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.
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.