Calculating Fuel Consumption | Boating Magazine

# Calculating Fuel Consumption

You can probably figure out your car's gas mileage in your head. It's a different story calculating a boat's fuel consumption.

Calculating Fuel Consumption

John Linn

It's a different story with a boat. Since sea conditions vary more widely than road conditions, the time it takes to cover a distance varies more, so fuel consumption is measured in gallons per hour. Also, while many engines have fuel flow readouts, the ability to estimate fuel burn while shopping for a boat or engine is important. You measure fuel efficiency in pounds of fuel used per horsepower developed per hour. The pros call it "brake-specific fuel consumption." This makes it important to know that gasoline weighs about 6.1 pounds per gallon and diesel fuel 7.2 pounds per gallon.

On average, an in-tune four-stroke gasoline engine will burn about 0.50 pounds of fuel per hour for each unit of horsepower. Likewise, a well-maintained diesel engine burns about 0.4 pounds of fuel per hour for each unit of horsepower it produces. These figures don’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.

Confused yet? Look at the mathematical examples below, and it should become clear.

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: 6.1 lb. per gal. | 7.2 lb per gal.

300-hp Diesel Engine Example
GPH = (0.4 x 300)/ 7.2 = 120/7.2 = 16.6 GPH

300-hp Gasoline Engine Example
GPH = (0.50 x 300)/ 6.1 = 150/6.1 = 24.5 GPH

Keep in mind that these formulas apply when the engine is making peak horsepower, which usually is near wide-open throttle. Fuel consumption will be decreased at cruising speeds. Also remember that engines with electronically-managed fuel injection and direct 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.

Another way is to take the total engine horsepower and divide it by 10 for gas engines or .06 for diesel engines. As you can see, this formula is simpler to calculate and easier to remember. You don't even need a pencil and paper. It's just not as accurate as the formulas above. The result represents the approximate gallons per hour the engine will burn at wide-open throttle. For example, a 150-horse engine will use about 15 gallons per hour. Though these figures represent averages and can vary from 10 to 20 percent, they'll put you in the ballpark so you can plan a long-distance cruise without fear of running out of gas.

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

### Expert Tips for Saving Fuel

Choose the Right Prop
The primary criterion for selecting a propeller is to make sure it allows the engine to "turn up" to a speed within the band designated by the manufacturer - usually within 500 revolutions of absolute top rpm. Within that band, you can select a variety of props that will allow the engine to turn up, yet have differences in pitch and number of blades, plus possess more subtle characteristics such as rake, skew and cup.

John Linn

Optimum Trim
Properly trimming out reduces the wetted surface of the hull by raising the bow.

John Linn

T-Tops, Hardtops, Towers
Opening or closing windshields, and raising or lowering canvas enclosures can help improve fuel efficiency.

John Linn

Back Off, Burn Less
Unless you're in a tournament, are racing to make a bridge opening or have that momentary need for speed that afflicts us all, slow down to save fuel without costing any real time.

John Linn

Pounds = A Real Drag
One of the quickest ways to get more miles per gallon is to get the lead out!

John Linn

Later Craters
Efficiency mavens select ablative paints, such as Interlux Micron or Petit Hydrocoat. These wear away, leaving a smooth surface. If your boat has sat idle for a while, it pays to hire a diver to scrub the bottom or to don a mask and fins and do it yourself.

John Linn

Electronics can also help improve your boat's efficiency and reduce the number of stops you make at the fuel dock. FloScan's CruiseMaster 5500 fuel meter is an effective way to record fuel consumed and find your most efficient speed.

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Connect Garmin's GMI 10 to its GFS 10 fuel sensor and you’ll have an economy and gallons-per-hour fuel monitor.

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Connect the Lowrance LMF-400 or LMF-200 to Lowrance’s EP-60R gas engine fuel sensor, and you have fuel flow, consumption, trip fuel used and seasonal fuel information at the touch of a fingertip.

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Maretron's FFM100 module, combined with an appropriate fuel sensor, delivers detailed fuel-management information to any NMEA 2000 standard display.

Boating Magazine

Mercury Marine offers the SmartCraft VesselView integrated control center. It displays approximately 50 functions, including fuel level, fuel used, fuel range, low fuel and fuel to a waypoint.

Boating Magazine

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