Framing.—The framing varies considerably with the size and type of the ship. In small vessels a frame usually consists of an angle bar, called a frame bar, extending from the centre line to the gunwale. To the frame bar is riveted another angle called a reversed bar, in such a way as to form a built up Z bar, while at the bottom the frame and reversed frame are separated in order that both may be attached to the floor plates which form deep girders across the bottom to give the required strength when the vessel is resting either on the ground or on the keel blocks. The usual procedure in constructing a complete frame and floor is as follows : From the scrieve board the shape of the section of the frame is transferred to the bending slabs, the outline being drawn in with chalk ; the frame bar is drawn from the furnace and while hot is bent to the required shape and given the necessary bevel. The reversed bar is prepared in the same way except that the inner edge of the frame and floor must be worked to. The floor plate requires to be cut to shape. The frame, reversed frame and floor all being prepared, are placed together in their respec tive positions over the outline of the frame on the scrieve board, the final adjustments made, and rivet holes marked and punched and the work secured and riveted up. In many instances the frame is formed of a bulb angle, in which case the reversed bar is fitted only on the upper edge of the floor plate and does not extend up the ship's side. In vessels fitted with double bottoms the frame bar extends from the tank side margin plate to the gunwale but the general process of marking and bending proceeds as described above.
Double Bottoms.—There are a considerable number of varia tions used in the construction and arrangement of double bot toms. At the centre line immediately over the flat keel plates there is a vertical girder the full depth of the double bottom, connected to the flat keel plate and to the centre plate of the inner bottom by continuous double angle bars. This centre girder may or may not be watertight, according to the desired tank arrangements.
The floor plates, which extend from the centre girder to the margin plate, are provided on their edges with angle bars for attaching them to the outer and inner bottoms, to the centre girder and to the margin plate. As will be seen, the margin plate cuts completely through the transverse frames, and special brackets are fitted to maintain the transverse strength. The chief advantages derived from cutting the frames at the margin plate are the ease with which watertight work is secured and the rapidity with which this part of the structure can be proceeded with. Except where it is desired that the floors should be water tight, manholes are punched for the purpose of providing ready access to all parts of the bottom. Between the centre girder and the margin plate one or more intercostal girders are fitted, these girders consisting of plates fitted in short lengths between the floors, to which and to the inner and outer bottoms they are attached by short pieces of angle bar.
With a view to rendering the bottom more easily accessible it is now customary to fit plate floors at every second or third frame only, the intermediate floors being built up of angles or bulb angles connected together by plate brackets. Such floors are known as bracket floors. Solid floors must be fitted on every frame under the machinery and at the forward end of the ship.
Decks.—The decks are very important parts of the structure from the point of view of both transverse and longitudinal strength, but their number and position necessarily vary consid erably with the size and type of the vessel. In bulk cargo ships the number of decks is reduced to a minimum, and some ships having a depth of about 32 ft. have been built with one deck only, while in a similar passenger ship there might be three decks.
Decks are supported primarily by the deck beams, which are usually formed of bulb angles and are attached to the side frames by a number of plate knees. The beams may be fitted either to every frame or to alternate frames, and are in turn supported by deck girders, the latter being carried by pillars which may be closely spaced, say at every second frame, or may be spaced as much as 3o feet apart. The decks are generally completely plated over, the thickness of the plating being greatest on the top deck, and each deck being less in thickness than the one above. Thus in a ship with three decks the top deck might be .5o inch in thick ness and the two lower decks .4o inch and .3o inch respectively. The strake of deck plating next the ship's side is called the stringer plate and is attached to the shell plating by an angle bar known as the stringer angle. In passenger ships it is customary to sheathe the steel decks with wood, usually pitch pine 21 inches in thick ness, where exposed to the weather, and to lay a composition of which the principal ingredient is sawdust about i1 inches thick inside the passenger accommodation. In cargo ships the steel deck is left bare except in the crew's accommodation.
Shell or Outside Plating.—The outside or shell plating forms the watertight skin of the ship and also contributes the major part of the structural strength. The plating is arranged lengthwise in a series of strakes about 6o inches in breadth, the overlaps of ad jacent strakes being called seams. The plates are usually about 3o ft. in length and adjoining pldtes in the same strake are over lapped, these joints being known to shipworkers as butts. Thick ness of the plating is governed by the necessity for providing suf ficient structural strength, and this thickness is usually maintained for half the vessel's length, from whence it is tapered off to the ends where the thickness is about two-thirds of the midship thick ness. The mould loft supplies templates and battens for the dif ferent plates in each strake, and it is not uncommon for a very large part of the shell plating to be shaped and punched before the framing is erected.