Most props are stamped on the forward end with diameter, pitch and shaft size. Diameter is always first, pitch second. Size is often repeated on the side of the hub with the manufacturer''s name, type prop, hand, and date of manufacture.
For your 20 x 13 prop’, the first number is the diameter (the prop’ is 20" tip-to-tip circle), and the second number designates the 13" propeller pitch.
Pitch is the angle of the blades, expressed in inches of theoretical travel in one revolution of the propeller. This propeller has a pitch that will theoretically* result in 13" of forward travel in one revolution.
*As capnjim02 says , the propeller experiences slippage, so that its actual travel per revolution is less than the stated pitch. “Slip” is the difference between the actual distance traveled by a boat, and the theoretical distance computed by the number of revolutions and propeller pitch.
Actually, there are several kinds of slip, including:
Apparent slip is the difference between the advance observed and that calculated by pitch times revolutions.
True slip is greater than apparent slip, due to wake moving with the vessel. The wake that affects slip is a body of water surrounding the propeller and moving along with the vessel. The amount of wake is determined by the amount of friction produced by the hull moving through the water.
Sailing auxiliaries generally experience about 35 to 40% slip.
Increasing or decreasing propeller pitch directly affects engine load throughout its RPM range.
A high load (high pitch and/or diameter) on an engine will result in lower engine RPM throughout its entire RPM range for a given throttle setting. Adding 1 inch of propeller pitch will reduce full-throttle (WOT) RPM by about 150 to 200.
A light load (low pitch and/or diameter) on the engine will result in higher engine RPM throughout its entire RPM range for a given throttle setting. Reducing pitch by 1" will reduce full-throttle (WOT) RPM by about 150 to 200.
More interesting info’ at:
RBBI PROP CALCULATOR: http://www.rbbi.com/folders/prop/propcalc.htm
THE PERFECT PITCH: http://www.nautica.it/info/motore/eliche_e.htm