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Circle of rotation of the ship
Ali Abd Alrahman
18200533
Term 4
SHIP HANDLING
Circle of rotation of the ship
Before venturing deeper into our topic of discussion, let us first understand what the term
‘manoeuvrability’ of a ship means.
Manoeuvrability is defined as the inherent ability of a vessel to change its course/path.
Understanding the factor of manoeuvrability of a vessel is extremely important to a designer or a
seafarer, along with other aspects such as structural design, machinery, propulsion, stability and
sea keeping.
As a ship has to endure long voyages through different weather conditions, it is important that
the performance of a vessel during the day-to-day instances of its voyage is not compromised on:
Turning about an obstruction such as landmass or any other marine vehicle.
Circling about a desired point of interest.
Surging, stopping and accelerating.
For maintaining/varying its course or direction.
To maintain a desired path/trajectory.
To promptly veer about an object (ship, iceberg, landmass, reef etc.) without costing
much in time.
Turning Circle for a ship
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 size of the ship's turning circle is influenced by
various factors, such as block coefficient, underwater
side shape, rudder area ratio, draft, trim and Froude's
number. Most of them are already fixed on departure
from a port. However, the ship's speed and the rudder
angle are controllable factors which operations are able
to change optionally during sailing. The DGPS
measured the turning circles according to the ship's
speed and the rudder angle.
1
The maximum advances by slow and full ahead were 302m and 311m, and the maximum
transfers were 460m and 452m, respectively. There occurs almost no difference in size of the
turning circle by variation of the ship's speeds. When the rudder angles were changed to, and, the
maximum advances were 447m, 271m and 202m, and then also the maximum transfers 657m,
426m and 285m, respectively. The diameter of the tuning circle was decreased exponentially
when the rudder angle was increased. The maneuverability was better when the direction of
turning and propulsion of propeller are in the opposite direction rather than in the same one
together. The distance of the maximum transfer was always bigger than that of the maximum
advance.
SHIP-TURNING CHARACTERISTICS IN
DIFFERENT WATER DEPTHS
The diameter of a ship's turn depends on several factors in addition to rudder angle, and one of
the most important of these is water depth. For manoeuvring, deep water can be assumed to have
a depth of more than five times the ship's draught; at three times the draught, the shallow-water
effects become significant, and as depth decreases from twice the draught these effects increase
rapidly. Water depth thus influences manoeuvring at time when misinterpreting its effects can be
disastrous. Although, in shallow water, resistance to ahead motion is increased, it cannot be
assumed that the ship can be stopped more quickly; the drag increase can be more than offset by
an increase in the ship's virtual mass. The Author discusses these matters and explains the
phenomena that occur when a ship is turning, in shallow water in particular. The drift angle
(relative to the direction of advance) during the turn will vary with water depth. The rate of turn
depends on the ship's directional stability, and, though the rate increases at first on leaving deep
water, it decreases as shallower water is reached. These changes in rate of turn are comparatively
small and could suggest to the ship handler that his ship is not significantly affected by the
shallow water, but their combination with a smaller speed-loss in the turn as underkeel clearance
decreases results in an increase in the ship's turning-circle diameter. Therefore, the rotation circle
of the ship in shallow water is 3 times that in deep water.
References
Marine Insight
Bulletin of the Korean society of Fisheries Technology
2020 National Academy of Sciences.
Ships business
