prop size 18 seacraft
 

I have a 1975 18ft Sea Craft original. I recently purchased a Suzuki df 175 from and individual that was on a small party barge. What prop size should I use. I have tried a 16x18.5 a 15x19, tact's at 6200 but 25 miles an hour.

sorry forgot I posted the question already. New to posting I mainly read. This is the 5th Sea Craft in our family. really need a 20 or 23. Thinking about selling the 18 an stepping up.