So in a nutshell, the two numbers on the prop are the diameter (1st number) and the pitch (second number) of the prop. Ideally, you want a prop with the same diameter as the one that was previously on the motor...assuming that was a good fitting prop. There are no numbers anywhere on the old prop? On the hub? Under the nut? On the side of prop? They may be hard to read or see, but there should be a number somewhere. Can you take some pics of the old prop?
Once you find one that is the correct diameter for the prop shaft (the 13 3/4 number on your new prop) the correct pitch is determined by the WOT rpm's of your boat with the prop compared to your motor's recommended max rpm's. As a general guideline, each inch you change the pitch of a prop, your motor will lose or gain about 200 rpm's. If you go down in pitch one inch (from 15 to 14) you would gain around 200 rpm's to your WOT rpm's. If you go up an inch (from 15 to 16) you would lose about 200 rpm's from WOT rpm's. Ideally, you would like the boats WOT rpm's to be as close to--without going over- your motors recommended WOT rpm. Make sense?
