Evinrude 4906B 4HP 1969 Owner's Manual page 11

1-6 SAFETY
If the power train
impact great enough to stop rotation,
for an instant, something would have to give
and be damaged. If a propeller is subjected
to repeated striking of underwater
it would eventually
under normal loads.
start to slip, a new hub and shock absorber
would have to be installed.
Propeller Rake
If a propeller blade is examined on a cut
extending directly through the center of the
hub, and if the blade is set vertical to the
propeller hub, as shown in the accompanying
illustration, the propeller
zero degree (00) rake.
back, the rake increases.
lers have a rake angle from
A higher rake angle generally improves
propeller performance
ventilating situation.
boats, higher rake often will increase per-
formance by holding the bow of the boat
higher.
Progressive Pitch
Progressive pitch is a blade design inno-
vation that improves performance
ward and rotational speed is high and/or the
propeller breaks the surface of the water.
Progressive pitch starts low at the lead-
ing edge and progressively
trailing edge, as shown in ,the accompanying
illustration. The average
is
entire blade the number assigned to that
propeller. In the illustration
sive
pitch, the average pitch assigned to the
propeller would be 21.
CONSTANT PITCH
a constant
Comparison of
right, changes to give the blade more thrust and therefore,
was to absorb an
even
objects,
slip on its clutch hub
If the propeller would
,
is said to have a
As the blade slants
Standard propel-
o
0
a
to 15 •
in a cavitating
On lighter, faster
when for-
increases to the
pitch over the
of the progres-
and progressive pitch propeller.
or
Propeller with
gives the propeller
Cupping
If the propeller is cast with a edge curl
inward on the trailing edge, the blade is said
to have a cup. In most cases, cupped blades
improve performance.
blades to "HOLD" and not break loose, when
operating in a cavitating or ventilating
ation. This action permits the engine to be
trimmed out further, or to be mounted high-
er on the transom.
high-performance
two adjustments
speed.
Notice how the pitch of the progressive pitch propeller,
the boat more speed.
a "cupped" leading edge.
a better "hold" in the water.
The cup helps the
This is especially true on
boats. Either
will usually add to higher
PROGRESSIVE PITCH
"Cupping"
situ-
of these