Boating and Sailing.pdf - Moja ladja

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Part 2: The Basics of Operation
Heavy displacement boats can benefit from surface-piercing propellers, too, especially
when operation in shallow water is required. But the propulsion system—if not the entire
boat—should be designed from the keel up for surface-piercing operation.
Bet You Didn’t Know
The redline is the upper limit in revolutions per minute (rpms) recommended by the
engine manufacturer. It’s indicated on the tachometer, the dash instrument that indicates
rpms. Most motors are redlined at well below the rpm that will destroy them, but
exceeding the redline regularly or for prolonged periods will definitely cause the powerhead
to start making ugly noise—or even worse, no noise at all.
Most outboards are redlined between 5,000 and 5,800 rpms, although some highperformance
versions are designed for sustained speeds in excess of 6,000 rpms, and
motors used in racing sometimes rev up to 10,000 (but they don’t have to live very
long between rebuilds). There is actually a red line on many tachometers indicating the
line beyond which you should not go if you don’t like spending time and money in the
company of mechanics.
Shallow Water Trim
You also trim up at idle speeds when crossing shallows or grassy areas—it gets the prop
higher and avoids bottom contact. Many times, when you think you’re stuck on a shoal,
you can get off by trimming up the lower unit of an outboard or stern drive to the point
where the water pickup is barely below the surface and the prop is actually breaking the
surface.
You lose a good bit of your propulsion with the prop in this position and you can hardly
back up at all, but the blades grab enough to ease you off the obstruction—usually. If not,
it’s time to shut off the motor, step over the side and put your shoulder to things.
Boating in a Bog
Avoid trimming up much at speeds between 1,000 and 2,000 rpms in the shallows because
this pushes the stern down. The boat actually runs deeper in the water than if you had the
motor in the full down position.
Remember that at about 1,500 to 2,500 rpms on outboards, many boats will “plow” or
bog severely. You’ll go slow, but you’ll actually make a much larger wake and burn more
fuel than if you went fast.