Ships manoeuvering- External forces affecting the drift angle
Structural design and length :
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.
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 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 vessel 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 degrees
P or S throughout this trial.
Motive power:
The relation between power and displacement will affect
the turning circle performance of any vessel 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.The turning circle will therefore 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 vessels 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:
A new vessel when engaged on trials will be on
an even keel when carrying out turning circles for recording the
ships 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 hull form,
will experience greater resistance when navigating in shallow water.
A form of interaction takes place 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 helm movement, probably increasing
the advance of the turning circle, as well as 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 which will cause
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
motion.Although the bow movement is what is desired, the resultant
motion of the vessel 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 pivot point of the
vessel in most cases describes the circumference of the turning circle.
Advice for Helmsman and Officer of the Watch
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.
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 actually 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 diameter of the turning
circle 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.
Related Topics
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.
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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 )....
