Calculating Fuel Burn
If you cruise on a powerboat, you pay attention to fuel consumption. Despite the importance of this topic, confusion and false assumptions abound. Many assume that an engine of moderate power runs more economically than a high-output version, but is that always true? Should you prefer a singleengine trawler over a twin-screw, semidisplacement boat because of fuel economy? If you have a twin-engine boat, can you run on one engine to reduce fuel burn?
Let’s begin by agreeing on the standard. While many boaters focus solely on gallons per hour (GPH), this is a meaningless number on its own. For example, here’s a question: Which is more efficient, Boat A burning 11 GPH or Boat B burning 22 GPH? Without calculating miles per gallon (MPG) it is impossible to say. Gallons per hour can be helpful when calculating range and determining whether you have enough fuel to get to your destination. If you are consuming 20 GPH and you will be running for five more hours, then you know you will burn 100 gallons of fuel before arriving. But that does not speak to fuel economy. When we compare cars, we all agree that MPG is our standard. So we will adopt the same standard for this article.
Let’s return to the question of which boat is more efficient, Boat A burning 11 GPH, or Boat B burning 22 GPH. Let’s include the speed and look at the math.: Boat A: 10 knots (nautical miles per hour) / 11 GPH = 0.9 miles per gallon
Boat B: 22 knots (nautical miles per hour) / 22 GPH = 1.0 miles per gallon
In this particular example, we see that although the difference is minor, the boat burning more gallons per hour achieves better mileage. For the purposes of this discussion, we will concentrate on nautical miles per gallon, which we will abbreviate to nMPG.
FUEL BURN BASICS
A given hull will require a certain amount of energy to move it through the water. The fuel contains the stored energy and we can describe that in terms of horsepower ( hp), kilowatts ( kW), or British thermal units ( BTU). One gallon of diesel fuel stores about