Re: best prop
We need to know the year of the boat,the brand and year of the motor.
If it has a prop you should do a test run and get us the prop size (Usually near the base of the blades or under the prop nut)
the wot rpm and gps speed.Some cells have a gps app.
If I had to guess with no real info I would think a good starting point would be a 19" prop.Probably under sized but better under sized than over. Then you can evaluate your setup and select a prop.