Practical Consideration Copy

27 May 2022
|By Maurizio Garipoli

Some Things to Consider

When it comes to swinging, keep in mind that there is a big difference between shallow water and deep water. The turning circle can be twice as in the second case in the first case.

A ship’s turn radius is affected by various factors:

  • Speed: the lateral movement of the vessel approaching at high speed widens the turning circle;
  • Draft & Speed: the heavier the ship, the greater the lateral shifting due to speed;
  • Under Keel Clearance: the less water you have under the hull, the wider the turning circle;
  • The shape of the hull: flat bottom, trim, bow and stern shape; these are all elements that make a ship “hard” or “manoeuvrable”;
  • The shape of the superstructures: in windy conditions, the structures above the bridge can significantly influence the turning circle. Let’s assume the 90 degrees starboard turn of a sizeable unloaded container vessel with aft accommodation: the hull has considerable windage concentrated on the aft part. A strong wind that hits the bow to the port could stop the ship’s rotation at mid-turn. For this reason, It is crucial to get to the dead point with a reasonable rate of turn and an engine power reserve to use when the latter begins to decrease;
  • Current: The current can also slow down or even prevent swinging as the wind. In this case, the more significant the draft, the more evident the effect.
  • Rudder: The rudder creates a turning moment. Its size and characteristics influence in a decisive way the turning circle;
  • Orography: When manoeuvring a ship inside a port, one cannot ignore the shelters from the wind and the current of land fixed structures. These can change the ship’s behaviour even at a distance of a few meters, making the effect of the propeller more or less effective or predictable.
  • The bow and stern thrusters and their effectiveness: These navigational aids make ship handling safer.
  • Propeller shape: The characteristic of these ships are high performance and low fuel consumption in forwarding motion, but they are dangerously weak in reverse. It is not uncommon to come across vessels of this type that require caution and handling precautions.

IMO MSC.137 76 standards for ship manoeuvrability