Underwater Gear; Propeller General Information - Sea Ray 260 Sundancer Owner's Manual

S
ECTION
3. U G
NDERWATER
A. P G
ROPELLER ENERAL
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
to carry an extra propeller aboard.
B P C
ASIC ROPELLER HARACTERISTICS
P P D
ROPELLER ITCH AND
(F . 4.6.1)
IG
PITCH
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.
4.6
4 • B
ILGE
EAR
I
NFORMATION
IAMETER
DIAMETER
& U
NDERWATER
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. This problem is normally
caused by air or aerated water entering the
propeller. (A damaged propeller can also cause
v e n t i l a t i o n . ) V e n t i l a t i o n c a n u s u a l l y b e
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).
C , I C
AVITATION TS AUSES AND
P C
ROPELLER AVITATION
(F . 4.6.2)
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 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 propeller inspection should be part of
routine maintenance.
G
EAR
C
AUSES AND ORRECTIONS
C
ORRECTIONS
CAVITATION
260 Sundancer (275 Sundancer
®
)
®