Here is the calculator we used to calculate the maximum thrust and size of engine required:
http://personal.osi.hu/fuzesisz/strc_eng/index.htm
As others have said the pitch of the props have no effect on static thrust, only the maximum speed at which the craft will be traveling it, and the pitch have enormous effect on the engine. To other teams attempting this method of propulsion, use the smallest pitch you can find (12.25x3.75 was the smallest we found). It will provide the same thrust and require less power.
Due to a loose belt during our first test run, we couldn't run the prop up to full speed
Spinning that blade at 10k rpm according to our calculator (from above) we will get about 6lb of thrust (12lbs total with 2 of them). Is 12 lbs of thrust enough to move the robots?
Our other possibilities for props are using a 28x4 (still havn't found a low pitch prop this big) spinning at much lower speeds to get more thrust. and one more question if anyone knows this, besides requiring more power, is there a reason to use a 2 prop vs a 4prop blade? Using a 4 blade 28x4 prop spinning at 3500 r, according to this calculator (above) would provide nearly 35lbs of thrust