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 $133 per day and $99 per day,
respectively at $4.07 per gallon. (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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