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