Fuel Flow vs Speed Calculator

How It Works

This calculator models how fuel consumption changes across different boat speeds and determines the most fuel-efficient cruising speed for your vessel. By understanding the relationship between speed and fuel burn, you can optimize your boating budget and trip planning by finding the sweet spot that doesn't waste fuel on unnecessary speed while still getting you to your destination in reasonable time.

The Formula

Variables

• Max Speed — The maximum speed your boat can achieve in knots (nautical miles per hour). This is typically found in your boat's specifications or owner's manual.
• GPH at Max Speed — Gallons per hour fuel consumption when operating at maximum speed. You can find this in your manual, or determine it empirically by running full throttle and measuring fuel consumption over time.
• Hull Type — The boat's hull design category: 1 = displacement (traditional monohulls), 2 = semi-displacement (moderate deadrise), 3 = planing (fast boats that climb onto plane). This affects how dramatically fuel consumption increases with speed.
• Speed — Any cruising speed between zero and your boat's maximum speed, measured in knots. The calculator computes fuel burn at various speeds to find the optimal efficiency point.
• Most Efficient Speed — The output: the speed at which your boat burns the least fuel per nautical mile traveled. Operating at this speed minimizes your fuel cost per mile but maximizes time underway.
• Fuel Burn Rate — Gallons per hour at any given speed. Unlike fuel consumption per mile, this shows absolute hourly burn and increases dramatically at higher speeds.

Worked Example

Let's say you own a 35-foot displacement cruiser with a maximum speed of 12 knots and a fuel burn of 8 gallons per hour at wide-open throttle. Using the calculator with hull type 1 (displacement), you input max speed = 12 knots and GPH at max speed = 8. The calculator applies the displacement hull formula (approximately GPH = 8 × (Speed/12)³·⁵) and tests fuel consumption across the speed range. At 6 knots (50% of max speed), the model estimates roughly 1.2 GPH, meaning you're traveling at 6 knots on 1.2 gallons—0.2 gallons per nautical mile. At 9 knots (75% of max), fuel burn jumps to about 4 GPH, or 0.44 gallons per mile. The calculator identifies that 6-7 knots delivers the best fuel economy, typically around 0.15-0.18 gallons per nautical mile, making it your most efficient cruising speed for this vessel.

Practical Tips

• Displacement hulls achieve best efficiency at 40-50% of maximum speed; semi-displacement at 50-65%; planing hulls at 60-75%. Operating significantly below or above these ranges wastes fuel per nautical mile.
• The relationship between speed and fuel burn is not linear—it's exponential. Increasing speed from 5 to 6 knots might use 20% more fuel, but increasing from 9 to 10 knots might use 40% more. Small speed increases at higher speeds cost disproportionate fuel.
• Real-world fuel burn varies with sea conditions, load, hull fouling, and engine tuning. Use the calculator as a baseline, then monitor your actual fuel consumption over several trips to calibrate expectations and adjust your optimal speed estimate.
• Factor in travel time value: if your most efficient speed saves $50 in fuel but adds 2 hours to your trip, determine if that's worthwhile. The calculator helps you make this economic decision by showing you both fuel consumption and implied travel times.
• Update your GPH at max speed figure annually or after engine maintenance. A fouled bottom, worn fuel injectors, or impeller degradation can increase fuel consumption by 15-30%, shifting your optimal speed downward and costing more per mile at any given speed.

Last updated: March 10, 2026 · Reviewed by the BoatCalcs Editorial Team