Sea Ray 270 Select EX, Owner'S Manual

Page: 61 / 126

4.7
S
ECTION
4 • B
ILGE
& U
NDERWATER
G
EAR
270 Select EX
OPTIMUM PERFORMANCE
CAVITATION
P
ROPELLER
C
AVITATION
(F
IG
. 4.7.1)
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. 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. Replace the damaged or incorrect
propeller with the recommended one.
2. With stern drives, set the outdrive at a
lesser trim angle (trim the unit downward).
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 propeller inspection should be part of
routine maintenance.