trolling motor prop

[shuorc]

After a recent rainstorm i noticed my trolling motor(motorguide 30) running and the prop is now 3/4" shorter on each blade(2 blade Ninja) went to ww and they had 2 and 3 bladed props each with it's reason for being better. Went to the flea market today and there was a 4 bladed prop again saying how good it was-- it was indeed good price wise $5.00, so which is the better prop to use? 2, 3 or 4 blades? am trying to spider rig for speckled perch(crappies).--shu