I have a 18' fiberglass 1969 larson with a 120 hp 2.5L 4cyl inboard. I have a couple of different props for it a 14, 15 and a 17 pitch which would be the best one to use?? I have the 14 on now and with two people in the boat it gets up ok and does about 25 mph at around 4500 rpm, with 4 poeple...