boatsandiego.com

Fuel for Fun

   Miles aren't easy to measure on water, so fuel consumption is measured in gallons per hour. You measure fuel efficiency in pounds of fuel used per horsepower developed per hour. This makes it important to know that gasoline weighs about 6 pounds per gallon.

   On average, an in-tune four-stroke gasoline engine will burn about 0.4 to 0.45 pounds of fuel per hour for each unit of horsepower. Likewise, a well-maintained two-stroke outboard burns nearly O.6 to O.8 pounds of fuel per hour for each unit of horsepower it produces. These figures apply to carbureted and fuel-injected engines, but not to direct injected engines such as Mercury's OptiMax and OMC's FICHT models.

   See the mathematical examples below that show the figures for a 90-horse engine, and it should help to understand.

Four stroke engines

90 hp x 0.43 lb. of fuel per horsepower per hour = 6.45 gals. per hour
6 lb. per gallon

Two-Stroke Gasoline Engines

90 hp x 0.7 lb. of fuel per horsepower per hour = 10.5 gals. per hour
6 lb. per gallon

   Though these figures represent averages and can vary from 10 to 20 percent, they'll put you in the ballpark so you can plan a long-distance cruise.

   The average boater will use the engine about half as much as the actual time spent on the water. This translates to 5 hours of running for each 10 hour day of boating. With a 90 hp. four stroke engine this equates to about 32.5 gals. of fuel at full throttle and about 24.4 gals of fuel at 3/4 throttle. This translates to $55 per day and $41 per day, respectively. (Have you priced a days trip to Disneyland, lately?)

   Keep in mind that these formulas apply when the engine is making peak horsepower, which usually is near wide-open throttle. Fuel consumption will be decreased at cruising speeds. A 25% savings in fuel can be obtained by using 3/4 throttle. Also, remember that newer engines with loop-charged intake systems, fuel injection, and direct injection will get higher fuel efficiency than older, cross-flow fuel systems.

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