How to calculate boat range
Divide the usable fuel by the burn rate to get hours of running time, then multiply by cruising speed. Always subtract a reserve: fuel gauges are imprecise and sea state, load and fouling increase burn.
Example
100 gal tank, 10% reserve, 8 GPH at 22 knots: 90 ÷ 8 = 11.25 hours × 22 = 248 nautical miles, at 2.75 nm per gallon. By the rule of thirds, don't plan to go more than 92 nm from your fuel source.
Estimating fuel burn
If you don't have a fuel-flow meter, a rough rule for gasoline engines at cruise is about 0.1 GPH per horsepower being used. Diesels burn roughly 0.05–0.055 GPH per horsepower. Real numbers vary with hull, load and trim, so log fill-ups against engine hours to get your own figure.
| Boat | Typical cruise burn |
|---|---|
| 18–20 ft bowrider, 150 hp outboard | 5–7 GPH |
| 22–24 ft center console, 250–300 hp | 10–15 GPH |
| 30 ft twin outboard | 20–30 GPH |
| 40 ft trawler, single diesel | 3–6 GPH |
Frequently asked questions
What is the rule of thirds for boat fuel?
Use one third of your fuel going out, one third coming back, and keep one third in reserve.
How do I convert knots to mph?
Multiply knots by 1.151. 22 knots is about 25.3 mph.
Why does fuel economy drop at high speed?
Drag rises much faster than speed. Most planing boats are most efficient just above planing speed, at about 60–75% throttle.