I have a 15' open bow tri-hull which had a '69 Merc 65hp O/B on it. I had a 15 pitch prop and the boat rode great (30 MPH by GPS). I recently put a '78 Merc 80hp on the same boat. First I started with a 19 pitch but couldn't get over 4800 rpm, so I replaced it with a 17 pitch (getting 5200 rpm now). 5200 rpm with a 17 pitch for 80hp on a 15' seems low. Other thing (much stranger) is the way the boat rides now. First, it hooked to the right something awful. A trim tab adjustment later, and that's OK. Thing is the boat is listing heavily to port when it planes and kicking up a lot more wake than it used to. The motors are virtually identical externally and I have the cavitation plate dead even with the bottom of the hull. I've tried adjusting the tilt, and other than affecting bow-rise (as expected) it does nothing for the list. According to any measuremaents I can come up with, the motor is centered and square to the boat. What gives? Sorry for the lengthy entry.