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Why fuel range is a safety number
Running out of fuel at sea is embarrassing in a harbour and dangerous offshore. Unlike a car, a boat cannot pull over and walk to a station, and a tow in rough weather is slow, expensive, and sometimes impossible. Fuel range — how far you can go on the fuel you carry — is therefore a safety number first and a planning number second. This calculator turns your tank size, burn rate, and speed into hours of endurance and miles of range, with a reserve you choose.
The arithmetic is simple, but the inputs deserve honesty. Tank capacity is not usable fuel: the last few percent sits below the pickup or sloshes away from it in a seaway. Burn rate rises with speed, head seas, and a fouled hull. Speed falls in bad weather. Every one of those effects shortens your range, which is why the reserve exists and why you should plan to use only part of the calculated range.
How the calculator works
Enter your fuel capacity in gallons, the reserve percentage you want to keep untouched, your fuel burn in gallons per hour, and your cruising speed in knots. The tool subtracts the reserve to get usable fuel, divides by burn rate to get endurance in hours, and multiplies by speed to get range in nautical miles and kilometres.
The reserve is the key input. Ten percent is a minimum for calm coastal hops. Twenty percent is sensible for passages, and many offshore sailors keep a third in reserve. The reserve covers the things you cannot predict: a headwind that doubles your passage time, a fouled propeller, a detour around weather, or a harbour entrance that forces you to wait for the tide.
A worked example
Take a boat with 100 gallons of fuel, a 10 percent reserve, a burn rate of 5 gallons per hour, and a cruising speed of 6 knots. Usable fuel is 90 gallons. Endurance is 90 divided by 5, or 18 hours. Range is 18 times 6, or 108 nautical miles — about 200 kilometres.
Now change one input. Raise the reserve to 20 percent and usable fuel falls to 80 gallons, endurance to 16 hours, and range to 96 miles. Or keep the reserve at 10 percent but raise the speed to 7 knots: if burn rises to 7 gallons per hour at that speed, endurance falls to about 12.9 hours and range to about 90 miles. Going faster can actually reduce your range, because burn usually rises faster than speed. That trade-off is the single most useful thing this calculator shows.
Plan with two-thirds. If the calculator says 108 miles, plan passages of 70 miles or less on that fuel. The unused third is your margin for weather, diversions, and the day your burn rate turns out worse than your logbook said.
Where burn rate comes from
The best burn rate is your own measured one. Fill the tank, run a known distance at a steady speed in ordinary conditions, refill, and divide fuel used by hours run. Do it at two or three speeds, because burn is not linear: doubling speed can triple consumption. Engine manuals give figures too, but they usually describe a new engine at a steady load in a test tank, which flatters the real number.
Keep a fuel log. Note the date, the hours run, the distance, the conditions, and the fuel added. After a season, your log gives burn rates you can trust for your boat, your propeller, and your cruising style. No calculator can replace that, but a calculator turns your log numbers into passage plans in seconds.
Common mistakes
The biggest mistake is using total tank capacity as usable fuel. Always subtract a reserve, and remember that the tank’s rated capacity may itself be optimistic. The second mistake is using a calm-water burn rate for a rough-weather passage. Head seas can raise consumption by a third or more. The third mistake is forgetting the generator, the heater, or the dinghy outboard, all of which drink from the same fuel or from jerry cans you meant to count.
Another common error is mixing units: gallons come in US and imperial sizes, and a US gallon is about 17 percent smaller than an imperial gallon. Make sure your tank figure and your burn figure use the same gallon, or your range will be wrong by that margin in one direction or the other.
Frequently asked questions
How much reserve should I keep? Ten percent for short coastal hops in settled weather, twenty percent for passages, and a third for offshore or unfamiliar waters. More reserve is never wrong; it only shortens your planned range.
Does going slower always extend range? Almost always, because burn falls faster than speed. There is a sweet spot — usually just below hull speed — where miles per gallon peaks. Below that, hotel loads like instruments and refrigeration eat into the gain.
What if I have two tanks? Add their usable capacities together, but remember that fuel in a tank you cannot draw from — a damaged or isolated tank — does not count.
Should I trust the fuel gauge? No. Boat fuel gauges are famously unreliable, especially at the low end. Trust hours run and a dipstick or sight gauge instead.