November 22, 2012

Estimating 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, 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 Thought
He is free from danger who, even when he is safe, is on his guard.
— Publilius Syrus, Sententiae.

“What’s Vanessa’s last name?”
“Vanessa who?”

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


Jack said...

this article reminds me of dinner I attended, where the after dinner speaker was the skipper of the local Coastguard vessel on Vancouver Island. He speech was somewhat dry, with statistics of work performed in the past year, with a thinly disguised warning of what not to do when putting to sea. The 1/3 rule was foremost in the man's mind. Saying lack of fuel was the most common reason mariners called
for assistance. He then added, in all his time assisting sailors who run out of fuel he had never came across one that had run out of beer!..... To some, forward planing appears to stop at the liquor store.

Anonymous said...

John, I keep exact track of fuel consumption on Islander and my records show that in over 800 hours of running my 40 hp Yanmar at an average 3/4 speed I have achieved .6 gal/hour. In conversations with other owners of diesel engines in the 35-40 hp range I have found that my .6 gal/hr is not unusual. As I interpret your column I calculate that I should be consuming 1.6 gal/hr. So I am hoping you can help me understand your theory as it applies to my boat and engine.

John Vigor said...

Hi Mike: This theory is an average for all kinds of boats at all speeds in all weather conditions. Now, a 36-foot sailboat like yours is very slippery and you can expect particularly high mileage if 3/4 throttle gives you less than hull speed. There are huge rewards in mileage for slowing down a knot or two, and even greater penalties for speeding up, especially if you are at or near hull speed.
Non-planing powerboat hulls usually offer far more resistance than sailboat hulls and pay the penalty in fuel consumption.
Sailboats most often use their engines in light or contrary winds anf often motor-sail with a raised mainsail, which saves fuel by adding its own power. Powerboats don't get this benefit, and have to motor in strong contrary winds if necessary, which increases fuel consumption. It also increases the theoretical fuel consumption averaged for all boats.
For planning purposes the theory is valuable because it ensures spare fuel if you consume less than bargained for.
Neverthess, 0.6 gallons an hour is splendidly frugal for a 36-foot Islander, and it proves that both hull and engine are well maintained.

John V.

Dave said...

I learned to sail/cruise in Puget Sound, San Juan Is., and the Gulf Is. I always think in terms of range in hours not miles ... with tides i feel hours/gallon is a better value to know as the 'distance over water' is an unknown (until you arrive). with a GPS telling me 'time to arrival' I quickly know if I have enough fuel.

- Dave