Editing Propeller (section)

===Cavitation===
{{main|Cavitation}}
[[File:Cavitating-prop.jpg|thumb|left|Cavitating propeller in [[water tunnel (hydrodynamic)|water tunnel]] experiment]]
[[Image:Cavitation Propeller Damage.JPG|right|thumb|[[Cavitation]] damage evident on the propeller of a personal watercraft]]
[[File:Propeller & anti-cavitation plate & Schilling rudder.jpg|thumb|Bronze propeller & anti-cavitation plate, & [[Schilling rudder]] (on a river barge)]]

[[Cavitation]] is the formation of vapor bubbles in water near a moving propeller blade in regions of very low pressure. It can occur if an attempt is made to transmit too much power through the screw, or if the propeller is operating at a very high speed. Cavitation can waste power, create vibration and wear, and cause damage to the propeller. It can occur in many ways on a propeller. The two most common types of propeller cavitation are suction side surface cavitation and tip vortex cavitation.

Suction side surface cavitation forms when the propeller is operating at high rotational speeds or under heavy load (high blade [[lift coefficient]]). The pressure on the upstream surface of the blade (the "suction side") can drop below the [[vapor pressure]] of the water, resulting in the formation of a vapor pocket. Under such conditions, the change in pressure between the downstream surface of the blade (the "pressure side") and the suction side is limited, and eventually reduced as the extent of cavitation is increased. When most of the blade surface is covered by cavitation, the pressure difference between the pressure side and suction side of the blade drops considerably, as does the thrust produced by the propeller. This condition is called "thrust breakdown".  Operating the propeller under these conditions wastes energy, generates considerable noise, and as the vapor bubbles collapse it rapidly erodes the screw's surface due to localized [[shock wave]]s against the blade surface. 

Tip vortex cavitation is caused by the extremely low pressures formed at the core of the tip vortex. The tip vortex is caused by fluid wrapping around the tip of the propeller; from the pressure side to the suction side. This [https://www.youtube.com/watch?v=GpklBS3s7iU&feature=PlayList&p=218220F6C5BD650E&playnext_from=PL&index=18 video] demonstrates tip vortex cavitation. Tip vortex cavitation typically occurs before suction side surface cavitation and is less damaging to the blade, since this type of cavitation doesn't collapse on the blade, but some distance downstream.