BoatCost estimates fuel burn from engine math, not from a brochure.
The SFC formula
gph at WOT = SFC × horsepower × engines / fuel specific weight
- Gasoline: SFC 0.50 lb/hp-hr, 6.2 lb/gal
- Diesel: SFC 0.40 lb/hp-hr, 6.9 lb/gal
A 300 hp gasoline outboard is about 24.19 gph at wide-open throttle. The same horsepower in diesel is about 17.39 gph.
Cruise burn is WOT times a load factor (default 0.65). That is a planning average, not your GPS speed.
Gasoline also shows the old rule of thumb: horsepower / 10 at WOT. Use it as a sanity check, not as the budget.
Trip cost
Trip fuel = cruise gph × hours × dollars per gallon. Annual fuel uses hours per year the same way.
This is an SFC estimate, not a sea trial. Props, hull fouling, load, and a heavy right foot will move the number. Confirm with a fuel-flow meter when it matters.
Open the fuel calculator and tap a Yamaha, Mercury, or Honda preset to start.