[Fr. Frank]

Originally Posted by uncleboo

Anti cav plat about 2-1/4" above keel.

That's about right, but JFTR, that flat surface on the lower unit above the prop? It's an anti-ventilation plate, not an anti-cavitation plate.

Here's a greatly simplified definition of each:

A propeller CAVITATES at high rpm's or under heavy load when the blade also has a high coefficient of lift. When this happens, the pressure on the inlet side of the blade can drop below the vapor pressure of the water, resulting in the formation of a pocket of vapor against the propeller surface, which can no longer effectively transfer force to the water. The air bubbles which make up the vapor are also destructive to the strength of the metal over a relatively brief period of time, causing pitting even in stainless steel.

A propeller VENTILATES when air is introduced to water around a prop, or to the water flow fed to a propeller. This is usually cause by a portion of the propeller blade surfacing beyond it's ability to carry water against the thrusting surface of the propeller during it's travel above the surface of the water. This is the very effect propeller cupping is designed to defeat. Ventilation can also be caused by propeller porting, or by improperly placed or mounted underwater gear such as transducers, pitot tubes and through hull devices.