The ship at sea lying in still water is being constantly subjected to a wide variety of stresses and strains, which result from the action of forces from outside and within the ship. Forces within the ship result from structural weight, cargo, machinery weight and the effects of operating machinery. Exterior forces include the hydrostatic pressure of the water on the hull and the action of the wind and waves. The ship must at all times be able to resist and withstand these stresses and strains throughout its structure. It must therefore be constructed in a manner, and such materials, that will provide the necessary strength. The ship must also be able to function efficiently as a cargo-carrying vessel.The various forces acting on a ship are constantly varying as to their degree and frequency. For simplicity, however, they will be considered individually and particular measures adopted to counter each type of force will be outlined. The forces may initially be classified as static and dynamic. Static forces are due to the differences in weight and buoyancy which occur at various points along the length of the ship. Dynamic forces result from the ship’s motion in the sea and the action of the wind and waves. A ship is free to move with six degrees of freedom- three linear and three rotational.FIGUREThese static and dynamic forces create longitudinal, transverse and local stresses in the ship’s structure. Longitudinal stresses are greatest in magnitude and result in bending of the ship along its length.
SHIPCON<br />SHIPBUILDING MATERIALS<br />
STEEL ARE ALLOYS OF IRON, WITH PROPERTIES DEPENDENT UPON THE TYPE AND AMOUNTS OF ALLOYING MATERIALS USED.<br />
THE SPECIFICATIONS OF SHIP BUILDING STEELS ARE LAID DOWN BY CLASSIFICATION SOCIETIES.<br />
SHIPBUILDING STEELS ARE TESTED AND GRADED BY CLASSIFICATION SOCIETY SURVEYORS WHO STAMP IT WITH APPROVAL MARKS<br />
MILD STEEL ARE GRADED “A” TO “E” IS USED FOR MOST PARTS OF THE SHIP<br />
HIGHER TENSILE STEELS MAY BE USED IN AREAS OF HIGH STRESS, SUCH AS THE SHEER STRAKE<br />
THE USE OF HIGHER TENSILE STEEL IN PLACE OF MILD STEEL RESULTS IN SAVING OF WEIGHT FOR THE SAME STRENGTH<br />