Hugh,

This is very true.

The assumption that more blades create airflow patterns that interfere with each other, created the counter-weighted, single blade, control-line speed props.

Even with migratory birds, this is untrue and the airflow from the the bird flying diagonally in front of another, actually aids the rear bird.

Some WW2 planes, with powerful engines resorted to 5 blades (Supermarine Spitfire XII, Hawker Tempest and Fury).
Many modern turboprop planes, the C-130K among them, have very efficient 5-12 blade props.

And nobody said anything about the "inefficiency" of the multi-blade configuration. APC and other manufacturers have the ability to make efficient props, with as many blades as they want.