Vessel’s Characteristics

Turning circle and terms

Advance

The distance traveled in the direction of the original heading measured from the point at which a helm was first applied. The maximum advance is usually between 3 to 5 ship lengths for a merchant ship of any size going at full ahead and using full helm.

Transfer

The distance of the center of gravity of the ship from the original track line measured in the 90º to the original heading. The transfer for a turn of 90º is about two ship lengths

Tactical Diameter

It is the transfer for a turn of 180º which is almost equal to the maximum transfer. It is usually about the same distance as the maximum advance.

Drift Angle

It is the angle between the tangent to the turning circle at any point and the fore and aft line of the ship.

Pivot Point

It is the point about which the vessel pivots with the bow swinging inwards and the stern swinging outwards. It is about one third (1/3) of the vessels length from forward when going ahead. When going astern the vessel pivots about approximately one quarter of the length from astern.

Final Diameter

It is the diameter of a circle traversed by a vessel after turning through 360º and maintaining the same speed and rudder angle. This diameter is always less than the tactical diameter. It is measured perpendicular to the original course and between the tangents at the points where 180º and 360º of the turn have been completed.