If your boat is running a bit slow or having trouble getting on plane you may be considering going to a larger propeller but you may actually need to get a smaller propeller to achieve the results you desire. It all comes down to matching the right prop to your motor. If your motor is not reaching the rpms it should under full throttle then a larger propeller will likely make things worse and lead to expensive repairs in the future. In this situation a smaller propeller would reduce load on the motor and allow it to reach higher operating rmps.  This will the allow the motor to make more power and spin the propeller faster allowing it to move more water. In this situation reducing the propellers size will result in higher speed, less wear and tear on the motor and might even improve fuel economy!