Page 271 - Introduction to Naval Architecture

256 MANOEUVRING

The turning circle
The motion of a ship turning in a circle is shown in Figure 10.1,
As the rudder is put over there is a force which pushes the ship
sideways in the opposite direction to which it wishes to turn. As the
hydrodynamic forces build up on the hull the ship slows down and

Figure 10.1 Turning circle

starts to turn in a steadily tightening circle until a steady state speed and
radius of turn is reached. A number of parameters are used to define
the turning performance. They are:

(1) the drift angle, which at any point is the angle between the ship's
head and its direction of motion. This varies along the length,
increasing the further aft it is measured. Unless otherwise
specified the drift angle at the ship's centre of gravity is to be
understood.
(2) the advance, which is the distance travelled by the ship's centre of
gravity, in the original direction of motion, from the instant the
rudder is put over. Usually the advance quoted is that for a 90°
change of heading although this is not the maximum value.