First question you have to ask yourself is, how does the boat run now with the 15P Prop? Does the motor run within its specified WOT RPM range when correctly trimmed and lightly loaded with the 15P? You don't mention the motor, so we can't tell you what the recommended RPM range is for your rig. In broad strokes, if you step up to a 21P prop, you will drop your RPMs significantly; like on the order of about 1000 RPMs. If you are within your recommended RPMs with the 15P, then the 21P will not do a thing for you except to lug your motor and cause it to die an early death. You would also have a harder time getting your boat up on plane.