Ships trial- Turning circle diameter & factors affecting a turn in manoeuver
A ships manoeuvring characteristics defined its ability of turning, yaw-checking, course-keeping and stopping distance.
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.
The following factors will affect the rate of turn and the size of turning circle:
Structural design and length of the vessel.
Draught and trim of vessel.
Size and motive power of main machinery.
Distribution and stowage of cargo.
Even keel or carrying a list.
Position of turning in relation to the available depth of water.
Amount of rudder angle required to complete the turn.
External forces affecting the drift angle
Draught and trim :
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 a draught is considerably
influenced by the wind.
The trim of a vessel will influence the size of the turning circle in
such a way that it will decrease if the ship is trimmed by the head.
However, vessels normally trim by the stern for better steerage and
improved headway and it would be unusual for a vessel to be trimmed
in normal circumstances by the head.
TCD manoeuvres. Ship run at full speed with rudder helm 35°
P or S throughout this trial.
Motive power:
The relation between power and displacement will affect
any vessel's turning circle performance in the same way that a light
speedboat has greater acceleration than a heavily laden ore carrier. It
should be remembered that the rudder is only effective when there is a
flow of water past it. Therefore, the turning circle will not increase by
any considerable margin with an increase in speed, because the steering
effect is increased over the same period. (The rudder steering effect will
increase with the square of the flow of water past the rudder.)
Distribution and stowage of cargo :
Generally, this will not affect the turning
circle in any way, but the vessel will respond more readily if loads
are stowed amidships instead of at the extremities. Merchant ship
design tends to distribute weight throughout the vessel's length. The
reader may be able to imagine a vessel loaded heavily fore and aft
responding slowly and sluggishly to the helm.
Even keel or listed over:
When engaged in trials, a new vessel will be on
an even keel when carrying out turning circles for recording the
ship's data. This condition of even keel cannot, however, always be
guaranteed once the vessel is commissioned and loaded. If a vessel is
carrying a list, it can be expected to make a larger turning circle when
turning towards the list, and vice-versa.
Available depth of water :
The majority of vessels, depending on the hull form,
will experience greater resistance when navigating in shallow water.
A form of interaction occurs between the hull and the sea bed, which may result in the vessel yawing and becoming difficult to steer.
She may take longer to respond to the helm movement, probably increasing
the advance of the turning circle and increasing over the transfer.
The corresponding final diameter will be increased retrospectively.
Rudder angle:
Probably the most significant factor affecting the turning
circle is the rudder angle. The optimum is one that will cause
the maximum turning effect without causing excessive drag.
If a small rudder angle is employed, a large turning circle will result,
with little loss of speed. However, when a large rudder angle is
employed, then, although a tighter turning circle may be experienced,
this will be accompanied by a loss of speed.
Drift angle and influencing forces:
When a vessel responds to helm movement,
it is normal for the stern of the vessel to traverse in opposing
the motion. Although the bow movement is desired, the vessel's resultant
motion is one of crabbing in a sideways direction, at an
angle of drift.
When completing a turning circle, because of this angle of drift, the
stern quarters are outside the turning circle area while the bow area is
inside the turning circle. Studies have shown that the vessel's pivot point in most cases describes the circumference of the turning circle.
Advice for Helmsman and Officer of the Watch
A deeply laden vessel will experience the little effect from wind or sea
when turning, but a vessel in light or ballasted condition will make
considerable leeway, especially with strong winds.
When turning, the pivot point of the vessel is often situated well forward
of the bridge and may produce the effect of the vessel turning at
a faster rate than she is.
A vessel trimmed by the stern will steer more easily but the tactical
diameter of the turn is increased.
A vessel trimmed by the head will decrease the turning
circle's diameter but will become difficult to steer.
If a vessel is carrying a list, the time taken to complete the turn will
be subject to delay. A larger turn will be experienced when turning
into the list.
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.
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