The terms Cavitation and Ventilation are often mistaken for each other.   Knowing the difference between the two can help you determine numerous different aspects of propping your boat correctly.  

Prop Cavitation:

Cavitation occurs when the pressure of a liquid changes rapidly to point that it is lower than the vapor pressure of the liquid.   This in turn causes small vapor bubbles or cavities in the fluid.  

This usually happens around the prop as it is spinning at high speeds causing pressure changes (areas of low pressure) therefore creating bubbles in the water.  These bubbles can then burst or collapse causing a decreased performance and possible damage to the prop.

Prop Ventilation:

Ventilation is caused by outside elements introducing air or gases into the prop.   

This normally occurs when the prop is too high (close to the water surface), hard turns that pull the prop out or near out of the water, and during hard acceleration.   Excess ventilation can cause the propeller to turn air instead of water which in turn can lead to over-rev and a reduction in performance.

Determining whether you are experiencing Cavitation or Ventilation: 

To determine what is occurring on your vessel you will need to pay close attention to the noises coming from the propeller, vibrations, & sudden jumps in rpms.

Sudden Jumps in RPM without an increase in speed would normally indicate excess ventilation.   

Cavitation will often times be accompanied by a high pitch whining noise and sometime vibrations.