hi ya Dave
try and keep it simples
Imagine, as the propeller turns - one blade always cuts fresh water and moves the boat forward one distance of the props pitch.
now, with slip of the prop and cavitation the next blade doesnt get quite the same bite of the water and soforth as the prop moves around.
That is why, if you look at high speed props, how the prop is kept small in diameter and is very coursely pitched.
Best way, is to put model in the bath, run motor, hold boat and put hand behind the prop - MAKING SURE YOU KEEP YOUR FINGERS OUT OF THE WAY .
aye
john