Determining pitch
Virtually all propeller pitch is measured at the 75% radius. I assume your prop diameter is 72" bansed on the 3 inch progression that they gave you. So 72'/2=36'. .75X36=27. So your 75% radius is the 27" station. They say the pitch in degrees at 27" is 20.25 degrees.
Graphic interpretation;
Draw a horizontal line on your paper representing about 70 inches. At one end of this line draw a vertical line representing about the same 70 inches. Keep both lines the same scale. Go up from the junction of these two lines vertically to the 27 inch station. This is your 75% radius. draw a line down from this point at an angle of 20.25 degrees and where it meets the horizontal line is the pitch in inches. Looks like your prop is about 68" pitch. That is how far the prop will advance in one revolution in a non compressible medium. Since air is compressable there is what is called slip. So if your ariplane advances 56.44" in one rev of the prop you have a propulsion effencicy of 83%. That is a good number for a goodly designed propellr operating in the 2400 to 2800 rpm range. Propellers operating in the 1100 to 1300 rpm range, (turboprops) many times can realize a little over 90% propulsion efficency. Lastly; when the tip speed starts getting around Mach .8 efficendy starts to drop. In that area a teal thin tip helps delay the Mach buffet, but then mechanical vibration comes into play.
Every few years Hartzell, McCaully and many other prop designers re-invent the propeller and they make all sorts of claims. Semitar, clip tip, super airfoils and on and on it goes. Prop technology was pretty much known by the late 1930s. The only real improvements since then is slow rpm props. If you are getting around 83% propulstion efficenty then be happy. About everything else is largely hype and sales material.
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