here is a video on how well a fishtail rudder can work.
Correct all rudder I have a bow thruster but don't use it much except to get away from the dock.
The backside of the fish tail is pointing way more than 90
"But I'm sure the extra drag of a fish tail rudder capable of the above performance would be noticeable."
"Seems to me the "fish tail" rudders frequently or most often have a wide rudder section in the vicinity of the rudder shaft. What role does that streamlined but fat part of the rudder play (if any) in the turning capabilities of the fish tail rudder?"
The bigger the rudder(s) the more wetted area. The more wetted area, the more drag. The more drag, the higher fuel burn. Comments?