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| What is Pitch? | Pitch is the distance that a propeller would advance in one revolution. The
two kinds of pitch are constant or true pitch, which means the pitch is constant throughout the blade, and progressive, which
starts lower at the leading edge and increases to the trailing edge. Pitch is measured in inches.
| | What is Rake? | Rake is the angle of the blade to the center of the shaft and is measured in degrees. Most performance
propellers are angled aft to give boats more lift without ventilating the propeller. The two common types of rake used in
performance boating are parabolic and straight rake. Parabolic rake is an off-center rake line that gives the blade an appearance
of a spoon-like effect. Straight rake is basically flat across the blade.
| | What is cupping? | Cupping is a slight curl on the trailing
edge to reduce ventilation and also gives more lift capability. It increases the overall pitch ½ " to 1".
Number
of Blades Three-bladed is
by far the most common propeller design for today’s boat engines. The three blade design offers good performance and
efficiency with the best “cost vs. benefit ratio” for most boating applications. Four-blade propellers are used
to enhance performance where increased engine heights are required, faster acceleration is needed or where the boat is being
operated in rough water conditions. Additionally, four-bladed propellers can increase overall performance in applications
where heavier boats and/or loads are typically being experienced. The four-bladed design can also enhance the ability to use
more positive trim angle without “slip” and cause the boat to have increased bow and/or stern lift. However, four
blades also mean increased drag on the engine, lower top speeds, and different handling characteristics. It’s beneficial
to note that if a three blade propeller is operating within the proper WOT RPM specification, a four-bladed propeller must
normally be one size pitch less in order to accurately maintain the correct WOT RPM.
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