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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.
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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.
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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.
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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.
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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. 
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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.
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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.
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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.
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