Few boat owners know their fuel consumption to any degree of
accuracy because it varies so much with boat speed, headwinds,
contrary currents, and the boat’s load. Most owners tend to
exaggerate their mileage figures, probably because the truth is
so depressing. Nevertheless, it’s important for any serious
boater to know at least roughly how far the boat will go on a
tankful of fuel, and a couple of simple formulas will help
establish that figure. Firstly, an inboard gasoline engine will
use roughly one gallon of fuel per hour for every 10 horsepower
expended. So, if a 40-horsepower engine is running at half speed
and expending 20 horsepower, it’s using about two gallons of
fuel every hour. Now, diesel fuel has more energy, by volume,
than gasoline, so a diesel engine needs about one gallon per
hour for every 18 horsepower expended. Incidentally, most marine
engines expend about 75 percent of maximum horsepower at
cruising speed. So if you have a 25-horsepower diesel engine and
you’re running at cruising spee