Lesson nine Ship motions, Manoeuvrability Ship motions are defined by the movements from the equilibrium position of the ship's centre of gravity along the three axes shown in Figure I and by rotations about axes approximately parallel to these. The linear displacements along the horizontal(x), lateral (y), and veritical(z) Fig. 1 Coordinate axes of ship motions(see text) axes are termed surge, sway, and heave, respectively. The rotations about the corresponding body axes are respectively termed roll, pitch, and yaw(veering off course). Roll, pitch, and heave are oscillatory because hydrodynamic forces and moments oppose them Ship motions are important for many reasons. A ship should be able to survive any sea that may be Encountered and, in addition, to behave well and to respond to control. In brief, a ship should respond to the action of the sea in such a manner that the amplitudes of its motions and its position never become dangerous, and so that the accelerations it undergoes are kept within reasonable limits. Propulsive performance, or heaving Hence these motions are made as small as possible Ship motions are excited by waves, whose growth is governed by the wind velocity at the sea surface, the area of water, or distance, over which the wind blows(the"fetch), and the length of time during which the wind has been blowing(the""). Any seaway is al ways a complex mixture of waves of different lengths, as wind itself is a complex mixture of gusts. All wave components do not travel in the same direction, but the directions of most of them in a single storm lie within 30 of each other. Regular trains of waves of uniform height and length are rarely, if ever, encountered. Most seas are confused and can be considered as made up of many separate component waves that differ in height and length Pitching, rolling, and heaving are all excited by the changing pattern of surface waves in relation to the speed and course of the ship. In practice, it is possible to damp one motion only--that of rolling. The fitting of bilge keels(finlike longitudinal projections along the part of the underwater body of a ship between the flat of the bottom and the vertical topsides) has this effect, and still more effective means are the activated for stabilizer(a device along the side of a ship activated by a gyroscope and used to keep the ship steady and the passive or flume stabilizing tank, filled with water inside the ship Manoeuvrability Increases in the size and speed of ships bring problems of safe operation in congested waters and control at high speed in waves. Therefore, designs necessarily represent a compromise between manoeuvrability and course-keeping ability. Ship operators desire maximum manoeuvrability in port to minimize the need for assistance from tugs and to reduce delays in docking. They also desire a ship that can hold a steady course at sea with the minimum use of helm. These aims, however, are mutually conflicting A ship is steered by means of one or more rudders arranged at the stern or, in rare cases, at the bow. There areLesson Nine Ship Motions, Manoeuvrability Ship motions Ship motions are defined by the movements from the equilibrium position of the ship’s centre of gravity along the three axes shown in Figure 1 and by rotations about axes approximately parallel to these. The linear displacements along the horizontal (x), lateral (y), and veritical(z) Fig. 1 Coordinate axes of ship motions (see text) axes are termed surge, sway, and heave, respectively. The rotations about the corresponding body axes are respectively termed roll, pitch, and yaw (veering off course). Roll, pitch, and heave are oscillatory because hydrodynamic forces and moments oppose them. Ship motions are important for many reasons. A ship should be able to survive any sea that may be Encountered and, in addition, to behave well and to respond to control. In brief, a ship should respond to the action of the sea in such a manner that the amplitudes of its motions and its position never become dangerous, and so that the accelerations it undergoes are kept within reasonable limits. Propulsive performance, or heaving. Hence these motions are made as small as possible. Ship motions are excited by waves, whose growth is governed by the wind velocity at the sea surface, the area of water, or distance, over which the wind blows (the “fetch”), and the length of time during which the wind has been blowing (the “duration”). Any seaway is always a complex mixture of waves of different lengths, as wind itself is a complex mixture of gusts. All wave components do not travel in the same direction, but the directions of most of them in a single storm lie within 30°of each other. Regular trains of waves of uniform height and length are rarely, if ever, encountered. Most seas are confused and can be considered as made up of many separate component waves that differ in height and length. Pitching, rolling, and heaving are all excited by the changing pattern of surface waves in relation to the speed and course of the ship. In practice, it is possible to damp one motion only---that of rolling. The fitting of bilge keels (finlike longitudinal projections along the part of the underwater body of a ship between the flat of the bottom and the vertical topsides) has this effect, and still more effective means are the activated for stabilizer (a device along the side of a ship activated by a gyroscope and used to keep the ship steady) and the passive or flume stabilizing tank, filled with water inside the ship. Manoeuvrability Increases in the size and speed of ships bring problems of safe operation in congested waters and control at high speed in waves. Therefore, designs necessarily represent a compromise between manoeuvrability and course-keeping ability. Ship operators desire maximum manoeuvrability in port to minimize the need for assistance from tugs and to reduce delays in docking. They also desire a ship that can hold a steady course at sea with the minimum use of helm. These aims, however, are mutually conflicting. A ship is steered by means of one or more rudders arranged at the stern or, in rare cases, at the bow. There are