rule of thumb drop 1" of dia.for each blade added the OS 91 will spin a 15" prop in the rec. range the 14/6 will spin up to 10,500 the 3 blade gets about 500 less rpm but I seem to get alot more speed out of it with the higher pitch prop at a lower rpm + I like the look on the ground I dipped the tips in wt paint the 3 blades come with no painted tips MAS has a pretty big selection of prop sizes I havn't settled on one particular size just trying diferent ones out to see what performance I get