What are factors affecting ship’s manoeuvring? |

What are factors affecting ship’s manoeuvring?

Written by Amit Sharma

Handling characteristics will vary from ship type to ship type and from ship to ship. Handling qualities are determined by ship design, which in turn depends on the ship’s intended function. Typically, design ratios, such as a ship’s length to its beam, determine its willingness to turn. However, desirable handling qualities are achieved only when there is a balance between directional stability and directional instability.

  1. Underwater hull geometry :

Length to beam ratio (UB),  beam to draught ratio (Bm, block coefficient,  prismatic  coefficient  (ratios of the ship’s volume of displacement  against the volume of a rectangular  block or a prism) and location of longitudinal centre of buoyancy, all give an indication  High values of UB are associated with good course directional  stability.
Container ships are likely to have a UB ratio of approximately 8, while harbour tugs, which need to be able to turn quickly and where course stability  is not required, have a value of 2.5 to 3. High values of BIT increase leeway and the tendency for a ship in a beam wind to ‘skate across the sea surface’. A BIT ratio of over 4 is large.

  1.  The pivot  point

A ship rotates about a point situated along its length, called the ‘pivot point’. When a force is applied to a ship, which has the result of causing the ship to turn (e.g. the rudder), the ship will turn around a vertical axis which  is conveniently  referred to as the pivot point. The position of the pivot point depends on a number of influences. With headway, the pivot point lies between 1/4 and 1/3 of the ship’s length from the bow, and with sternway, it lies a corresponding  distance from the stern. In the case of a ship without headway through the water but turning,  its position will depend on the magnitude  and position of the applied force(s), whether  resulting from the rudder, thrusters,  tug, wind or other influence.The pivot point traces the path that the ship follows.

  1.  Lateral  motion

Ships move laterally when turning  because the pivot point is not located at the ship’s centre. When moving forward  and turning to starboard, the ship’s lateral movement is to port. When moving astern and turning to starboard, lateral movement is to starboard. It is important  to understand where the pivot point lies and how lateral movement can cause sideways drift, knowledge of which is essential when manoeuvring close to hazards.

  1. Propeller   and  rudder

The rudder acts as a hydrofoil. By itself, it is a passive instrument  and relies on water passing over it to give it ‘lift’.  Rudders are placed at the stern of a ship for this reason and to take advantage of the forward pivot point, which enhances the effect. Water flow is provided by the ship passing through the water and by the propeller forcing water over the rudder in the process of driving the ship. The optimum  steerage force is provided by water flow generated by a turning  propeller. Water flow is vital in maintaining  control   of the ship. While water flow provided by the ship’s motion alone can be effective, the effect will diminish speed is reduced. Obstacles that deflect flow, such as a stopped propeller in front of the rudder, particularly  when the propeller is large, can reduce rudder effectiveness.  Reduced or disturbed flow will result in a poor response to rudder movements.
Conventional rudders are described as ‘balanced’;  part of the rudder area is forward  of the pintles to help    the rudder turn and to ease the load on the steering motor.
This arrangement  provides for better hydrodynamic  loading. A flap (Becker rudder) can be fitted to the rudder’s trailing  edge. The flap works to increase the effective  camber of the rudder and to increase lift. Rudders can be defined by what is known as the ‘rudder area ratio’, which  is a ratio of the surface area of the rudder divided by the ship’s length and draught. The rudder area ratio gives an indication  of the likely effectiveness  of a rudder. Merchant ship ratios range from 0.016 to 0.035. The larger the ratio, the greater the effect the rudder will have.

About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.

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