Boating and Sailing.pdf - Moja ladja

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Part 1: Getting Started in Boating
manage excellent performance. The test results run in magazines such as Boating, for
whom I work, give you exact numbers for comparison.
Boat Bytes
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.
Better at Cruising Speed
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 100gallon
fuel tank:
Safety margin or reserved fuel: 10 percent × 100 gallons = 10 gallons
Total tankage minus reserve: 100 – 10 = 90 gallons
Range at best cruising speed: 90 gallons × 4.0 mpg = 360 miles
Range at full speed: 90 gallons × 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.)