best to give propsol a call. as far as im aware if you increase the diameter and reduce the pitch the revs should change ie 15x15 prop on a 270 leg with a 2.3 sierra engine giving 67hp with a 2.15 reduction was revving to 4200 and changing it to a 15x17 dropped the revs to 3200 and increased the speed by a knot we can all have our own ideas about things but sometimes it is best to seek help from the experts .