Why is my inboard diesel using more fuel than usual?
My yacht's inboard diesel seems to be using noticeably more fuel at our normal cruising speed. We're filling up more often, although the engine still starts normally. What should I check before booking a mechanic?
Answer
Your inboard diesel usually uses more fuel because hull or propeller drag is making it work harder, you are running it faster, fuel is leaking, or combustion is poor. First measure consumption over recorded engine hours and check for leaks, then compare boat speed, engine revs and exhaust smoke with your usual figures.
Measure fuel consumption
A dashboard fuel gauge is too imprecise to diagnose increased consumption, especially with an irregularly shaped tank. Measure litres added over several engine hours, filling to a repeatable level with the boat at roughly the same trim. Avoid filling into the expansion space.
- Litres per hour: divide fuel added by engine hours since the previous fill.
- Litres per nautical mile: divide fuel used by distance travelled, allowing for tide when comparing trips.
- Other consumers: account for a diesel heater or generator sharing the tank.
For example, 24 litres over eight hours is 3 litres per hour. That figure alone does not establish a fault. Displacement, engine size, cruising revs and conditions determine whether it is reasonable.
Check the tank fittings, primary filter bowl, lift pump, injector leak-off pipes and return hose for wetness or a diesel smell. Stop the engine before checking fuel connections. Never use your fingers to find a leak in high-pressure injection pipes. Investigate a rising engine-oil level or oil smelling strongly of diesel before running the engine again.
Check hull and propeller load
Suspect fouling when consumption rises gradually through the season. Weed and barnacles on the hull increase resistance; even modest growth on propeller blades can noticeably reduce thrust. Rope around the shaft or a folding propeller that fails to open properly can cause similar symptoms.
Compare performance at your usual engine revs in calm water. If boat speed has fallen, inspect the underwater surfaces before paying for injector work. Use speed through the water where available, or compare reciprocal GPS runs to reduce the effect of tide.
Other common changes include:
- More throttle to maintain speed into wind, waves or current.
- Extra displacement from cruising stores, full tanks or water collecting in the bilge.
- Heavy battery charging after starting, particularly with a large alternator.
- A different propeller pitch, diameter or gearbox ratio following repairs.
On a displacement yacht, pushing beyond its comfortable cruising speed can burn substantially more fuel for little extra speed. On a planing motorboat, struggling below an efficient planing speed can also increase fuel used per mile.
Read smoke and engine revs
Black smoke under load suggests more fuel is being injected than the available air can burn cleanly. Check propeller loading and the intake silencer or air-cleaner element first, following the engine manual's cleaning or replacement instructions. Restricted engine-room ventilation, exhaust restriction and poor injector spray are further possibilities. On turbocharged engines, check the boost hoses and turbo system too.