Bang on course even at high speeds
The twin rudders – each of which is smaller – underwater have the advantage that they keep the ship on the right course more reliably at high speed and that the leeward rudder in particular continues to work effectively, while a rudder located in the middle projects far out of the water and very quickly becomes inefficient. We are familiar with this from earlier generations of very wide sailing yachts: the normal consequence of this is a tendency to broach. The boat gets out of control, because the rudder no longer works, and heads towards the wind in uncontrolled fashion.
Bow and stern thrusters for greater manoeuvrability
For every advantage there is a disadvantage. In the case of twin rudders, manoeuvrability when motoring is what is affected. A ship can be turned most effectively and within the smallest of spaces when the propeller engages the rudder directly. This is not possible with twin rudders. On modern and, in particular, larger yachts, this is compensated for by bow thrusters – to an increasing extent combined with stern thrusters, so that these yachts can be docked at right angles to the direction of the ship – almost like a ferry – at least when there is not much wind.