Turning circle diameter trials:
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
Related Topics
Ship manoeuvering-turning circle advices A deeply laden vessel will experience little effect from wind or sea when turning, but a vessel in a light or ballasted condition will make considerable leeway, especially with strong winds.
....
Various factors affecting ships turning circle while manoeuvering When a vessel fitted with a right-hand fixed 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.
....
External factors that affecting ships turn The deeper a vessel lies in the water, the more sluggish will be her response to the helm. On the other hand, the superstructure of a vessel in a light condition and shallow in draught is considerably influenced by the wind.
....
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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