Shipbuilding Mercantile

The Isherwood system has also been applied to ordinary cargo vessels. The success which has attended this system has led to introduction of designs on which the framing at the bottom and deck is longitudinal, while ordinary vertical framing is retained at the ship's side. A number of ships have been built on this combination system and have proved satisfactory.

The most recent development in ship construction has been the introduction of the Isherwood bracketless system, which is a modification of the normal longitudinal framing in which the brackets attaching the longitudinals to the bulkheads are dispensed with. This has necessitated a rearrangement of the spacing of the transverses and the provision of special strengthening of the shell and deck plating in the neighbourhood of the bulkheads, and results in a great simplification of the structure and of the work of erection (see fig. to).

Laying Off.

This is the name given to the process of drawing the lines of a ship to full size in plan and elevation in order to determine the exact dimensions of the more important parts of the ship's structure, as the sizes of the various members must correspond with one another in order that, when assembled, there may be no irregularity or unfairness in the surface of the ship. The process is carried out on a specially planed and black ened wooden floor of such a size as to take in the full depth of the ship in its width. The room in which the floor is situated is called the mould loft, and is an important adjunct of the ship yard drawing office.

The principles of the methods of the projections of the various lines and planes are exactly to those followed in practical solid geometry, and do not call for any detailed explanation.

In different localities and in the construction of different types of ships, the extent to which the process of laying off to full size is employed varies considerably. In some yards laying off on a large scale on paper is relied on almost entirely, and very little full size work on the floor is considered necessary, particu larly in the case of ships the lines of which have very little curva ture over the greater part of their length.

The primary object in laying off a ship is to determine the exact shape of each of the frames, and these are drawn down on the scrieve board, which is an auxiliary mould loft floor con structed conveniently near the frame bending shop, which has copied on it all the information necessary for the correct shaping of the frames in the ship. All the frame lines are shown on the scrieve board, and the complete section of the frame surface for both sides of the ship is shown for each frame.

Special wood moulds are prepared giving the spacing of the rivet holes in the frames and floors, while wood battens are pre pared on which are marked off the spacing of the rivet holes in the floors and keelsons.

Great progress has been made in recent years in the art of laying off, and wood moulds and battens can be prepared to suit the requirements of the different deck and shell plates, frames and beams so that it is possible to shape and punch the rivet holes in about 90% of the material before the vessel's keel is actually laid.

On account of the sharpness of form at the ends of the ship it is usual to make a wooden pattern of the exact shape of the plates at these parts from the structure after the frames have been erected, but in some instances even this has been unnecessary, and practically the whole of the material has been prepared in advance.

Materials.

Ships of the present day are almost invariably constructed of the material known as mild steel, which consists of iron with a small percentage of carbon, manganese, phosphorus and sulphur, a typical mild steel containing Mild steel is very tough and ductile, and differs from the hard steel out of which tools are made in that it will not take a temper; i.e., if heated and plunged into oil or water the sudden cooling has very little effect upon it, whereas with tool steels a great change takes place—the steel becoming very hard and usually brittle. This quality of tempering depends chiefly on the amount of carbon in the steel, mild steel containing less than .25%. Before being accepted for use in shipbuilding, steel is re quired to be submitted to a tensile and to a bend test. For the purpose of the tensile test, strips are cut from the plates or bars and are machined to give a parallel part about 2 inches in width of at least 8 inches in length. Two marks are made, 8 inches apart, and the strip is secured in a testing machine constructed so that the ends of the strip can be gripped by strong jaws beyond the parallel parts. The jaws are then gradually pulled apart, the amount of the pull required to break the strip being registered, and also the extent to which the strip stretches in the length of 8 inches before breaking. The tensile strength varies between 26 and 32 tons per square inch calculated on the original sectional area of the parallel part before breaking, and the elongation in the 8 inches is not less than about 20%. In addition to the tensile test, sample strips 2 inches in width are cut and are bent double by hammering or in a press until the bend is a semi-circle the diameter of which is r -1 times the thickness of the plate. As an additional test the strips are sometimes heated and plunged into water to cool them suddenly before bending.