Hey Sal,
I just put a 70hp Yamaha 4-stroke on a 1978 170 and made a couple runs. I'm still messing with the prop configuration but will try to get back to this thread as I have updates. A few things that may give you some direction in the mean time:
WOT for that engine is spec'd at 6300rpm tops. The Coast Guard rating on the hull says 1235 lbs max. I loaded up the boat with jugs of water, a couple coolers full of water, kettlebells, etc to bring the total with motor, fuel, persons, etc to 1500 lbs and took her out a couple weeks ago on a very calm day. Trying to get a 'worse' case for the torque I need from the prop to get on plane. Obviously, it took a bit longer to get planed out than with a more appropriate load but still did fairly well. On plane, was able to rev the motor beyond the Yamaha rating still by a smidge. Tack topped out at 6500rpm. This test all with an aluminum 13 5/8 x 13 pitch.
This would indicate that I can go up in prop pitch which will hurt my hole shot but keep the WOT below max. I'm trying to locate a 15 pitch to try.
