RE: Cool
Hate to say this cause usually I agree with Bill, but the effeciencies of a single blade prop has nothing or little to do with disturbed air. As the aircraft is moving foward or even on the ground the air that the prop has disturbed is moving behind it, that is one reason why you have to add right rudder on take off, the disturbed air is spiriling around and hits the top left side of the rudder turning the plane to the left. Since the disturbed air is behind the plane the next prop blade is in clean air no matter how many blades are used. The reason less blades are more efficient are two fold. The drag of the blade tips are very large, more blades more loss, the area where the prop goes into the spinner is another smaller loss that is affected by the number of blades. In fact if you use a ducted fan this loss is greatly reduced and you can increase thrust by adding blades right on top of each other such as used on jet turbine engines. The other reason is as downunder described, the more blades the smaller the prop arc, the area of the prop arc is more important than the blade area for generating thrust. More area is more efficient. However sometimes when going really fast, such as around 400 hp you have to reduce the disc diameter to gain more air velocity and add more blades, though most multi blade props are used simply for prop clearance.