Hello,
Having made some trials of rotation of Titanic, full rudder, the speed settled to appr. 14.2 kts (some 7.30 m/s), and the rate of turn became stable at 58°/min (6 min and 20 s for a full turn), that means a tactical diameter of some 3.4 lengths, 880m.
I think having read it was 3850ft (1173m), so she turns indeed somewhat (1/3) too quickly, perhaps is it due to a rudder angle of 45° instead of 35°?
Regards,
Luc