How to Calculate Vessel Fuel Consumption

Three common ways to calculate vessel fuel consumption

Vessel fuel consumption is usually worked out one of three ways: from engine running hours and a known consumption rate, from engine RPM using a load curve, or directly from a flowmeter on the fuel line. Each is a trade-off between how easy it is to set up and how close it gets to what the engine actually burned. Running-hours math is the simplest and the least accurate, since it assumes every hour at a given load burns the same fuel. RPM-based estimation is closer, because it accounts for how hard the engine is working, but it is still a curve, not a measurement. A flowmeter reads the fuel itself, as it moves through the line — it is the only one of the three that measures rather than infers.

Why main and auxiliary engines are usually calculated differently

Main engines drive the propeller and are the largest single fuel cost on most voyages, which is why they are the ones most often fitted with flowmeters where accuracy matters. Auxiliary engines, running generators for hotel and deck loads, are more often estimated from running hours, since the cost of instrumenting every genset rarely matches the value of the extra precision. On MANTRAZ, main engine fuel is measured by flowmeters on the supply and return lines, while auxiliary engine fuel is usually estimated from running hours — and reports keep the two figures apart instead of adding them into one number that hides which part is measured.

Turning RPM into litres per operating mode

A vessel's fuel consumption is not one number, it changes with how the engine is being run. MANTRAZ lets an operator name RPM bands per engine — idle, maneuver, cruise, full — and classifies every minute of engine running time into one of them, then shows how many litres each mode burned and what share of running time it represents. That answers a more useful question than "how much fuel did we use today": it shows whether the fuel went into unavoidable running time or into time spent at full RPM that coaching the crew could reduce.

Consumption per voyage, not just per day

A single trip's fuel consumption is usually expressed as litres per nautical mile (L/NM) or fuel per running hour, so it can be compared across voyages of different lengths. MANTRAZ's voyage reports record distance, average speed, L/NM, fuel per running hour, time in each operation mode and the wave conditions met along the way, and two voyages on the same route can be placed side by side to see the delta in percent. That is how a fleet finds out that holding a steady speed on one voyage used noticeably less fuel than pushing and slowing down on another, on the same route, in a similar time.

Keeping a running fuel balance

Consumption numbers answer "how much did we burn," but a fleet also needs to know how much fuel is left on board before planning the next voyage or ordering the next bunker. MANTRAZ keeps a running balance per vessel — Remain On Board (ROB) — calculated as the start balance plus every bunker, minus consumption, with adjustments so the balance matches the tanks: ROB = start balance + bunker − consumption ± adjustment, logged daily, weekly or monthly.

Breaking it down further

For a closer look, MANTRAZ's datalog records fuel flow, RPM and speed per minute, hour, day or week, downloadable as Excel, and a custom chart can overlay up to 8 series across vessels or periods to compare fuel flow, RPM or speed hour by hour.

Getting started

Request a demo to see fuel consumption calculated on data similar to your own fleet's.