Enter the engines and the fuel on board, and the calculator estimates the burn at full power, how long the fuel lasts and how far it goes. If you know the real burn at cruise, from a fuel flow meter or the engine display, enter it and the range uses that instead.
The burn estimate
Boating Magazine works it out from brake-specific fuel consumption, the pounds of fuel an engine burns for each horsepower each hour:
GPH = specific fuel consumption × horsepower ÷ fuel weight
| Fuel | Pounds per hp per hour | Pounds per gallon |
|---|---|---|
| Gasoline, four-stroke | 0.50 | 6.1 |
| Diesel | 0.40 | 7.2 |
So a 300 hp diesel burns about 16.6 gallons an hour flat out, and a 300 hp gasoline engine about 24.5. Litres use NIST's 3.785 412 litres to the US gallon.
Boating Magazine is clear about the limits. The formula applies near peak power, consumption drops at cruise, and it leaves out hull drag, sea state and losses in the drive, so real figures vary by 10 to 20%. A measured burn always beats the estimate.
The rule of thirds
One third of the fuel to go, one third to come back, one third in reserve, as Boating Magazine puts it. The calculator gives the turning point: a third of the full range.
On the survey
The estimate gives you a sanity check on the sea trial. A diesel burning far more than the formula at full power, or failing to reach rated rpm, is worth a closer look: the fuel supply and filters, and the propeller. Note the fuel on board and how you measured it.
Citing it in a report
"Twin 300 hp diesels. Fuel on board 300 US gal by the owner's log. Burn at 18 knots cruise 22 US gal/h total from the engine displays, giving about 13.6 hours and 245 nm. Full power burn not measured." Say where each figure came from.