Shipbuilding Mercantile

Outline drawings must first be prepared, based on dimensions which are considered suitable, and the various calculations made for this assumed design. These calculations will include the vari ous factors to which reference has been made, and, if it is not intended to class the vessel with one of the recognized Classifica tion Societies, questions of strength will have to be considered. If, however, the vessel is to be so classed, it may be assumed that the scantlings required by the Rules of such Society will provide generally sufficient strength. If the calculations show that the dimensions assumed do not enable the required conditions to be fulfilled, the dimensions must be modified and the calculations repeated, the process being continued until a satisfactory result is obtained. As soon as the dimensions selected for the vessel are found to be appropriate, more complete drawings are put in hand and the final calculations pertaining to the displacement, weights of hull and equipment, deadweight and capacity, centre of grav ity and trim, while metacentric diagrams and curves of stability are also made.

In the case of the construction of large passenger vessels, com plete drawings and specifications are prepared by the shipowners or by naval architects employed by them. In other cases, ship builders work in close connection with the shipowning companies and the business relations are of a very simple character, the ship owner being content to send a note of the principal dimensions, deadweight, capacity and type of ship required and to stipulate that the ship shall receive the highest class of one of the recog nised Classification Societies, leaving the determination of the details of the design in the hands of the builders.

In any case complete design drawings and detailed specifications are necessary for the shipyard operations, and if not supplied must be prepared by the shipyard staff. The principal plans required are the sheerdraught and the profile and deck plans which show the general arrangement. The sheerdraught consists of an elevation showing the vessel's longitudinal contour, the position of the decks, the water line or line at which she will float, and certain other lines parallel to this and equally spaced below it, which are also called water lines, and of a series of vertical lines equally spaced from stem to stern called square stations ; of a body plan showing the sectional form of the ship at the square stations—supposing her to be cut by transverse planes at these stations; and of a half breadth plan showing the form of the ship at the several water lines, sup posing her to be cut by horizontal planes at the level of these lines. The profile and deck plans show all the internal arrangements of the vessel, the holds and spaces set apart for cargo, the position of the engines and boilers, the accommodation provided for the pas sengers and crew, and all the principal fittings throughout the ship. The midship section shows the structural arrangements of the vessel and the dimensions—or scantlings—of the more important parts of the structure. The specification is a statement of all the

particulars of the vessel, including what is shown on the drawings as well as what cannot be shown on them. The quality of the materials to be used is carefully defined, and it is clearly stated how items not manufactured by the shipbuilders are to be obtained.

Practical Shipbuilding.

Practical shipbuilding requires a knowledge of the properties of materials used in the construction of ships, and also an acquaintance with the methods, means, and machinery by which, after delivery in the shipyard, the materials are brought to the required shapes, erected in their proper rela the general principles of construction remain very much the same in all cases. The exterior parts—the bottom, sides and decks— supply the strength required for the structure as a whole. The bottom and sides are spoken of as the shell or outside plating and are, with the decks, kept to the proper shape by means of frames running across the ship like the rafters in a roof or the ribs of a body. These are called transverse frames where attached to the shell or beams where they run under the decks. At the bottom of the ship where special strength must be provided to support the structure against grounding and in dry dock, the frames are con siderably increased in depth and are known as floors. The tops of the floors are held upright in their correct relative positions by girders running lengthwise ; one at the middle line being called the centre keelson, and others nearer the sides side keelsons.

In all merchant vessels except those of small size an inner bot tom is provided, the space be tween the inner and outer bot toms being utilised for carrying either water ballast or oil fuel. In such cases the centre keelson is called the centre girder and the side keelsons are called side girders. The centre girder is made continuous, and the deep trans verse plates forming the floors extend from the centre girder to the ship's side. The side girders are fitted in pieces between the floors, and are said to be intercostal. Occasionally in large ves sels one of the side girders is made continuous and the floors in that case are fitted intercostally. In modern vessels it is the prac tice to fit solid plate floors on every second or every third frame only, the remaining floors being built up of bulb angle bars inside tive positions, connected together and completed so as to form a structure which shall fulfil the intentions of the design. The vari eties of ships are very great and are constantly changing, and thus new problems are continually presented to the shipbuilder. There is also an ever increasing demand for rapid production which necessitates a constant search for simplification of methods of work, for labour-saving and time-saving machinery, for improved means of handling material in the shipyard, and for workshops which will more completely prepare and finish their products be fore dispatch to the shipyard.