I have an 04'640LX with an 05' 75hp Optimax that I bought new. Mine also came with a 19" aluminum prop and was a ***** to drive. Even with the motor tucked all the way on takeoff the bow would rise to the sky for about 30 seconds or more. The boat would porpoise so bad you had to trim and slow way down and try again to get it to stop. With some work it now gets compliments from friends that ride in it as to how well it rides and handles for a 16' boat. First get rid of the old aluminum prop and replace it with a 20" Laser II with 3 medium vent plugs in it. Raise the motor to the next to the highest mounting holes. Try it you'll like it. Mine runs 46mph GPS@5800 rpm all day long with that set-up.
