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 speed, you’re using about 18 horsepower and your engine is swallowing about one gallon of fuel an hour.
Outboard engines use far more fuel than inboards, of course — often as much as 50 percent more — and two-strokes use considerably more than four-strokes.
Finally, here’s a handy tip that will make the Coast Guard love you: When you’re going on a trip, plan to use one-third of your fuel on the outward leg, one-third to get back, and one-third for a safety reserve.
Today’s ThoughtHe is free from danger who, even when he is safe, is on his guard.
— Publilius Syrus, Sententiae.
Tailpiece“What’s Vanessa’s last name?”
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