Page 275 - Introduction to Naval Architecture

260 MANOEUVRING

The spiral manoeuvre
This is a manoeuvre aimed at giving a feel for a ship's directional
stability. From an initial straight course and steady speed the rudder is
put over say 15° to starboard. After a while the ship settles to a steady
rate of turn and this is noted. The rudder angle is then reduced to 10°
starboard and the new steady turn rate noted. This is repeated for
angles of 5°S, 5°P, 10°P, 15°P, 10°P and so on. The resulting steady rates
of turn are plotted against rudder angle.

(a)
Figure 10.4 Spiral manoeuvre

If the ship is stable there will be a unique rate of turn for each
rudder angle. If the ship is unstable the plot has two 'arms' for the
smaller rudder angles, depending upon whether the rudder angle is
approached from above or below the value. Within the rudder angles
for which there is no unique response it is impossible to predict which
way the ship will turn, let alone the turn rate, as this will depend upon
other disturbing factors present in the ocean. The manoeuvre does
not give a direct measure of the degree of stability, although the range
of rudder angles over which response is indeterminate is a rough
guide. To know the minimum rudder angle needed to ensure the ship
turns in the desired direction is very useful.