Practical Ship Hydrodynamics

Ship manoeuvring 171
it is important to consider that the ship is faster at the beginning of the
turning circle and slower at sustained turning. The heeling angle exceeds
dynamically the statical heel angle due to forces listed above.
The turning circle test is used to evaluate the turning ability of the ship.
2. Spiral manoeuvres
We distinguish between:
– ‘Direct’ spiral manoeuvre (Dieudonne)
With the ship on an initial straight course, the rudder is put hard to one
side until the ship has reached a constant rate of change of heading.
The rudder angle is then decreased in steps (typically 5°, but preferably
less near zero rudder angle) and again held until a steady condition is
reached. This process is repeated until the rudder has covered the whole
range to the maximum rudder angle on the other side. The rate of turn
is noted for each rudder angle. The test should be performed at least for
yaw unstable ships going both from port to starboard and from starboard
to port.
– ‘Indirect’ (reverse) spiral manoeuvre (Bech)
The ship is steered at a constant rate of turn and the mean rudder angle
required to produce this yaw rate is measured. This way, points on the
curve rate of turn vs. rudder angle may be taken in any order.
The spiral test results in a curve as shown in Fig. 5.5. The spiral test is used
to evaluate the turning ability and the yaw stability of the ship. For yaw
unstable ships, there may be three possible rates of turn for one given rudder
angle as shown in Fig. 5.5. The one in the middle (dotted line) represents
an instable state which can only be found by the indirect method. In the
direct method, the rate of turn ‘switches’ at the vertical sections of the
curve suddenly to the other part of the curve if the rudder angle is changed.
This is indicated by the dotted arrows in Fig. 5.5.
Rate of turn Rate of turn
Right (stb)
Curve obtained
from reversed
spiral
Rudder angle d
Rudder angle d
Left (port) Right (stb) Left (port)
Right (stb)
Left (port)
Figure 5.5 Results of spiral tests for yaw stable and yaw unstable ship
Right
Left
(stb)
(port)
Curve obtained
from direct or
reversed spiral
The spiral test, especially with the direct method, is time consuming and
sensitive to external influences. The results show that a linearization of the
body force equations is acceptable only for small jrj (Fig. 5.5). For yaw
stable ships, the bandwidth of acceptable rudder angles to give small jrj is
small, e.g. š5°. For yaw unstable ships, large jrj may result for any υ.