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rupture of the deck that would otherwise occur. The side plating adjacent is cut by the gangway ports.

Transverse Strains.-The above remarks apply to the foreand-aft structural strength; the transverse strength is also of importance. The rolling of the ship will tend to rack or distort the section, and this tendency is resisted by the transverse bulkheads, which are numerous in a war-ship because of the W.T. subdivision, and also by the connections of the beam arms to the frames, which are made specially strong for this purpose.

A ship when placed in dry dock, especially one with heavy weights of armour at the side, will have severe strains tending to tear the decks. Such a ship, however, is carefully shored as the water leaves so as to obtain plenty of support. It should be noted that the shores must be placed on transverse or longitudinal frames, or at bulkheads, in order to prevent the plating being injured.

2. Local Strains.-The above strains are termed "structural," because they come on the ship regarded as a complete structure, but there are a number of isolated strains that are brought to bear only on a portion of the vessel. Such strains are termed local.

Panting. This term is used to denote the in-and-out working of the plating. It is especially found at the forward end of a ship which is subject to blows from the sea. If necessary, extra stiffening is added forward. In the large ships of the Navy panting is avoided because the forward end is made exceedingly strong to stiffen the bow and stem for ramming purposes, and also in some cases the armour protection is carried to the bow. The plating at the forward end is also well stiffened by the more closely spaced frames, and by the flats and platforms.

Severe local strains exist in the neighbourhood of heavy weights. Thus in way of an armoured barbette for the 12-in. guns, a tremendous weight is localized over a short portion of the length. Beneath such places the framing of the ship is made very strong, and bulkheads and pillars are arranged to distribute the strain throughout the structure.

Under the engines, where we have machinery in motion and not simply a dead weight, extra strength is necessary, and the framing, both longitudinal and transverse, is made very massive.

The thrust from propellers has to be transmitted to the structure of the ship by means of the thrust block. The foreand-aft framing under the thrust is constructed very strong.

In way of gun mountings also, special strengthening is necessary, not merely to support the weight of the gun, etc., but also to withstand the recoil.

The security of masts at the decks where they are wedged has to be made of sufficient strength.

Bulkheads, both longitudinal and transverse, must be built of sufficient strength to withstand the strains due to the compartment on one side being full of water to several feet above the waterline. This would bring very severe strains on the bulkhead, and the safety or control of a ship might very conceivably depend upon one of the main bulkheads remaining intact under such circumstances.

Armour is found to be much more effective in resisting penetration when provided with a rigid support, and because of this the framing, etc., behind armour is of great strength. This framing is thus far stronger than would be necessary for the ship herself at such a part. It is locally very strong for the particular purpose of supporting the armour.

Another place of exceptional strength in a large ship is the stem and the structure adjacent. Here the strength is necessary for ramming purposes. This will be referred to at some length in Chapter VII.

CHAPTER II.

TESTS OF STEEL, etc., sECTIONS, RIVETS, JOINTS, ETC.

AT the present time steel is the material almost universally employed in the building of ships. The steel employed is known as mild steel, which is a very pure form of iron, containing less than per cent. of carbon. Mild steel is quite a different material from the steel that is used for knives and tools. This tool steel is capable of taking a temper and hardening. Mild steel is produced of uniform quality and very reliable, and it is admirably adapted to stand the rather rough treatment in the shipyard, necessary in the formation of a ship's structure. A property of mild steel, which has frequently been found of immense service, is its capability of bending without fracture. There are many cases on record of steel ships having received severe injuries to the skin plating, but remaining quite intact. With wrought iron as formerly used, which is not so ductile, fracture is much more easily obtained. High tensile steel is being used in certain parts of cruisers and destroyers instead of mild steel. It is of greater strength, but is more expensive than mild steel, and it has to be carefully treated in working it into the ship to prevent reduction of the strength.

Tests of Material.-It is obviously of the greatest importance that the steel, etc., employed in the construction of the ships of the Royal Navy should be of first-class uniform quality, and in order to ensure this, a complete system of testing and inspection of material is arranged for at the works at which it is made. Only firms of good standing and having works of sufficient capacity are asked to tender, these firms being on the Admiralty List. The process of manufacture is carefully watched by the resident Admiralty Overseer; the following is an abstract of the tests carried out by the firm under his supervision.

