Diameter vs pitch
Larger diameter moves more air/water per turn — more thrust and load. Higher pitch moves it faster — more theoretical speed and load.
Start from pitch speed
Compute the theoretical ceiling in the Pitch Speed Calculator: RPM × pitch / 1056. Target a pitch that gives headroom above your desired speed.
Match the motor
Higher-pitch or larger props load the motor harder — verify with the motor manufacturer's prop table. For a geared setup, convert with the Propeller RPM Calculator first.
Boats
Boat props slip heavily — pitch speed is a ceiling, not a boat speed. See pitch explained and the boat calculator.
Safety
Never exceed the prop's rated RPM. Balance new props before first run.