Lifted from another site:
Cavitation is often confused with ventilation. Cavitation occurs when there is an extreme reduction in pressure on the front side of the propeller blades. Under normal conditions, water boils at 212 degrees, but if you reduce the atmospheric pressure sufficiently, water can also boil at room temperature.
As your propeller begins turning through the water at an ever-increasing rate of speed, the pressure on the front side of the blades is reduced, and if that pressure is reduced low enough, the water will begin to boil and form water vapor on the blades. This usually occurs near the outer or leading edge of the blade.
The water vapor bubbles migrate toward the center of the blade, where the pressure is higher, and the boiling stops. The vapor bubbles implode against the blade’s surface. The resulting energy release is so great that it can chip away at the blade surface, leaving a cavitation burn.
There are numerous possible causes of cavitation. Nicks or damage to the leading edge of the propeller blades are one of the most common causes. If your propeller cannot cut through the water smoothly, it will cause disturbances in the water flow, and many times will result in cavitation.
Ventilation is the result of air or exhaust gases being pulled into the propeller blades. This causes the blades to lose their grip on the water, the engine RPMs go up and the forward speed of the boat is reduced.
Sometimes when the propeller RPMs increase, it can also cause a massive cavitation, which only compounds the problem. Most outboard and stern-drive engines have anti-ventilation plates made onto the lower unit housing directly above the propeller. Many people mistakenly call them cavitation plates or possibly anti-cavitation plates, but that is incorrect.
The anti-ventilation plate does not stop or help prevent cavitation. Its sole purpose is to prevent surface air from being sucked into the negative pressure side of the propeller blades.
Ventilation can also come from exhaust gases being introduced into the blades of the prop. Most propellers have some sort of ring around the trailing edge of the exhaust hub. This ring creates a high-pressure barrier that prevents exhaust gases from being sucked back into the blades.
Other sources for ventilation include the leading edge and sides of the motor’s lower unit and the bottom of the boat. Inspect your lower-unit housing for any imperfections in the surface directly in front of the propeller. Knicks, gouges, scratches or barnacles can all disturb the water flow to the propeller, and cause ventilation.
