IN Prop numbers, the first is teh prop diameter, 11=11" diameter

the second number is teh pitch, or the amount of 'bite' the prop has in teh air. the higher the number, the more 'bite'.

Ideally, the second number indicates how far the prop would travel in one rotation. So, the 11-7 would travel 7" forward. At least taht's teh idea. Also, the prop tip travels faster than the center, so the 'angle of attack' is les than at teh vcenter, where the prop is not turning as fast as teh tips. But air can be a funny thing, and swish around in eddies, currents, turbulence, and vorticies. all these add to the 'inneffeciencies' of teh prop.

But with more bite comes more drag. and that stresses the engine. Changing to big a diameter, and/or too much 'bite' and the engine will be under powered. Same is true for an undersized and/or little pitch. the engine has no resistance (load) and works overtime!