this is pretty neat technology.
The rotors rotate by a electrical motor and as a ship sails with the wind abeam, the wind will interact with the rotor and give forward propulsion. It's affect lessen to the ships quarters and is non existing at the head and stern. So, with voyage planning, good winds there can be fuel savings.

http://www.hansa-online.de/maersk-and-s ... i/?p=59588

That's the sort of thing Anthem of the Seas is thinking about having fitted so I hear.
It's when an MV becomes a BM ( for the non technical that's a Blow Merchant )
Not only is it cheaper to run but the savings on Clutch Plates are said to be quite substantial .