The circle is the path of the ship’s pivot point as it executes a 360° turn. In shallow water, the rate of turn is likely to be decreased, so the vessel will have a larger turning circle.
Once trials of a new ship are complete, operators will need to know how the vessel can expect to perform in a variety of sea conditions. The ship handler, for instance,should be aware of how long it will take for a vessel to become stopped in the water from a full ahead position or how far the vessel will advance in a turn.
Turning circles and stopping distance (speed trials) provides such essential information to those that control today’s ships.
Merchant ships usually turn in a circle having a diameter of about 3–4 times the length between perpendiculars (LBP). The larger the rudder, the smaller will be the Turning circle diameter(TCD). During the TCD manoeuvre, the ship will experience transfer, advance, drift angles and angle of heel.
Advance – Defined by the forward motion of the ship, from the moment that the vessel commences the turn. It is the distance travelled by the vessel in the direction of the original course from commencing the turn to completing the turn. It is calibrated between the course heading when commencing the turn, to when the vessels head has passed through 90°.
Transfer – Defined by that distance which the vessel will move perpendicular to the fore and aft line from the commencement of the turn. The total transfer experienced during a turn will be reflected when the ship’s head has moved through a course heading of 180°. The amount of transfer can be calibrated against the ship’s change of heading and is usually noted at 90° and 180°.
Tactical diameter – Is defined by the greatest diameter scribed by the vessel from commencing the turn to completing the turn.
⦁ When the vessel is trimmed by the stern, the tactical diameter of turn is increased
⦁ When the vessel is trimmed by the head, the tactical diameter of turn is decreased
⦁ The vessel with a list will take longer to execute the turn, and when turned into the list, will develop a larger circle
⦁ The vessel with a right-hand turn propeller, if making a turn to port, will end up with a smaller diameter than starboard due to the effect of transverse thrust
Final diameter – Is defined as the internal diameter of the turning circle where no allowance has been made for the decreasing curvature as experienced with the tactical diameter.
Drift angle : it is the angle between the axis of a ship when turning and the tangent to the path on which it is turning.
The factors affecting turning circle of ships are :
- Draught and trim .
- Distribution and stowage of cargo .
- Even keel or listed
- Rudder angle.
- Available depth of water
- Drift angle and influencing forces.
- Structural design and length