Turning Circle Diameter Trials for a Container ship
Container ships manoeuvring characteristics defined their ability of turning, yaw-checking, course-keeping and stopping distances.
When a vessel fitted with a fixed right-hand propeller, she would benefit
from the transverse thrust effect, and her turning circle, in general, will be
quicker and tighter when turning to port than to starboard.
The longer the ship generally, the greater
the turning circle. The type of rudder and the resulting steering effect
will decide the final diameter, with the clearance between rudder and
hull having a major influence. The smaller the clearance between rudder
and hull the more effective the turning action.
Merchant ships usually turn in a circle having a diameter of about 3–4
times the length between perpendiculars (LBP). The larger the rudder, the
smaller will be the Turning circle diameter(TCD). During the TCD manoeuvre, the ship will experience
transfer, advance, drift angles and angle of heel (see Figure ).
The maximum angle of heel must be recorded. If the ship has Port rudder
helm this final angle of heel will be to Starboard and vice versa. Again, this
is due to centrifugal forces acting on the ship’s hull.
This manoeuvre is carried out with the ship at full speed and rudder helm
set at 35°. The ship is turned completely through 360° with say Starboard
rudder helm and then with Port rudder helm (see Figure ).
Fig. TCD manoeuvres. Ship run at full speed with rudder helm 35°
P or S throughout this trial.
Turning circle diameter :There will
be two TCD of different diameters. This is due to the direction of the rotation
of the propeller. For most single screw Merchant ships, the propeller
rotates in a clockwise direction when viewed from aft to forward part of the
ship. It does make a difference to the Turning circle diameter (TCD).
It should be observed in Figure that at the beginning of the Port
turning manoeuvre, the ship turns initially to Starboard. There are reasons
for this. Forces acting on the rudder itself will cause this move at first
to Starboard. Larger centrifugal forces acting on the ship’s hull will then
cause the vessel to move the ship on a course to Port as shown in this
diagram.
Ship model tests and Ship Trials have shown that the TCD does not
change if this trial is run at speeds less than full speed. If these trials had
been carried out in shallow waters, the TCD could have been double that
measured in deep-water conditions.
Turning circle - Normal full loaded condition with maximum rudder angle full ahead RPM
Turning circle - Normal ballast condition with maximum rudder angle full ahead RPM
Turning circle - Loaded condition with maximum rudder angle half ahead RPM
Turning circle - Normal ballast condition with maximum rudder angle half ahead RPM
Fig:Turning circle – examples ‘Modern Container Vessel’ 4,318 TEU
(comparing Loaded Condition against Ballast Condition with a maximum
rudder angle or hard over at 35°).
Details of ship and operation:
Length OA 292m Deadweight at design Draught 61,787MT Calm weather.
Breadth 21.7m Lightship tonnage 20,560MT No current.
Draught 13.5m Service speed 24.3 knots Clean hull.
(Water depth for turn example twice that of the draught)
Diameters
The greatest diameter scribed by the vessel from starting the turn to completing
the turn (ship’s head through 180°) is the tactical diameter. The
internal diameter of the turning circle where no allowance has been made
for the decreasing curvature as experienced with the tactical diameter is
the final diameter.
Transfer
This is defined by that distance which the vessel will move, perpendicular to
the fore and aft line at the commencement of the turn.The total transverse
movement lasts from the start of the turn to its completion, the defining limits
being known as the transfer of the vessel when turning
Further Reading
The IMO resolution MSC.137(76) "Standards for Ship Manoeuvrability" identify the following characteristics:
Inherent dynamic stability - A ship is dynamically stable on a straight course if it, after a small disturbance, soon will settle on a new straight path without any corrective rudder action. The resultant deviation from the original heading will depend on the degree of inherent stability and the disturbance's magnitude and duration.
Course-keeping ability – The course-keeping quality is a measure of the steered ship's ability to maintain a straight path in a predetermined course direction without excessive oscillations of rudder or heading. In most cases, reasonable course control is still possible where there exists an inherent dynamic instability of limited magnitude.
Initial turning/course-changing ability –
The initial turning ability is defined by the change-of-heading response to a moderate helm, in terms of heading deviation per unit distance sailed or in terms of the distance covered before realizing a particular heading deviation. (such as the "time "to execute second demonstrated when entering the zig-zag maneuver).
Yaw checking ability –
The ship's yaw checking ability is a measure of the response to counter-rudder applied in a specific state of turning, such as the heading overshoot reached before the counter-rudder has canceled the yawing tendency in a standard zig-zag maneuver.
Turning ability – Turning ability is the measure of the ability to turn the ship using hard over the rudder. The result is a minimum "advance at 90° change of heading" and "tactical diameter" defined by the "transfer at 180° change of heading".
Stopping ability – Stopping ability is measured by the “track reach” and “time to dead in water” realized in a stop engine-full astern manoeuvre performed after a steady approach at full test speed.
Related Topics
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Ships navigation -Factors Affecting Turning circle diameter Merchant ships usually turn in a circle having a diameter of about 3–4 times the length between perpendiculars (LBP). The larger the rudder, the smaller will be the Turning circle diameter(TCD). During the TCD manoeuvre, the ship will experience transfer, advance, drift angles and angle of heel (see Figure )....
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