Tests of Steel.-(a) Tensile test of mild steel to determine the

ultimate strength.-A strip is cut from the plate or bar selected by the Overseer, and it is planed to the shape shown in (a) Fig. 6, having a parallel width of 11

[blocks in formation]

tensile strength of between 26 and 30 tons per square inch before fracture, and the stretching or elongation in the length of 8 in. should be 20 per cent., i.e. when the parts are put together, as in (b) Fig. 6, the original 8 in. should be 93 in.

(b) Bending test of mild steel to determine the ductility.—This property of ductility is of importance for the reasons already stated. Strips 1 in. wide, heated to a low cherry red and cooled in water of 80° Fahr., must stand bending double to a curve of which the inner diameter is three times the thickness. test can be done cold if preferred by the Overseer.

This

For the high tensile steel used in cruisers, a tensile test of 34 to 38 tons per square inch is specified, with an elastic limit1 of 20 tons. Similar results as to elongation and ductility are required as for mild steel. For the high tensile steel used in destroyers, a tensile test of between 37 and 43 tons per square inch is specified. The elongation and ductility required are rather under those given above for mild steel.

The Overseer selects one plate or bar for testing purposes out of every batch of fifty, or less than fifty.

In addition to the above specified tests, the Overseer makes an examination of the plates and bars to see that there is no lamination or flakiness, and that there are no objectionable hollows or other surface defects. For angle bars, etc., the Overseer can also test the bars at his discretion to ascertain the ductility under ordinary working conditions.

It is the Admiralty practice to specify plates by weight per

1 Below the elastic limit the material is elastic, i.e. it will resume its original length if the strain be removed. Above the elastic limit this is not so, and "permanent set" takes place.

square foot, and angles, etc., by weight per lineal foot. Thus a steel platein. thick weighs 20-4 lbs. per square foot. A plate is ordered as 20 lbs. per square foot, and is thus really rather under

in. thick (0-49 in.). A steel angle bar specified as 3 in. × 3 in. of 7 lbs. per foot is rather under in. There is a distinct advantage in ordering steel by weight in this way. We have an exact check on the thickness supplied, because by a simple measurement of the area or length we can tell what a plate or bar ought to weigh of the specified weight per square or lineal foot. Comparing this with the actual weight supplied, any excess or defect of thickness is at once apparent. Such variations of thickness are difficult to determine by actual measurement. Small excesses of thickness would amount in the aggregate to a considerable weight in a ship's structure. With plates 20 lbs. per square foot and over, and also bars, the manufacturer is allowed a latitude of 5 per cent. under the specified weight, and no latitude above. For plates under 20 lbs., a latitude of 5 per cent. below and above is allowed.

Treatment of Mild Steel, etc.-There are certain precautions necessary in the treatment of mild steel. Plates and bars should be heated as little as possible; when heated, no work must be done when the temperature has fallen to a blue heat (600-400° Fahr.). At this temperature the steel is very brittle, and the plate or bar must be reheated to complete the work.

When holes are punched in steel, it is found that the strength per square inch of the material left is considerably less than the original strength. The forcible entry of the punch makes the steel round the hole brittle. It is found, however, that the process of riveting partially restores the strength, the influence of the hot rivet and the subsequent hammering must alter the structure of the steel round the hole. The strength is restored if the plate is annealed (heated to redness and allowed to cool slowly under a heap of ashes). For the above reasons it is laid down that butt-straps of important parts of the inner and outer bottom plating, stringers, etc., should either (a) have their holes drilled, or (b) be annealed after the holes are punched. The former is not adopted in ordinary work on account of the greater cost.

The high tensile steel mentioned destroyers has all the holes drilled. serious depreciation of the strength.

above as used in cruisers and Punching is found to cause

There are many holes in a ship's structure which have to be countersunk, i.e. the hole is made conical to take the point of the

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