I also do not think that thrust hp is that close to the real world. In your example, you will never reach 50mph on a 20x6 at 6300 rpm. The effective angle of attack will have reached zero and actually be negative (pushing) after going over 34mph. That is what thrustHP is telling you with a max speed. I did a small calculation and determined that after such a prop, spinning at that speed, reached 35.795 mph, it would be producing zero thrust. In the real world, however, the prop would encounter less drag the closer it got to this speed, so the rpm will climb. The top theoretical speed with the rpm increase would be somewhat higher.