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Prop Terminology

Prop Terminology

Pitch is the distance that a propeller would move in one revolution. When a propeller is identified as 14 1/2 x 24, it has a 14 1/2″ (diameter with 24″ of pitch. Theoretically, this propeller would move forward 24″ in one revolution.)
Repitch This is where we increase or decrease the pitch by 1 inch. A repitch will change the RPMs by approximately 200.
Diameter is the distance across the circle made by the blade tips as the propeller rotates, determined by the RPM at which the propeller will be turning.
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.

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.

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.
Lab finishing, refers to the process used to insure the accuracy of the propeller rake angle, pitch, progression, pitch variation and cup. This hand process involves thinning the blades and sharpening the leading edges.

Blueprinting refers to the process used to ensure the accuracy of the propeller rake angle, pitch, progression, pitch variation, and cup. This hand process involves thinning the blades and sharpening the leading edges.

The advantage of blueprinting is that it reduces the horsepower required to turn the propeller and generates more usable horsepower for thrust. On an accurate, well-built propeller the speed gain may be 2-3 miles per hour. On a lesser quality propeller, the increase may be 3-4 miles per hour.

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