I don't have a 115 on my 170. But I do have a sweet running 90 evinrude. And I have played a lot with props. You should stick with a 17" pitch. I would use a factory style OMC SST 13 7/8"x17" prop. They perform good and and are priced right. Do you have any needs you want to change that a prop may be able to cure? Holeshot, porpoise, over-reving, ventilation/cavitation?