Re: Easy question for the pro's!!!!!
Here is a quote from another web site:
"Figuring out fuel mileage on a car is simple. Fill the tank completely, jot down the current mileage and drive until the tank is near empty. At the next fill-up, note the miles traveled and the gallons of fuel it takes to refill the tank. Then divide the number of gallons of the second fill-up by the miles traveled.
It's a different story with a boat. 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. The pros call it "brake-specific fuel consumption." 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 0.6 to 0.8 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.
Confused yet? Look at the mathematical examples below that show the figures for a 50-horse engine, and it should become clear.
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. Also remember that newer engines with loop-charged intake systems, fuel injection and direct injection will yield higher fuel efficiency than older, cross-flow fuel systems that were manufactured as late as 1990. Cross-flow systems are antiquated and wasteful compared with the technology available today.
To apply these formulas to your boat, just plug in its horsepower rating and multiply the top, then divide the result by the bottom. For two-stroke engines, another way is to take the total engine horsepower and divide it by 10. As you can see, this formula is simpler to calculate and easier to remember. You don't even need a pencil and paper. It's just not as accurate as the formulas above. The result represents the approximate gallons per hour the engine will burn at wide-open throttle. For example, a 150-horse engine will use about 15 gallons per hour. 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."
