changing prop

[48tony]

hi,I am planning on changing my prop size.I am running a aluminum 13 3/4 x 15 on a 1970 60 hp johnson on a 14 ft fiberglass boat.The motor is turning about 4700 rpm at wide open at 30 mph.I am looking at the stainless power tech prop.I am a bit unsure what to drop my pitch to.Should I try a 11 pitch or would I be better off going to a 13 pitch?The boat is on the heavy side do to all my gear 2 batt 6 gall tank small tool box fish gear.I would like to see the motor turn about 5500 as the book states between 4500-5500 rpm.This is kind of like black science to me and I am looking for some help Thank you Tony