# Calculating Fuel Consumption

Calculating fuel consumption on a boat is much different than your own car. Weather and sea conditions vary more than road conditions, the time it takes to cover a distance varies more, so fuel consumption is measured in gallons per hour rather than miles per gallon. Many of today’s marine engines have fuel flow readouts. This gives the ability to estimate fuel usage based on the throttle position. Fuel efficiency is measured in gallons of fuel used per horsepower by per hour. This makes it important to know that ethanol free gasoline weighs about 6.2 pounds per gallon and diesel fuel about 6.9 pounds per gallon.

On average, a four-stroke gasoline engine will burn about 0.50 pounds of fuel per hour per horsepower. Likewise, a well-maintained diesel engine burns about 0.40 pounds of fuel per hour per horsepower. These figures donâ€™t take drag of the boat, sea conditions, or efficiency losses through transmissions and bearings into account. But they provide an excellent relative difference between engines when shopping.

Let’s look at the mathematical calculation below for an explanation. They are a rough estimate of what your fuel consumption will be.

Formula To Estimate Maximum Engine Fuel Consumption
GPH = (specific fuel consumption x HP)/Fuel Specific Weight

Constants | Gas | Diesel
SFC: .50 lb. per HP | .40 lb. per HP
FSW: 6.2 lb. per gal. | 6.9 lb per gal.

300-hp Diesel Engine Example
GPH = (0.4 x 300)/ 6.9 = 120/6.9 = 17.4 GPH

300-hp Gasoline Engine Example
GPH = (0.50 x 300)/ 6.2 = 150/6.2 = 21.8 GPH

Keep in mind that these formulas apply when the engine is making peak rated horsepower, which usually is at wide-open throttle (WOT). Fuel consumption will be decreased at cruising speeds. Also, remember that engines with direct and electronic fuel injection will yield higher fuel efficiency.

To apply these formulas to your boat, just plug in its horsepower rating and multiply it by the specific fuel consumption average, then divide the product by the fuel specific weight. These are rough estimates of your fuel consumption and you should always refer to your engine’s manufacturer for more accurate fuel consumption.

Keep track of your boat’s fuel consumption by installing a fuel monitor.