Practical Ship Hydrodynamics

Ship manoeuvring 185
attack ˛ of nearly the same size in initial turning, of smaller size during steady
turning and of larger size in the yaw checking phase.)
Because of the different stall angles ˛s and lift curve slopes of rudders of
different aspect ratios it would be advantageous to use an effective rudder
angle υeff instead of the geometrical rudder angle υ for rules, e.g., about the
maximum rudder angle and the rudder rate of the steering gear, as well as
for nautical use. This would be ‘fairer’ for rudders of different aspect ratio;
it would also make better use of rudders of smaller aspect ratio (today their
greater stall angle ˛s is frequently not realized because of a too small maximum
rudder angle υ) and would lead to more equal response of different ships on
(effective) rudder angles. If geometric and effective rudder angles are defined
to coincide for a normal aspect ratio of 3 D 2, their relationship is (Fig. 5.15):
δ eff/δ geom
υeff D
1.5
1.0
0.5
0
2.2 Ð 3
Ð υ
3 C 2.4
1 2 3
L
Figure 5.15 Ratio between effective and geometrical angle of attack
For aspect ratios 3