Running out of fuel is one of the most common reasons boaters call for help. Planning fuel is simple once you know your boat’s burn rate at cruising speed. This calculator works out the fuel a trip burns, how much to carry with a reserve, the range of a full tank and the cost.
The formulas
- Running time = distance ÷ speed
- Trip fuel = running time × fuel burn per hour
- Fuel to carry = trip fuel × reserve factor
- Safe range = tank capacity ÷ reserve factor × speed ÷ burn rate
The rule of thirds is the classic reserve rule: use one third of the fuel going out, one third coming back, and keep one third in reserve for wind, current, waves or a detour. It means carrying 1.5 times the fuel the trip needs.
Example
A boat cruising at 25 knots burning 10 gallons per hour runs 40 nautical miles in 1 h 36 min and burns 16 gallons. With the rule of thirds, carry 24 gallons. A 60 gallon tank gives a safe range of 100 nautical miles. At $5 a gallon, the trip costs about $80 in fuel.
Typical fuel burn
As a rough guide, a gasoline outboard burns about 0.1 gallon per hour per horsepower at wide-open throttle, and about half that at an efficient cruise. A 150 hp outboard might burn 15 gph flat out and 6 to 8 gph at cruise. Diesels use roughly 30 to 40% less. Your engine’s maker or a fuel flow meter gives the most accurate figures.
Getting a reliable burn rate
- Fill the tank to the top before a trip.
- Note your engine hours or log the time and average speed.
- Refill to the top and divide the fuel added by the hours run.
Burn rate rises sharply above the boat’s most efficient speed, and with weight, a fouled hull, rough water and headwinds.
Before you leave
- Tell someone your route and return time.
- Check the weather and the tides or currents.
- Know where you can refuel along the way.
- Never run the tank close to empty: sediment at the bottom can clog filters, and a sloshing tank can starve the engine.
Fuel gauges on boats are often inaccurate, so trust your own calculations and logs.