Course of Construction.—On the receipt at the shipyard of the design drawings and specifications, steps are taken to put in hand the detailed drawings of the structural arrangements which will enable materials for the various parts to be ordered from the manufacturers and will provide information for the guidance of the workmen in the erection of the structure.
A wooden model of half the exterior surface of the ship, called the half block model, is immediately prepared from the sheer drawing, generally to a scale of 4 inch to the foot. On its surface are carefully drawn the main frames, the edges and butts of the shell plating, the positions of the decks, and other features which will influence the detailed arrangement of the framing and plat ing. The work on this model is carried out concurrently with the laying-off of the ship so as to be complete by the time the latter is sufficiently far advanced to enable full size measurements of the breadth of the plates to be obtained. The lengths of the plates are then measured from the model and the breadths from the mould loft floor (a small surplus on the net measurements being allowed to provide for inaccuracies), and the whole of the shell plating is ordered from the steel works.
For flat or nearly flat surfaces such as keel plate, bulkheads and decks, the detailed arrangements are made on drawings from which the dimensions are taken for ordering the material while the drawings themselves constitute working plans which are issued for general guidance in building the ship. In addition to these principal structural drawings a very large number of detailed plans showing the arrangement of passenger and crew accommo dation, systems of piping and ventilation and numerous other details are necessary. Very much of the success achieved in actual building will depend upon the efficiency of the drawing office, which must supply accurate and detailed working plans, which must be ready as soon as required. Each firm has its own system of work in these departments but experience shows that the more thorough and systematic the work in the drawing office and its adjunct, the mould loft, the better the general result.
A very important record kept during the building of the ship is the cost of materials and labour, a very careful account being kept of the workmen's time whether employed on piece or by the day. Many different systems are in vogue but the aim in all cases
is to record the cost of the labour in each trade and the detailed cost of the various parts of the ship.
The first stage in the actual erection of the vessel is the laying of the keel blocks, a task undertaken by the shipwrights assisted by labourers. The blocks consist of several pieces of rough, rec tangular timber about 12 inches square and 4 to 6 feet in length, laid on top of each other to the height, required. The top block is called the cap piece and is of oak. The spacing of the blocks depends to some extent on the size of the ship, but is usually about 4 feet. It is essential that the ground under the keel blocks should be firm and hard, otherwise the blocks may sink when weight becomes concentrated on them during building and the keel may consequently droop from a straight line. The upper surface of the blocks must be at such a height from the ground that men, especially riveters, can do their work with facility under the bottom of the vessel and that when launched the vessel may move down into the water without striking the ground. The last named is a very important consideration, and thus it happens that the first thing to be settled before the blocks are laid is how the vessel is to be launched. The tops of the blocks are securely adjusted to a slope of about inch per foot run from bow to stern. The shipwrights at the same time pre pare the uprights for the staging and erect them in suitable posi tions round the building berth. The platers begin to prepare the keel, framing and bulkheads as soon as the material is delivered and the laying off and mould making are sufficiently advanced for the purpose.
Details of Structure: Keels.—The keels of small vessels usually consist of a stout flat bar placed vertically and attached to the garboard strakes by through rivets. In larger ships the keel usually consists of a wide horizontal plate running along the centre line of the bottom, the sides being turned up as necessary to follow the shape of the bottom.