People are confusing the term efficiency. The differance between a two blade prop and three blade is very low, 1 to 4 percent. However having the right match for the airplane is a differant issue. To keep the same diameter with a three blade you have to reduce pitch by about 1 inch. If you found the perfect two blade prop this probably wouldn't work as well. The perfect match would probably be to reduce diameter by 1/2 inch and increase by 1/2 inch, but you won't find such a prop.

Don't believe me ask this guy.

http://www.djaerotech.com/dj_askjd/d...fourblade.html