'89 gt100 johnson outboard prop?

[lyncraft_mechanic]

I have a 1989 gt100 johnson outboard on a Tyee 5.5 Lund boat and it is equipped with a 13 3/8 x 17 pitch prop.(cupped) my average speed is roughly 38MPH. I just bought the boat this year and would like to replace the prop. because it has alot of chips and dings out of it! I was wondering what I could replace it with for a little more speed, the only problem is i don't have a tachometer, any suggestions?