Table of Contents
Advertisement
S
ECTION
C
, I
C
AVITATION
TS
AUSES AND
P
V
ROPELLER
ENTILATION
(F
. 4.10.1)
IG
OPTIMUM PERFORMANCE
Cavitation is a phenomenon that occurs in all
propeller-driven craft under certain conditions. The
surface of propeller blades are not perfectly flat,
and as water is drawn through the blades to be
discharged aft into the propeller's slip stream, the
water flowing over the curved surface of the blade
encounters areas of greater and less pressure.
In those areas of reduced pressure, air bubbles are
formed. When they move out of the low pressure
area these bubbles collapse. If they collapse while
in contact with an object, such as part of the
propeller blade or trim plane, the bubbles create
such highly localized forces that they erode the
surface of the object. In the case of the propeller,
such damage is somtimes called a "burn". It may
be caused by an irregularity in the propeller's leading
edge, and it should be corrected by reconditioning
the propeller or by replacement.
Cavitation is a normal occurrence in modern sport
boats, and propeller inspection should be part of
routine maintenance.
P
T
ROPELLER
ORQUE AND
Some of the more powerful motors create a
considerable torque effect; that is, a twisting motion
causing the boat to ride with one sheer lower than
the other. This twisting reaction is caused by the
direction of propeller rotation lifting one side of the
boat. This causes an uneven drag, so that a boat's
bow may tend to fall off in one direction or the other
from the intended course given by the wheel.
4.10
4 • B
ILGE
C
ORRECTIONS
VENTILATION
I
C
TS
ORRECTION
& U
NDERWATER
Torque action may occur when maximum or close
to maximum rated horsepower is applied. Any slight
torque may be offset by shifting passenger or gear
weight laterally to the high side of the boat.
C
-R
OUNTER
OTATING
On twin engine yachts, one propeller turns in a
clockwise direction while the other turns
counterclockwise in order to maintain a straight
course through the water.
When removing or replacing propellers, be sure to
install the correct propeller on the correct drive.
C
-R
OUNTER
OTATING
(F
. 4.10.2)
IG
G
EAR
P
ROPELLERS
P
ROPELLERS
®
(375 Sundancer
340 Sundancer
®
)
Advertisement
Table of Contents
