Evinrude 4906B 4HP 1969 Owner's Manual page 10

Ventilation
Ventilation is the forming of voids in the
water just ahead of the propeller
Marine propulsion designers are constantly
fighting the battle against the formation of
these voids due to excessive blade tip speed
and engine wear. The voids may be filled
with air or water vapor, or they may actual-
ly be a partial vacuum.
caused by installing
too close to the lower unit, such as the knot
indicator pickup, depth sounder, or bait tank
pickup.
Vibration
Your propeller should be checked
ularly to be sure all blades are in good
condition. If any of the blades become bent
or nicked, this condition will set up vibra-
tions in the drive unit and the motor. If the
vibration becomes very serious it will cause
a loss of power, efficiency,
mance. If the vibration
tinue over a period of time it can have a
damaging effect on many of the operating
parts.
Vibration in boats can never be com-
pletely eliminated, but it can be reduced by
keeping all parts in good working condition
and through proper maintenance
cation. Vibration can also be reduced in
some cases by increasing
blades. For this reason, many racers
Rubber hub removed from
removed because the hub was slipping
blades.
Ventilation may be
a piece of equipment
reg-
and boat perfor-
is
allowed to con-
and lubr i-
the number of
use
a
propeller. This hub was
in
the propeller.
two-blade props and luxury cruisers
four- and five-blade props installed.
Shock Absorbers
The shock absorber in the propeller plays
a very important
shafting, gears, and engine against the shock
of a blow, should the propeller
underwater object.
lows the propeller
instant of impact
continues turning.
How much impact the propeller is able
to withstand before causing the clutch hub
to slip is calculated
force needed to propel the boat, but less
than the amount that could damage any part
of the power train. Under normal propulsion
loads of moving the boat through the water,
the hub will not slip. However,
the propeller strikes an object with a force
that would be great enough to stop any part
of the power train.
I
~~
illustration depicting
explained in the text.
PROPELLERS
have
role in protecting
strike
The shock absorber al-
to stop rotating at the
while the power train
to be more than the
it will slip if
0
- 0 RAKE
of a
the rake propeller,
1-5
the
an
as