Boating and Sailing.pdf - Moja ladja

The Art of Cornering
Chapter 5: Getting Underway for the First Time
Boat Bytes
What if your boat porpoises all the time? Chances are you have too much weight
forward, making it impossible for the prop to adequately lift the bow. A quick fix
is to move passengers, ice chests, or other heavy gear aft. Or you may want to
switch from a three-blade prop to a four-blade—the added blade area sometimes
helps in giving more lift.
Lots of trim make steering light and easy and adds speed, but it also makes the boat more
inclined to skid in sharp turns. So if you’re faced with running through a narrow, winding
creek or making sharp turns, the lower unit needs to be trimmed down. Otherwise, you
may wind up like the bad guy in Miami Vice, boating on dry land.
Don’t trim in too much or the boat may “bow steer,” with the front section of the keel
grabbing the water and pivoting the aft section so sharply that gear and passengers may be
endangered.
Ventilation: Good in Motel Rooms, Bad in Props
Excess trim can lead to “blow out” or prop ventilation, in which the prop sucks in surface
air as it breaks the surface. This is most likely to occur on turns as the boat heels or leans
into the turn. The prop loses its grip on the water and the motor howls, winding up very
fast while the boat slows quickly. Ventilation sometimes occurs when a fast boat jumps
over a large wave as well.
You know you’re at maximum trim when rpms go up but speed does not. On most performance
boats, this can be quite high, to the point where the motor howls and there’s a
healthy “rooster tail” or white-water spray upward from the prop. (This looks cool, but it
wastes fuel.)
Some performance props are what are called surface piercing, which means that at the top
part of the stroke, each blade pops free of the water. Since it takes less power to turn out
of the water than in, the prop load decreases and the engine winds higher.
Surface-piercing propellers use ventilation to advantage. Each blade carries an air
cavity into the water with it so that cavitation—a much more damaging condition than
ventilation—is completely suppressed. Also, surface-piercing installations can be designed
to keep most of the lower unit out of the water, significantly reducing drag on very fast
boats.
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