I have been running a 1965 Merc 500 longshaft on my 14 footer for the past 6 years. It has a 11P merc prop and I experience no ventilation when turning or under WOT. I get 5400 RPMs and 30 MPH. Well, I just rebuilt a 1968 Merc 500 longshaft and installed it on the boat. According to old manuals...