July 4, 2013

The truth about fuel consumption

IF YOU THINK fuel-mileage figures on new-car stickers are misleading, wait till you to try to find out how many miles you get to the gallon on a boat.

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 anyway, 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.

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.

Now I have come across many sailors who will challenge these figures. They will claim in all honesty that their mileage is much better than that, and in certain cases, and on certain occasions, it might be. A following wind or a favorable current will certainly do wonders for your mileage. A light-displacement boat with a slippery hull, steaming at slow speed in calm seas might amaze you with the distance it can cover on a gallon of juice. But the gallons-per-hour figures quoted above were calculated by engineers and designers with true-life experience of a wide variety of boats and engines. They deserve respect.

I think the disparity arises because fuel consumption increases dramatically with boat speed and few people realize how little power is needed to move a hull at, say, half the designed hull speed, especially when the water is flat and the wind is either calm or blowing from astern. It’s all a question of horsepower expended.

In any case, here’s another tip the professionals like to pass on:

Plan to use one-third of your fuel on the outward leg of a trip, one-third to get back, and one-third for a safety reserve.

Today’s Thought
Believe those who are seeking the truth; doubt those who find it.
— AndrĂ© Gide

A friend tells me that the professional at his golf club just quit his job because every time he put his arms around a woman to demonstrate the grip, a dog came rushing out of the clubhouse and threw a bucket of cold water over them.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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