Though air can be considered a fluid for all practical purpose, there is no cavitation, becuase there is no fluid-gas transition.

It is not my intention to give a complete lecture on aerodynamics, one question at a time. Check out http://www.aerodyn.org/Propulsion/propeller.html and zillions of other publications where you will find your answers.
For the quick and dirty work, use [link=http://mvvs.nl/prop-power-calculator.xls]my thrust calculator [/link]
You will find most answers there, % relationships included if you program the two engines with different data.