Practical Ship Hydrodynamics

170 Practical Ship Hydrodynamics
The main manoeuvres in sea trials are:
1. Turning circle test
Starting from straight motion at constant speed, the rudder is turned at
maximum speed to an angle υ (usually maximum rudder angle) and kept at
this angle, until the ship has performed a turning circle of at least 540°. The
trial is performed for both port and starboard side. The essential information
obtained from this manoeuvre (usually by GPS) consists of (Fig. 5.4):
Advance
(at 90° change of heading)
Tactical diameter
(at 180° change of heading)
Transfer (at 90° change
of heading)
Turning
radius
Rudder execute
Approach course
Drift angle
Path of centre
of gravity
Figure 5.4 Definitions used on turning circle
– tactical diameter
– maximum advance
– transfer at 90° change of heading
– times to change heading 90° and 180°
– transfer loss of steady speed
Typical values are tactical diameter of 4.5–7L for slender, 2.4–4 for short
and full ships. Decisive is the slenderness ratios L/ 3p r,wherer is the
volume displacement.
Fast displacement ships with Fn > 0.25 may feature dangerously large
heel angles in turning circles. The heel is always outwards, i.e. away from
the centre of the turning circle. (Submarines and boats with dynamic lift
like hydrofoils are exceptional in that they may heel inwards.) The heel
is induced by the centrifugal force m Ð u Ð r acting outwards on the ship’s
centre of gravity, the hull force Yvv C Yrr acting inwards, and the much
smaller rudder force Yυυ acting outwards. For manoeuvring predictions