Turning Characteristics The standard method of finding any ship's turning characterist is to turn her in a number of complete circles under different conditions and to record the results for each turn. The variables used are (1) right or left rudder of various degrees, (2) steady speeds of different value, and (3) differences in draft and trim. The effects of wind and sea must be wrote down and allowed for. Most turns, are not as much as 360 degrees, but by looking at the complete turning circle, the ship's behavior for turns can be predicted. Here are some common terms used to define turning characteristics.
Turning Circle - The path followed by the pivoting point of a ship in making a turn of 360 degrees or more. For the ordinary ship, the bow will be inside, and the stern outside this circle.
Pivoting Point - That point about which the ship turns when the rudder if put over.
Advance - The distance gained in the direction of the original course.
The advance will be a maximum when the ship has turned through 90 degrees.
Transfer - The distance gained at right angles to the original course when the ship has turned through 90 degrees.
Tactical Diameter - The distance gained to the right or left of the original course when a turn of 180 degrees has been completed.
Final Diameter - The distance perpendicular to the original course between tangents drawn at the points where 180 and 360 degrees of the turn have been completed. Should the ship continue turning with the same speed and rudder angle, she will keep on turning in a circle of this diameter, which will always be less than the tactical diameter.
Kick - The distance the ship moves sidewise from the original course away from the direction of the turn after the rudder is first put over. The term is also applied to the swirl of water toward the inside of the turn when the rudder is put over to begin the turn.
Drift Angle - The angle at any point of the turning circle between the tangent to the turning circle at that point and the keel line of the vessel.
The turning circle is the path followed by the pivoting point during the turn. The pivoting point is in the horizontal centerline of the ship, and its position on that line depends on the shape of the underwater hull.The pivoting point moves forward if the ship is trimmed down by the head and moves aft if it is trimmed down by the stern.
The pivoting point may also move aft along the keel line to some extent if the ship is deep in the water and forward if she is light. It is normally in the forward one-third length of the ship. When this is determined, it does not vary enough in the ordinary ship under different conditions of load and trim to cause any problems in shiphandling.
