QUOTE: A single prop would have needed very large blades, and the tips would have been very close to the fuselage and rotating with a velocity which was very close (if not exceeding) the speed of sound. Contra rotating props give the power needed at a shorter diameter.
The Griffon has similar ratings to late R-2800's on the DC-6. The wide bladed props on that and CV-440 kept the diameter down and there was always the option of going to a four-bladed prop too as on the DC-7. None of those had any sonic tip problems although the C-46 did.back in its day
I understand why the big turboprops as on the A400 and TU-95 Bear use them. That much power can't be handled by a single rotation prop. I do notice the C-130 (smaller engine) continues to truck on with single-rotation props although they now look like old-fashioned electric fan blades. Anyway, America never did field a successful production contra-prop.