This would best be asked of the Marine Trader Forum.
Note however that what was on the boat when new will NOT be suitable now without doing some serious checking to ensure you do not overload the engine with too much prop.
You will need a handheld laser pointer digital phototachometer along with some of the reflective tape. E-bay for ~ $50.00. Put a piece of Tape on the crankshaft dampener and when out on a run use the tach to shoot the tape on the dampener. Put one stripe of tape on the whole width of the dampener so it is easy to aim at. Not a tiny little dot of tape.
With a helper to run the boat take it out for a run and push it to wide open throttle [WOT] only for long enough to get a stable rpm reading and a boat speed reading. If you are bucking current or a lot of wind you will have to reverse course to average out the boat speeds. While reversing course, if needed, slow down untill ready again.
These tachs are accurate to a few rpms. Your dash tach is likely accurate to a few hundred rpm. Accurate readings are important for good results.
You will also need:
- the boat weight fully loaded as you will cruise
-boat length at water line, approx.
-the gear ratio
-engine hp and rated rpm
-existing propellor, diameter, pitch and #of blades
-boat speed now with the existing prop
Then a good prop shop can come up with a good recommendation. Otherwise you are flying totally blind and it could be expensive.
There are also online calculators which can guide you.
-Boatdiesel has one although you will need to join for $25.00 - well worth it
Keep in mind though that good as calculators are they are a guide only and must have good info.
So to start get the tach and test your boat as it is now.