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Firstly a propeller will have two sizes, diameter and pitch, and are usually measured in inches. For example 13" x 19". The first dimension, in our example 13" refers to diameter and the second 19" is the pitch. Diameter is simply the distance across a circle swept by the extreme tips of the propeller blades. Pitch is the theoretical distance travelled for one revolution of the propeller, in this case 19". However as water is not a solid substance this figure is less, due to slip. A propeller blade has a leading and trailing edge. The leading edge cuts into the water first. The trailing edge is the edge from which the water flows away. Many propellers have a slight upturn on the trailing edge, this is known as cupping. Cupping has the effect of reducing cavitation. Cavitation is commonly confused with bush failure, as the effects can be very similar, IE. Sudden rises in engine rev's and loss of speed. Cavitation is caused when the surface of the blades become covered in tiny vacuum..