I have a 15.5 ft fiberglass bass boat. It is a small boat and I have a 70hp Johnson on it. The motor runs great idles smooth does not cut out. I have played with my tilt and have the motor where it should be. My tach does not work so I have no rpms but the motor seems to be struggling to push a 17 pitch prop. With this prop I am only running between 25-30 mph. The motor had a 21 pitch on it when I bought it and that prop about killed the motor. The 17 pitch prop that I have running is new with no problems. Should I go down to a 15 pitch or 13 pitch. I know this would be a guess without rpms. I just want an educated guess from someone smarter than me. Thanks