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.
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.
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.