Marine Log reports that,
“Wärtsilä has signed an agreement that will enable it to integrate patented gate rudders into its propulsion product designs. Unlike the traditional arrangement of the rudder in the propeller slipstream, the gate rudder is a twin arrangement around the propeller, allowing improved fuel efficiency and reduced emissions. The concept is applicable to all current vessels equipped with conventional propellers.”
“…the rudder will realize synergies in capital and operational savings for ship owners by increasing fuel efficiency, improving maneuverability and course stability in both calm and rough sea conditions, while also reducing noise and vibration.”
You can see from the photo, when no rudder is required, that the rudder would not be in the accelerated flow behind the prop, thereby reducing drag, but when more extreme turns are required it would be, making it more effective.
The large bosses where the rudder shaft leave the hull might cause more drag than those of a normal rudder. There are of course two required, and it appears they would have to be more strongly built to deal with the torque resulting from the off-center drag on the rudder.