Underwater Gear; Propellers - Sea Ray 52 Sundancer Owner's Manual

S
ECTION
5. When pumping is complete, shut the pump off
and close the valve leading to the unit being
serviced. Check oil level and adjust if
necessary.
6. Repeat for each engine, generator or
transmission to be serviced.
REFER TO OWNER'S MANUAL PACKET FOR
INSTRUCTIONS AND WARRANTY INFORMATION.
13. U
NDERWATER
A. P
ROPELLERS
Propellers should be free of nicks, excessive
pitting and any distortions that alter them from
their original design. Badly damaged propellers
should be replaced, but those that are chipped,
bent or merely knocked out of shape can be
reconditioned by your marine dealer.
When doing extensive cruising, it is advisable
t o c a r r y e x t r a p r o p e l l e r s a b o a r d . E x t r a
propellers can be stored on the optional
propeller mounts in the engine room.
B P C
ASIC ROPELLER 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 (Figure 4.9.1).
P P D
ROPELLER ITCH AND
(F . 4.9.1)
IG
PITCH
®
52 Sundancer (525 Sundancer
4 • B
ILGE
G
EAR
IAMETER
DIAMETER
®
)
& U
NDERWATER
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 , I C
ENTILATION TS
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 4.9.2). 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 one or more of the
following:
1. R e p l a c e t h e d a m a g e d o r i n c o r r e c t
propeller with the recommended one.
2. With stern drives, set the outdrive at a
lesser trim angle (trim the unit downward).
P V
ROPELLER ENTILATION
(F . 4.9.2)
IG
OPTIMUM PERFORMANCE
C , I C
AVITATION TS AUSES AND
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
G
EAR
C
AUSES AND ORRECTIONS
VENTILATION
C
ORRECTIONS
4.9