"Charles Momsen" wrote in message
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What is pitch?
Propeller pitch is the theoretical forward movement of a propeller for one
revolution, assuming that there is no prop slip. For example, a 21 pitch
propeller will theoretically move 21 inches for every revolution. Propeller
slip occurs with every propeller, but the amount of slip varies depending on
propeller design. More aggressively and efficiently designed propellers will
slip less.

When selecting a propeller pitch for your boat, it is important that the
propeller runs at the upper end of your engines wide-open-throttle RPM
range. If you want your RPM's to increase, go down in pitch. To decrease
RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's will
change approximately 400 RPM's.

For water sports or extra people on board, you should generally drop 2" of
pitch to help compensate for the added weight and drag on your boat. It
makes a noticeable difference in your boat's hole shot, fuel efficiency,
RPM's, and overall performance. You should ALWAYS carry a spare propeller on
board, and if you're into water sports or occasionally load the boat with
extra people, a spare prop with a lesser pitch is a good idea. When thinking
a propeller pitch, compare it to a gear on car - lower gear, higher RPM's.

The same pitch from different manufacturers will run slightly different
RPM's due to a difference in blade design. Speed differences among the same
pitch from various manufacturers will vary even more. For example a 17 pitch
from one brand could run up to 7 mph faster than a 17 pitch from another
brand.

Pitch is the 2nd two digits that are listed in a propeller item description
(14-1/4 x 19, 19 is the pitch, 14-1/4 is the diameter)



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What is diameter?
Propeller diameter is distance across the imaginary circle that a spinning
propeller makes. It can be easily determined by measuring the distance from
the center of the hub to the tip of one of the blades and multiplying that
number by 2.

Diameters between different propeller manufacturers may vary slightly. For
example for a V6 outboard application, Turning Point uses a 14-1/4" diameter
where a different manufacturer may use a 14-1/2" diameter. This small
variance does not affect your performance as much as the pitch and overall
design. However, larger diameter propellers - 15" or 16" diameter - are
designed for larger boat applications - 23 ft +, and are not ideal for 15" -
22" foot run-a-bouts. These larger boats need more blade area to push more
water.



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What is rake?
Propeller rake is the degree that a propeller blade is angled in relation to
the hub. Props with higher rake typically have better speeds and greater
lift. The better performing propellers typically have between 20 - 30 degree
rake angles.




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What is cupping?
Propeller cupping is the curved lip at the trailing edge and/or tip of the
propeller. Cupping helps the propeller to get a better grip in the water for
better holding at higher trim and on turns. Cupping also increases the
efficiency of a propeller and can result in higher top end speeds when
properly designed.



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What are vent holes?
Vent holes are holes behind each blade designed to aid in the hole shot of a
propeller. When accelerating from neutral, these holes allow some exhaust
bubbles to flow through and flood the blades. The prop is then spinning
through more turbulent water, thus gaining RPM's and speed more quickly.
After approximately 1000 RPM's, the exhaust flows rapidly enough that is
will bypass the holes and flow through the hub, eliminating any further
slippage from the holes.

This performance feature can only be found in stainless steel propellers.
However Turning Point is the only one to offer this feature in aluminum
propellers as well.