter; blue smoke just after an over-generous oil change.
• To have looked at quickly: thick white smoke when warm, especially with coolant loss; persistent blue smoke with oil consumption; any smoke accompanied by an alarm, a new noise or a loss of power.

4. What you can safely check yourself

• Watch and film the exhaust when cold, then after fifteen minutes at cruising revs: does the smoke disappear as it warms up? Does it only appear under load?
• Note the smell: diesel, hot oil, sweetish smell.
• The oil level, engine stopped and boat level: too high, milky, or higher than last time (water or diesel is mixing in)?
• The coolant in the expansion tank, engine cold only: does it drop from one outing to the next?
• The air filter and the air intakes to the engine compartment.
• The water flow at the exhaust.
• The water separator bowl: water or sludge at the bottom?
• The hull and propeller: date of the last antifouling, shells on the propeller.
• The maximum revs reached in forward gear, compared with the figure in the manual: an engine that no longer reaches it is overloaded.

5. When to stop and call a professional

Persistent white smoke when warm, coolant loss, oil that is milky or whose level is rising, continuous blue smoke, black smoke that remains after cleaning the hull and changing the air filter: this is the time to call a marine mechanic. The measurements that settle the question — compression, checking and calibrating the injectors, the condition of the head gasket and the exhaust elbow, oil analysis — are their job. Give them the make and model of the engine, its hours, the colour and smell of the smoke, when it appears (cold, warm, under load, since when), how the oil and coolant levels have changed, the date of the last work on the injectors and the elbow, and what you have already checked. Your video of the exhaust will be useful to them.

6. Stopping it from happening again

• Run the engine under load regularly rather than for long hours at idle.
• Keep to the due dates for the air and fuel filters, the injectors and the exhaust elbow.
• Change the oil with the right quantity, to the dipstick mark, and note any top-ups between changes.
• Keep a clean hull and propeller: see the guide to antifouling.

The full calendar is in the guide to maintaining an inboard engine; the one on laying up explains how to prepare the engine for winter. Record in the Hullteam log what was found and done: the cause, the parts, the date and the engine hours. A well-kept history is what reassures a buyer and gives the boat its value at resale — see which proof really counts.