4.7
S
ECTION
4 • B
ILGE
& U
NDERWATER
G
EAR
44 Sedan Bridge (
455 Sedan bridge
)
will, in a single rotation, advance a boat 12 inches.
This variance is referred to as slippage.
V
ENTILATION
,
ITS
C
AUSES
AND
C
ORRECTIONS
While often called “cavitation,” ventilation is
really a different effect. At times when a boat
enters or leaves a sharp turn, the propeller
seems to slip and lose thrust and the engine
may over-speed (Figure 4.7.1). This problem
is normally caused by air or aerated water
entering the propeller. (A damaged propeller
can also cause ventilation.) Ventilation can
usually be corrected by replacing the damaged
or incorrect propeller with the recommended
one.
C
AVITATION
,
ITS
C
AUSES
AND
C
ORRECTIONS
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 sometimes 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.
C
OUNTER
-R
OTATING
P
ROPELLERS
(F
IG
. 4.7.2)
OPTIMUM PERFORMANCE
VENTILATION
P
ROPELLER
V
ENTILATION
(F
IG
. 4.7.1)
Cavitation is a normal occurrence in modern sport
boats, and prop inspection should be part of routine
maintenance.
P
ROPELLER
T
ORQUE
AND
ITS
C
ORRECTION
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.
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
OUNTER
-
ROTATING
P
ROPELLERS
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.
P
ROPELLER
I
NSTALLATION
For proper rotation, the installation of propellers on
inboard engine boats requires the right hand
propeller to be installed on the starboard side and
the left hand propeller to be installed on the port
side. Install in the following manner:
1. Inspect the key. It must be chamfered so that
the corners of the key do not touch the keyway
fillets.
Summary of Contents for 44 Sedan Bridge
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