The correct propeller size is one that achieves your boats maximum designed speed while operating within your engines maximum RPM range. There are many propeller styles available to confuse you but it all comes down to this. What propeller achieves your needs? Generally for maximum speed a three blade is your best choice. In some boats a four blade will be a good choice as they perform well and are smoother and offer better acceleration. For boaters that require maximum acceleration with out a significant speed loss the mercury high five is a good choice. In summary it really comes down to you and what your expectations and budget are.