This is a thought I asked myself often, but never did real efforts to get an answer. Barsmonsters question about number of fans of a wind turbine made me think of it again
Why do blades of aircraft propellers or wind turbines cover only a small fraction of the area they circle? Propellers of ships or Kaplan turbine propellers cover almost the entire circle. (Independent of number of blades). Same for steam turbines and jet turbines.
I am shure that the forms of those propellers are very close to optimum, due to decades of experience.