Gasoline two-cycle outboards put out about 10 horses per gallon of fuel burned at full throttle. For example, a 200-horse motor will consume somewhere near 20 gallons an hour, while a 150-horse motor will eat somewhere around 15 gallons an hour. It's not uncommon for motors to be a couple of gallons over on this, with a 150 taking, say, 18 gallons an hour at 5,800 rpms. Four-cycle outboards can cut this fuel use up to 60 percent in the mid-ranges, though they use nearly as much as two-cycles at full speed.
While all gasoline motors of a given horsepower are similar in top speed at maximum rpms, they're not the same. Some do better on fuel and still
Rpms, or revolutions per minute, refers to the number of times an engine's output shaft rotates in 60 seconds.
manage excellent performance. The test results run in magazines such as Boating, for whom I work, give you exact numbers for comparison.
The idea that you get there faster at full throttle, therefore don't run the motor as long and don't burn as much fuel, doesn't work out in reality. It's common to see fuel usage double from mid-range to full throttle. Of course, speed goes up, too, but the miles-per-gallon figure usually shows 30 to 50 percent better efficiency at the lower rpms.
What we find is that, true to the operating manual, most outboard motors get their best fuel economy—that is, their best miles per gallon—somewhere between 3,500 and 4,500 rpms, with most gas inboards best between 2,500 and 3,500. For diesels, the economy range is likely to be around 1,800 to 2,400 rpms. (Big diesels are never run as fast as gasoline motors—most redline at 2,200-2,700 rpms. However, some of the smaller models may be run close to 4,000 rpms without damage.) The most fuel-efficient speed for a given boat is called its "cruising speed."
The range of most boats is longer if you keep the rpms somewhere around the 60 to 70 percent level. Most wise captains allow a 10 percent safety margin on fuel. Here's the basic math a captain might work out to estimate his range at various speeds with a 100-gallon fuel tank:
Safety margin or reserved fuel: 10 percent x 100 gallons = 10 gallons Total tankage minus reserve: 100 - 10 = 90 gallons Range at best cruising speed: 90 gallons x 4.0 mpg = 360 miles Range at full speed: 90 gallons x 2.25 mpg = 202.5 miles
Note that in this case, you can travel more than 100 miles farther at cruising speed than at full speed. (I have seen one or two lightweight performance rigs that came close to matching their mid-range numbers at top end or full speed. These boats ride so high in the water at full speed that friction is reduced, greatly increasing their efficiency.)
Running your engines at moderate speeds gives the best fuel economy. Higher speeds can nearly double fuel usage on some boats.
(Photo credit: Grady-White Boats)
It's common for boats to get good mileage at idle speed, and then very bad mileage as they "squat" or drop into the hole before planing. This is because they are pushing uphill— there is a lot of water pressure against the bottom pushed forward at an angle of up to 10 degrees from the horizontal. Even though you're going slow, the fuel gauge is going fast, toward empty.
When the boat planes off, this angle drops rapidly to one or two degrees, which greatly reduces friction. Fuel economy and speed increase. Thus, if you want to save fuel, the worst thing you can do is what many first-timers do: Get the boat into a bog at somewhere between 1,500 and 2,000 rpms and motor along, blissfully sinking all boats tied to the docks with their enormous wakes as they pass.
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.