I have a 1968 aluminum Crestliner Nordic 17' with a 1975, 115hp Merc, at WOT it runs at 30mph (gps) and 4200 rpm with two men in the boat. I don't care that much about increasing the speed but I would like to get the rpm's up closer to max which is 4800 - 5300. The current prop, and I don't know what brand it is, is an aluminum 13.25 X 17 and I was thinking about going to a 13.5 X 15. I think that will increase the rpm's but not sure if it will be enough. Any thoughts or other ideas? Thanks