I got a new Michigan Wheel Vortex XHS 13x19 prop to test, thinking the newer style could get me a few more 10th of a mph but lost over a half mph.
Have also tried a 13 x 19 SST prop and got the same mph as my old style OMC prop.
If I can find a 21P locally for cheap I might give it a try, but think I'll try other brands of 19P.
By old style I mean the old OMC with the convergence hub on the end ... see picture...to hard to explane ...LOL
Dave found it for me, $35 rebuilt and the fastest prop I have tried to date.
What brand is your 21P ?
First picture is rebuit OMC 19P with convergance hub , best one so far.
Second picture from left to right, OMC 17P alm. with convergance hub , OMC 19P alm. with convergance hub and last is a OMC 19P SST.