I wonder how contra-rotating schemes compare to something like PBCF? https://www.youtube.com/watch?v=a8PjWSeLUzY https://www.youtube.com/watch?v=a8PjWSeLUzY I

I wonder how contra-rotating schemes compare to something like PBCF? https://www.youtube.com/watch?v=a8PjWSeLUzY https://www.youtube.com/watch?v=a8PjWSeLUzY It seems like it should achieve almost the same de-vortex efficiency gains, but without all that added mechanical complexity, skin drag, and underwater noise (PBCF is actually quieter than having no fuel-saving device). Honestly I'm surprised Sharrow doesn't already use it on their regular toroidal propellers. I notice a lot of hub vortex in their underwater videos...