Whatever the size of the ship or the type to which she belongs, the outer bottom and under the inner bottom with short bracket plates between them, the bracket plates being attached to the centre and other fore and after girders by short angle bars. Floors built up in this way are called bracket floors. Where, however, special local stresses have to be met, as in the machinery space or at the forward end of the ship, solid plate floors are fitted on every frame.
Besides the ordinary framing, the transverse strength of the ship is much increased by the partitions fitted to divide up the internal spaces of the ship, which are called bulkheads, and which may be watertight or non-watertight as the circumstances of the case require. At the extreme ends of the ship the shell plating extends from the keel to the weather deck. In order to provide for the expansion of the oil due to varying temperatures, an ex pansion trunk is provided usually by fitting a continuous fore and aft bulkhead between the upper deck and the deck below. The space between the expansion trunk and the ship's side is known as the summer tank, and is used as an additional space for oil cargo when light oils are carried. Vessels of this type are nearly on the two sides is attached to forgings or castings which are known as the stem at the fore end and the sternframe or stern post at the after end. Towards the bow of the vessel particularly, addi tional supports are introduced to enable the ship to withstand the heavy blows of the sea in bad weather and are called panting stringers and panting beams, panting being the term applied to the movements which occur in the side plating if sufficient stiff ness is not provided. At the stern a deep floor, called the transom; is attached to the upper part of the sternframe to form a base for the overhanging part of the stern which is known as the counter. To assist the beams in holding the decks in their correct position, vertical pillars are introduced in large numbers, but to avoid loss of space and inconvenience in handling cargo ordinary pillars are often dispensed with and a few strong pillars widely spaced with deep girders under the deck are fitted instead.
The general spacing of the frames varies from about 20 inches in small ships to 36 inches in large vessels; 36 inches is however common in ships of moderate size.
The whole tendency of modern shipbuilding is in the direction of simplifying the construction by redistributing the material, concentrating on the more important parts of the main structure, the remainder being treated largely on the basis of local consider ations. The changes which have occurred over a period of years will be appreciated by a study of the construction of typical vessels from 186o to 1928. (Figs. i to 7.) The foregoing description may be regarded as covering mer cantile vessels generally, with the exception of that large part of the mercantile marine devoted to the carriage of oil in bulk, a trade which has grown to such dimensions that in 1927 about of the total world's tonnage consisted of oil-carrying vessels. These ships differ in many important particulars from ordinary cargo carrying vessels. The oil carrying space. which usually extends for about half the total length of the ship, is divided by transverse bulkheads into tanks about 3o feet long. These tanks are sub divided by a continuous longitudinal centre line bulkhead which all built on the longitudinal system of framing. In this system, designed by Sir Joseph Isherwood, instead of closely spaced trans verse frames, deep frames formed of plates and angles known as transverses are fitted at intervals of about to feet. The shell and deck plating is supported by continuous longitudinal frames called longitudinals, which pass through slots in the transverses but are cut at the bulkheads, to which they are bracketed. The middle line and transverse bulkheads are stiffened in a similar manner, so that at each longitudinal there is a continuous hori zontal girder right round the tank. The main features of this system will be clearly understood by reference to fig. 9, which shows the construction of a typical tanker.