Frequently Asked Questions
How is marine engine fuel burn estimated?
GPH ≈ HP × specific fuel consumption (SFC) ÷ fuel density. Gasoline at SFC 0.50 lb/hp·h, 6.1 lb/gal: GPH ≈ HP × 0.50 ÷ 6.1 ≈ HP × 0.082. A 250 HP outboard at WOT ≈ 20.5 GPH; cruise (60% load) ≈ 12 GPH.
What about diesel?
Diesel SFC ≈ 0.40 lb/hp·h, 7.1 lb/gal density: GPH ≈ HP × 0.057. A 200 HP diesel at cruise (70% load) ≈ 200 × 0.7 × 0.057 ≈ 8 GPH - roughly 35% better range than gasoline of the same HP.
How much reserve should I carry?
The "1/3 rule": ⅓ out, ⅓ back, ⅓ in reserve. A 100-gal tank gives 33 gal usable each leg. Offshore voyages should plan 30–50% reserves and use a separate auxiliary tank for true range.
How does sea state affect burn?
Head seas can double GPH at the same throttle by killing efficiency. Always plan with realistic conditions, not flat-water spec sheets, and verify with a flow meter or tank dipstick.
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This calculator is for educational and informational purposes only and is not a substitute for official flight or navigation planning. Always use current performance charts, an approved POH/AFM, certified navigation tools, and follow all applicable FAA, ICAO, USCG, and other regulatory guidance. Verify all results independently before operating any aircraft or vessel.