The most reliable figure is measured consumption on your boat at a defined rpm, load, displacement and sea state. Manufacturer fuel curves are the next-best starting point. A rated-power figure alone is not.
Core calculations
Endurance hours = usable fuel litres ÷ litres per hour
Range nautical miles = endurance hours × speed in knots
Trip fuel = distance ÷ speed × litres per hour
Then add a reserve suitable for weather, diversion, tank geometry, fuel unusable below the pickup and the consequences of running short. Do not count the entire nameplate tank volume as usable.
Worked example
A yacht carries 150 L, but planning allows 80 per cent as usable fuel: 120 L. Measured consumption at the chosen cruise is 2.6 L/h and speed is 6.2 knots.
- Endurance:
120 ÷ 2.6 = 46.2 hours - Still-water range:
46.2 × 6.2 = 286 nautical miles
That is a planning baseline, not a promise. Head seas, current, fouling, alternator load and manoeuvring reduce it.
Measuring actual consumption
Fill consistently, record engine hours and separate generator use if both draw from the same tank. Better still, use calibrated fuel-flow data that accounts for return fuel. Build a table of rpm, speed and L/h in clean-hull conditions. The most economical rpm per hour may not be the most economical per nautical mile.
For twin engines, calculate one-engine and two-engine scenarios only after confirming the boat can safely operate and manoeuvre that way and that the single-engine propeller is not windmilling contrary to its instructions.