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.
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.
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.