You're correct, 20'shaft. Im gathering from your collection of props that it doesn't really matter what size prop you use, just whatever one gets you as closer to your ideal w.o.t with average load. which in this case should be 5750 rpms. I am going to try to john/rude 13.5 dia instead of the michigan 13.75 dia. maybe it will make it a little better. Your striper is really close in weight to my 19 arima so your info is very valuable. Im not looking for any special performance, I just have the health of the motor in mind! Thanks all for the input. T.Fox