I have a 1985 boat with a 3.8 OMC I/O. I live at an elevation of about 4500 feet. At this elevation, my boat maxes out at about 32 mph. I went to a lake two weeks ago at an elevation of 7600 feet. My boat maxed out at about 26 mph. The prop on my boat now is a 15 x 17. I'm considering a...