Eight knots at four units per hour
Observed economy is 2.0 nautical miles per unit.
Boat Fuel Economy & Best Cruise Calculator
Created by: Daniel Hayes
Last updated:
Compare entered sea-trial RPM, speed, and fuel-burn rows to find the best observed nautical-mile economy and cost per nautical mile.
Boat Fuel Economy & Best Cruise Calculator
BoatingCompare entered sea-trial observations without extrapolating beyond the RPM, speed, and burn rows you measured.
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What is a Boat Fuel Economy & Best Cruise Calculator?
This calculator compares repeatable entered sea-trial rows using nautical miles per fuel unit, fuel units per nautical mile, and optional cost per nautical mile.
“Best” means only the highest observed economy among the rows entered. It does not establish safe, approved, or mechanically suitable cruise RPM.
Comparisons are useful only when speed and burn were measured together under comparable load, trim, water, weather, and instrument conditions.
How the Boat Fuel Economy & Best Cruise Calculator Works
For every row, speed in knots is divided by fuel units per hour. The reciprocal gives units per nautical mile.
The greatest entered distance-per-unit row is highlighted and compared with the selected row. Fuel price is used only for cost per nautical mile.
No curve fitting or extrapolation is performed outside the observations.
Formulas and assumptions
nm per unit = knots ÷ units per hour
units per nm = 1 ÷ nm per unit
cost per nm = price ÷ nm per unit
Example Calculations
Twelve knots at five units per hour
Observed economy is 2.4 nautical miles per unit, twenty percent better than the first row under the model.
Common Applications
- Sea-trial comparison
- Cruise-setting review
- Cost-per-mile measurement
- Before/after maintenance checks
Fuel-Planning Tips
Use reciprocal runs to reduce environmental bias.
Retest with normal cruising load.
Follow engine and vessel operating guidance.
Frequently Asked Questions
Where should the burn-rate input come from?
Use repeatable measurements from the actual vessel or documented manufacturer data for the relevant engine, RPM, load, and conditions. Tank gauges and one short outing can be misleading. Keep the source, units, sea state, load, trim, fouling, and test method with the value.
Is calculated range or fuel use guaranteed?
No. Wind, waves, current, loading, fouling, trim, manoeuvring, idling, fuel quality, transfer problems, leaks, generator use, and mechanical condition can materially change consumption and usable fuel.
What does best cruise mean here?
It means the entered row with the highest observed nautical miles per fuel unit. It is not a recommendation to operate at that RPM and does not assess engine loading, manufacturer guidance, handling, planing, vibration, weather, noise, emissions, or safety.
Can I extrapolate between the entered RPM rows?
No. The tool compares only entered observations. Hull resistance and engine loading are nonlinear, so untested RPM and load combinations should not be inferred from a straight line.
Why can a faster row use less fuel per mile?
Fuel per hour can rise while speed rises faster, improving distance per unit. The opposite can also occur. Repeatable trials across suitable conditions are needed before treating a difference as meaningful.
How should sea trials be compared?
Use the same load, trim, water, wind, current correction, warm-up state, direction averaging, instruments, and measurement method. Record conditions and repeat measurements rather than selecting one favourable run.
Sources and References
- Royal Yachting Association. Passage Planning, accessed July 16, 2026; https://www.rya.org.uk/water-safety/passage-planning-and-navigation/passage-planning/.
- U.S. Coast Guard Auxiliary. Fueling and recreational boating safety resources, accessed July 16, 2026; https://wow.uscgaux.info/content.php?unit=B-DEPT.
- National Weather Service. Safe Boating and marine forecast resources, accessed July 16, 2026; https://www.weather.gov/safety/safeboating-marine.
- Mercury Marine. Propeller slip explanation and theoretical-speed convention, accessed July 16, 2026; https://www.mercurymarine.com/us/en/lifestyle/dockline/how-to-calculate-propeller-slip.
- Unit conversions and arithmetic assumptions are documented in each calculator method.
Planning limitation
This observational comparison does not recommend RPM, diagnose propulsion, predict untested performance, or override engine, gearbox, propeller, or vessel guidance.