Why is my marine diesel engine consuming more fuel than normal?

A sudden increase in diesel consumption almost always means your engine is working harder to achieve the same boat speed due to underwater drag, or it is starved of air and over-fuelling. Start by checking the condition of your hull and propeller, as marine growth is the culprit in nine out of ten cases.

Hull and Propeller Fouling

The most common reason for burning more fuel is physical drag. A light layer of slime along the waterline can increase fuel consumption by 10 to 15 percent. If you have hard growth like barnacles or tubeworms on the hull, your engine might be working twice as hard just to maintain your usual cruising speed.

Propeller fouling is worse. A fouled prop loses its hydrodynamic shape, turning water into turbulence rather than thrust. Even a single large barnacle on a blade tip destroys efficiency. If you have a folding or feathering prop (like a Gori, Max-Prop or Flexofold), ensure the blades are fully deploying. If marine growth or a lack of grease stops them opening completely, the engine will rev high, burn fuel and push you nowhere.

Airflow and Exhaust Backpressure

A marine diesel needs massive volumes of air to burn fuel efficiently. If the air intake or exhaust is restricted, the engine will run rich, wasting unburnt fuel and typically throwing black smoke from the transom.

• Air intake: Check the air filter. Many older marine engines have simple wire mesh screens, but newer units use foam or paper elements. If a foam element is saturated in oil vapour from the crankcase breather, it will choke the engine.
• Exhaust mixing elbow: This is a classic failure point on engines like the Yanmar YM/GM series and Volvo Penta D-series. Hot exhaust gas meets cold raw water here, leading to a build-up of hard carbon and salt crust. A 50mm exhaust bore can easily choke down to 15mm over five years. Unbolt the elbow and inspect it; replacement cast iron elbows cost around £150 to £250, whilst aftermarket stainless steel versions run closer to £350 but last much longer.

Injector Wear and Fuel Leaks

If the hull is clean and the engine breathes perfectly, your fuel delivery system is the next suspect. Fuel injectors are precision components designed to pop open at a specific pressure (typically between 130 and 150 bar on small mechanical diesels) to atomise the diesel into a fine mist.

After 1,500 to 2,000 hours, the injector nozzles wear out and the internal springs weaken. Instead of a fine mist, the injector dribbles raw fuel into the cylinder. This burns poorly, drastically increasing consumption. You will usually see a sheen of diesel on the water behind the exhaust or heavy black smoke under load. Pull the injectors and take them to a local diesel specialist; having them rebuilt and pop-tested usually costs around £50 to £75 per injector.