Page:Self-righting boat design.pdf/2

 curve in comparison with external, usually stipulated, heeling moments. The major efforts are focused either on establishing requirements for the shape and values of the GZ curve (calculated in calm water or on a wave crest) so that a required safety level can be maintained during ship operations in unspecified sea conditions, or on establishing the nominal heeling moments which the ship has to withstand (statically or dynamically or both).

The required safety standards are usually a result of a detailed stability analysis of a particular type of ship or some statistical studies of safe ships and casualties, and reflect so-called “good marine practice.” They may provide a reasonable safety level for some classes of ships or some operational situations but in other cases they are not satisfactory or fail completely.

The second trend represents efforts to base the stability criteria on more sound theoretical models. However, the studies are focused on some selected simplified cases which are related to ship behavior either in beam or following seas. Although the selected situations are realistic, they do not represent he most severe scenarios of ship operation in oblique extreme seas, and none of the proposed approaches is general enough to represent the whole range of phenomena affecting ship stability. Many competing requirements of the design of small craft such as minimum resistance and windage, adequate power from engine and sails, sufficient strength and structural redundancy, low weight, maximum comfort below and protection on deck, good aesthetics, and reasonable costs, are all objectives that the designer has to balance in order to achieve a compromise solution. Adequate stability is also a requirement of every boat design.
 * 2. Design And Stability Objectives

Ships are designed on the basis that capsize must be prevented, and its occurrence is considered a catastrophe that will inevitably lead to the loss of the vessel, and in all probability the loss of life. This approach cannot be taken with small craft. On many occasions the energy available in the environment in which the craft is operating is overwhelmingly greater than the work required to capsize the vessel, and so the designers of small craft have to countenance this possibility and design accordingly. An understanding of the fundamentals of stability was established by Bouguer in 1746, and independently by Euler in 1749, since when research has continued around the world into many complex and subtle developments of stability theory. However it is only in the small craft world that stability at all angles of heel, including 180 degrees, is of crucial concern. Since 1980, and as a result of actual or near disasters, there has been much research undertaken into the stability of yachts and small craft at all angles of heel. The availability of economical stability modelling programs for PCs since the mid 1980s has greatly facilitated the exploration of high angle stability, both in a research and design context, so that now the behaviour of vessels in the inverted state is widely understood.
 * 2.1 Ship Stability Basis

Ships are designed for stability when the vessel is operating normally and upright, but also to design for instability in the crisis situation where the vessel is inverted and we want it to rapidly