RE: "Unloading a prop"
This subject has already been covered well, I just want to add that the so called correct prop for your engine doesn't take into account the airframe and flying style you prefer. For years I used 10x6s without question on my .40 sized sport planes. I switched to a 10x4 and everything about the plane I was flying improved greatly, and the all out speed difference wasn't very noticeable.