//SYS21///INTEGRA/BST/VOL2/REVISES 31-7-2001/BSTC13.3D ± 553 ± [539±589/51] 30.7.2001 3:52PM
Manoeuvrability 553
Fig. 13.9(b) Presentation of spiral manoeuvre results (unstable ship) T H E P U L L -O U T MA N O E U V R E
This manoeuvre is used to determine the directional stability of a ship. The rudder is put over to a predetermined angle and held. When the ship is turning at a steady rate the rudder is returned to amidships and the change of rate of turn with time is noted. If the ship is directionally stable the rate of turn reduces to zero and the ship takes up a new straight path. If the ship is unstable a residual rate of turn will persist. The manoeuvre can be conveniently carried out at the end of each circle trial during ship trials. It has been found that for a stable ship a plot of the log of rate of turn against time is a straight line after an initial transient period. It was shown in the section on theory that the di erential equation of motion had two roots m1 and m2 both of which had to be negative for directional stability. It has been argued that the more negative root will lead to a response which dies
Fig. 13.10 Pull-out manoeuvre, rate of turn on time base