You can only swing a 19" blade? Are you running a cage or what? Rails? (I am guessing that would be a no)
If your Cat is tired drop in a Sisu, slap a 2:1 on it and wind the pitch to your blade if you are limited on the diameter. Adding blades won't give you more speed or power, just a somewhat smoother arc through the water as the propeller turns.
When was the last time the bottom was done? And by done I mean removal of all paint down to the original gelcoat, smoothed and repainted. You might be surprised by the results.