Sea Ray 390 Sundancer, Owner'S Manual

Page: 88 / 184

390 Sundancer
®
( 425 Sundancer
®
) 4.8
S
ECTION
4 • B
ILGE
& U
NDERWATER
G
EAR
B
ASIC
P
ROPELLER
C
HARACTERISTICS
Propellers have two basic characteristics:
• Diameter
• Pitch
Diameter is that distance measured across the
propeller hub line from the outer edge of the 360°
that is made by the propeller’s blade during a single
rotation. Pitch is that distance in inches that a
propeller will travel if rotated one revolution without
any slippage.
For example, a propeller with a 12-inch pitch, when
rotated 360° would, theoretically, advance 12
inches through the water. Actually, no propeller
applied to any boat is 100% efficient. No 12-inch
pitch blade will, in a single rotation, advance a boat
12 inches. This variance is referred to as slippage.
V
ENTILATION
, I
TS
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
4.8.1). 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
, I
TS
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.
Cavitation is a normal occurrence in modern sport
boats, and prop inspection should be part of routine
maintenance.
P
ROPELLER
T
ORQUE AND
I
TS
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
-R
OTATING
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.
OPTIMUM PERFORMANCE
VENTILATION
P
ROPELLER
V
ENTILATION
(F
IG
. 4.8.1)
C
OUNTER
-R
OTATING
P
ROPELLERS
(F
IG
. 4.8.2)