Usually the data relating to the type of propeller are indicated directly by the site, but if over the years you have made changes or want to make them, here is advice on how to understand if the choice is correct or not. If the propeller is undersized compared to the engine, the result is that it runs too fast at the expense of its efficiency. In practice, it fails to discharge all the power of the engine into the water when running at the speed for which it was designed.
If you have an inline shaft drive and do not feel the boat reactive as you would like, check that the propeller is in line with the axis of the motor and that its inclination is constant. This operation is obviously done with the boat on the reservoir: useful time to check if it is worn. If there are Thoracicas, remove them: they are one of the most common causes of excessive noise.
If you are absolutely not satisfied with the performance, you can even think of cavitation problems. This is caused by the incorrect ratio of engine speed to propeller size, which causes turbulence that is harmful to the blades. Small hyper-comprehensive air bubbles form and when they explode they emit a lot of energy, ruining the surface and the ends of the propeller and causing the propeller to work badly. One way to understand if your’s is well sized is to do some tests by checking the number of turns of the engine.
If the engine doesn’t take all the declared revolutions at full throttle, the propeller probably has too high a pitch. If, on the other hand, even at low speeds the number of revolutions is always high, it may be undersized.