The IMO resolution MSC.137(76) “Standards for Ship Manoeuvrability” identify the following characteristics:
- Inherent dynamic stability - A ship is dynamically stable on a straight course if it, after a small disturbance, soon will settle on a new straight course without any corrective rudder action. The resultant deviation from the original heading will depend on the degree of inherent stability and on the magnitude and duration of the disturbance.
- Course-keeping ability – The course-keeping quality is a measure of the ability of the steered ship to maintain a straight path in a predetermined course direction without excessive oscillations of rudder or heading. In most cases, reasonable course control is still possible where there exists an inherent dynamic instability of limited magnitude.
- Initial turning/course-changing ability – The initial turning ability is defined by the change-of-heading response to a moderate helm, in terms of heading deviation per unit distance sailed or in terms of the distance covered before realizing a certain heading deviation (such as the “time to second execute” demonstrated when entering the zig-zag manoeuvre).
- Yaw checking ability – The yaw checking ability of the ship is a measure of the response to counter-rudder applied in a certain state of turning, such as the heading overshoot reached before the yawing tendency has been cancelled by the counter-rudder in a standard zig-zag manoeuvre.
- Turning ability – Turning ability is the measure of the ability to turn the ship using hardover rudder. The result being a minimum “advance at 90° change of heading” and “tactical diameter” defined by the “transfer at 180° change of heading”.
- Stopping ability – Stopping ability is measured by the “track reach” and “time to dead in water” realized in a stop engine-full astern manoeuvre performed after a steady approach at full test speed.