Your engine has a rated maximum rpm. On the ground you lose about 15-20% of the rpm’s a prop will turn in the air due to reduced drag on the prop. In the case of the ST 75 red line is 16000 rpm. You want a prop that turns about 80% of that on the ground, or approximately 13,000 rpm after the engine is broken in.

Some engines produce maximum hp at much lower rpms than the maximum rpm. On those engines, this should also be taken into account. In the case of the ST 75 the max hp is at 15,600 and you can pretty much treat the hp and max rpms as the same.

The pitch on a prop is the number of inches it will move forward in one revolution. An 8 pitch prop will move forward 8 inches, in the air, not taking minor losses due to slippage into account. A 10 pitch prop would move forward 10 inches.

In order to keep the rpm where you want it, you adjust the diameter of the prop. A low pitch, larger diameter prop will produce more thrust (what you need for hovering or vertical performance). A higher pitch, with a smaller diameter will produce higher speed. More thrust also means a shorter takeoff run, while more speed means a longer takeoff run.

There are empirical formulas that predict which prop to use for what application you desire. One website that does this for you is located at http://pages.sbcglobal.net/limeybob/ . There is also an online manual that gives much more information and explanations of the formulas.