Why is my marine diesel engine using more fuel than usual?
My yacht's diesel seems to be using noticeably more fuel at our usual cruising revs, and we're slightly slower than last season. How do I work out whether it's hull fouling, the propeller or an engine problem before paying for an engineer?
Answer
Your marine diesel usually uses more fuel because the boat needs more power to move, you are running it harder, or combustion has deteriorated. Confirm consumption with a measured refill and check for fuel leaks, then compare boat speed, engine revs and exhaust smoke with your normal figures.
Check measured fuel consumption
Tank gauges measure consumption poorly, especially with irregularly shaped tanks or when the boat's trim changes. Refill to a repeatable level and record engine hours. At the next refill, divide litres added by hours run. Avoid filling fuel into the vent.
For example, 24 litres over eight engine hours is 3 litres per hour. At 6 knots, that is 0.5 litres per nautical mile; at 5 knots, the same hourly burn becomes 0.6 litres per nautical mile. A foul tide can make a passage less economical without changing the engine's hourly consumption.
Allow for diesel heating or a generator sharing the tank, and compare runs at similar revs, loading and sea conditions. On motorboats, extra passengers, tender weight or poor trim can prevent clean planing and sharply increase consumption.
Inspect the bilge, fuel filter bowls, hoses, injector return pipes and connections for diesel staining or wetness. Stop the engine and eliminate ignition sources before investigating a fuel leak.
Inspect hull and propeller drag
If the boat is slower at its usual revs, inspect the underwater gear before blaming the injection pump. Weed, slime and barnacles increase resistance. Growth on propeller blades can have a marked effect.
- Check the hull, keel, rudder, shaft and propeller for fouling or a trailing rope.
- Check that a folding or feathering propeller opens correctly and that its blades move as the manufacturer specifies.
- Consider recent changes: a replacement propeller, altered pitch, additional equipment or heavily loaded tanks.
- On a twin-engine motorboat, compare both engines' revs, temperatures and fuel readings under the same conditions.
A propeller with excessive pitch overloads the engine, often causing black smoke and preventing it from reaching its specified maximum revs under load. That figure depends on the exact engine and installation. Use the handbook's loaded-speed specification, not the neutral maximum. Do not carry out a full-power test while the engine is overheating or running roughly.
Read smoke and performance
Black smoke points towards too much fuel for the available air or load. Typical causes include propeller overload, a dirty intake silencer or air filter, poor injector spray and incorrect valve clearances. On turbocharged engines, also check for leaking boost hoses and insufficient boost.