reelapeelin: I hooked up my merc tach yesterday and ran the 3 blade aluminum 15.5 by 17 and it was a real eye opener. Red line is 5,600 for the Merc 150. I ran it up to a little over 5,400 and the GPS speed was right at 41 MPH solid with a half tank of gas , trimmed up and just me. I am now using the 4 blade comprop as a spare. I think I am going to leave well enough alone...Many thanks for your prop advice , Frank