The rudder it self was not the main problem.
One of the problem was the center propeller.
When engines where reversed, the center propeller was out of function, and redused the flow to the rudder.
The turbine where not reversable.
The senter propeller where powered by a stemturbine that used the exhaust steam from the main steam engines. The steam engines rotated the port and starboard proplellers and where reversable. 
Are we talking about the famous 14th April 1912 or in general? I still maintain that the rudder was sufficient.
I think so because I doubt that H&W would make such a mistake to underrudder their ship. Other ships was build the same way.
I know that the turbine was cut on the night of 14th April 1912. That was part of the procedure to go "Full astern"
But consider this: There was around 38 seconds from the spotting to the hit. In these 38 seconds the engineer had to bring down the steam, engage the change over walve to disengage the turbine and then engage the full reverse.
Turbine engine had no breaks. How long time would it take for it to go from 178 (?) revolutions per minute to just "windmill" in the slipstream? I doubt that the turbine had come to a halt or "windmill" speed within 38 seconds. I don't know it for a fact but I doubt it.
Also the Titanic still had forward momentum and this still had a flow of water over its rudder. In my opponion the wing propellers was too far away from the rudder to affect it when going full stern.
I agree that it was not designed to turn like a modern speedboat and that flow over the rudder should be considered, but I don't think it was underruddered. I also agree that the more speed the more effective the rudder.
