My 20' Glasply came with a 1.65 ratio 165 Merc stern drive with an 18P prop mated to my 1981 470. My boat will not go over 3800 RPM at WOT but runs nicely at 3000 rpm. Its when I go above 3000 rpm that things go awry. It runs rough and surges. Could this be due to the wrong gear ratio / prop pitch? The 470 stern drive's have a 1.84 gear ratio so maybe I should replace the whole drive or just the gears? Hopefully a new prop will be all that's needed!
