Do you have a Tach? Ideally you want the engine to turn around 5500 RPM at WOT with your normal load. Not knowing the size, type and weight of the boat makes it hard to determine what size prop you should be using.
A 13" prop sounds like maybe a pontoon or a brute of a boat.?
If you read "read this first" at the top of the page it will help you get us the info we need.