Estimate marine fuel consumption, calculate gallons per hour (GPH), and compute fuel costs for fishing trips, cruising, and offshore boating.
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Unlike road vehicles that measure fuel economy in distance per gallon (MPG), marine vessels operate under constant water drag load. Therefore, boat fuel consumption is primarily measured in Gallons Per Hour (GPH) or Liters Per Hour (LPH) based on engine RPM and throttle position.
Cruising GPH Rule of Thumb = Engine Horsepower (HP) ÷ 13.5
| Engine Horsepower | Idle GPH (1000 RPM) | Cruise GPH (4000 RPM) | WOT GPH (6000 RPM) |
|---|---|---|---|
| 90 HP Outboard | 0.4 GPH | 6.2 GPH | 9.0 GPH |
| 150 HP Outboard | 0.6 GPH | 10.5 GPH | 15.0 GPH |
| 250 HP Outboard | 0.8 GPH | 17.2 GPH | 25.0 GPH |
| Twin 300 HP Outboards (600 HP) | 1.8 GPH | 41.0 GPH | 60.0 GPH |
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A standard industry rule of thumb for 4-stroke gasoline outboard engines at wide-open throttle (WOT) is dividing maximum horsepower by 10 (GPH = HP ÷ 10). At typical cruising speeds (75% throttle), divide maximum HP by 13 to 15.
At wide-open throttle (WOT), a 200 HP gas engine burns approx 20 GPH. At cruising speed (~4,000 RPM), it burns approx 13 to 15 GPH.
Yes. Marine diesel engines burn 25% to 35% less fuel per hour than gasoline engines of equivalent horsepower because diesel fuel has higher energy density per gallon.
Displacement hulls push water aside and require exponential power to exceed hull speed ($1.34 × \sqrt{\text{LWL}}$). Planing hulls rise onto the water surface above 15–20 knots, drastically reducing hydrodynamic drag and improving fuel economy.
Marina gas includes marine fuel additives (valvtect/ethanol treatment), storage tank maintenance, water safety compliance, and low-volume retail markups, making marine gas $0.50 to $1.50/gal more expensive.