|
The
following information will provide you
with answers to some of the most frequently asked questions
about propellers.
What
is propeller diameter?
Diameter is the distance across the
circle made by the blade tips
as the propeller rotates.
What
is propeller pitch?
Pitch is the distance that a propeller
would move in one revolution
if it were moving through a soft solid,
like a screw in wood. When a
propeller is identified as 13 3/4 x 21,
it has a 13 3/4" (35 cm) diameter
with 21" (53 cm) of pitch.
Theoretically, this propeller would move
forward 21" in one revolution.
What
is propeller cupping?
When the trailing edge of the blade is
formed or cast with an edge curl
it is said to have a cup. Cupped props
will usually allow a faster top
speed and more midrange efficiency by
allowing more positive trim with
less prop slip. Cupping benefits are so
desirable that nearly all modern
recreational, high-performance or racing
propellers contain some degree
of cup. Cupping will usually reduce
full-throttle engine speed about
100 to 200 RPM below the same pitch
propeller with no cup. A propeller
repair shop can increase or decrease cup
to alter engine rpm to meet
specific operating requirements on most
propellers.
What
is propeller ventilation?
Ventilation occurs when air from the
water's surface or exhaust gases
from the exhaust outlet are drawn into
the propeller blades. The normal
water load is reduced and the propeller
over-revs, losing much of its thrust.
This action most often occurs in turns,
particularly when trying to plane in a
sharp turn or with an excessively
trimmed-out engine or drive unit.
Ventilation
can also be caused by aerated water from
step bottom hulls.
What
is propeller cavitation?
As a shape passes through water at an
increasing speed, the pressure
that holds the water to the sides and
back of the shape is lowered.
Depending upon the water temperature,
when the pressure reaches
a sufficiently low level, boiling (i.e.,
the formation of water vapor) will
begin.
The collapsing action, or implosion, of
the bubbles releases energy that
chips away at the blades, causing a
"cavitation burn" or erosion
of the metal.
The
initial cause of the low pressure may be
nicks in the leading edge, too
much cup, sharp leading edge corners,
improper polishing, or, sometimes,
poor blade design. Massive cavitation by
itself is rare, and it usually is
caused by a propeller that is severely
bent or has had its blade tips broken
off resulting in a propeller that is far
too small in diameter for the
engine.
Will
a stainless steel propeller make my boat
faster?
Generally, yes, a steel propeller will
be faster. If the designs are the same,
the steel propeller can be made with
thinner blades that run faster and with
greater strength. Also, most stainless
steel propellers take advantage of
performance enhancing designs to gain
even more advantages over aluminum.
How
should I maintain and service my
propeller?
Essential to good propeller maintenance
is periodic inspection to detect even
small dings, which can lead to blade
failure if not dressed or repaired. A
damaged propeller, even one that only
appears slightly damaged by running
through silt and sand, can significantly
reduce performance efficiency and fuel
economy, and can more severely damage
itself through cavitation erosion
emanating from the blades' irregular
leading edges. In one test with a
damaged propeller, top speed fell more
than 13%. Acceleration was off over 37%.
Optimum cruise miles slowed 21%. Worse
yet, damage usually is not done to each
blade uniformly and, therefore, the
damage can set up imbalance vibrations
that can cause fatigue damage to other
parts of the engine or drive. If you
boat in shallow or rocky waters, you
will want to check your propeller more
frequently for possible damage.
How
does propeller torque produce boat roll?
When observing from behind a boat, the
propeller turns clockwise when underway
with a normal right-hand propeller. As
water resists the clockwise rotating
propeller, it causes the boat to roll
slightly in the opposite direction
(counterclockwise) or down on the left
(port) side and up on the right
(starboard) side. To offset this slight
imbalance, the driver's seat is placed
on the starboard (right) side. Boats
differ significantly in the degree of
their reaction to prop torque.
|
