Practical Ship Hydrodynamics

172 Practical Ship Hydrodynamics
3. Pull-out manoeuvre
After a turning circle with steady rate of turn the rudder is returned to
midship. If the ship is yaw stable, the rate of turn will decay to zero for
turns both port and starboard. If the ship is yaw unstable, the rate of turn
will reduce to some residual rate of turn (Fig. 5.6).
Rate of change
of heading
Left (port) Right (stb)
STABLE SHIP UNSTABLE SHIP
Rudder returned
to midships
Rate of turn at
midship rudder
Rate of change
of heading
Left (port) Right (stb)
Time t Time t
Figure 5.6 Results of pull-out manoeuvre
Rudder returned
to midships
Residual rate of
change of heading
The pull-out manoeuvre is a simple test to give a quick indication of a
ship’s yaw stability, but requires very calm weather. If the yaw rate in a
pull-out manoeuvre tends towards a finite value in single-screw ships, this
is often interpreted as yaw unstability, but it may be at least partially due
to the influence of unsymmetries induced by the propeller in single-screw
ships or wind.
4. Zigzag manoeuvre
The rudder is reversed alternately by a rudder angle υ to either side at a
deviation e from the initial course. After a steady approach the rudder is
put over to starboard (first execute). When the heading is e off the initial
course, the rudder is reversed to the same rudder angle to port at maximum
rudder speed (second execute). After counter rudder has been applied, the
ship continues turning in the original direction (overshoot) with decreasing
turning speed until the yaw motion changes direction. In response to the
rudder the ship turns to port. When the heading is e off the initial course
to port, the rudder is reversed again at maximum rudder speed to starboard
(third execute). This process continues until a total of, e.g., five rudder
executes have been completed. Typical values for e are 10° and 20°. The
test was especially developed for towing tank tests, but it is also popular
for sea trials. The test yields initial turning time, yaw checking time and
overshoot angle (Fig. 5.7).
For the determination of body force coefficients a modification of the
zigzag manoeuvre is better suited: the incremental zigzag test. Here, after
each period other angles υ and e are chosen to cover the whole range of
rudder angles. If the incremental zigzag test is properly executed it may
substitute all other tests as the measured coefficients should be sufficient
for an appropriate computer simulation of all other required manoeuvring
quantities.
Figure 5.8 shows results of many model zigzag tests as given by Brix
(1993). These yield the following typical values:
– initial turning time ta: 1–1.5 ship length travel time
– time to check starboard yaw ts: 0.5–2 ship travel length time (more for
fast ships)