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Ships manoeuvering- External forces affecting the drift angle

  1. 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.

  2. 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.

  3. Turning cicle diameter for a typical containership
    TCD manoeuvres. Ship run at full speed with rudder
    helm 35 degrees P or S throughout this trial.

  4. 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.)

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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

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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 34 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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