rivet (see D, Fig. 9). In this case the injured material round the punched hole is removed by the countersinking drill, and the strength of the remaining material is fully equal to that of the original.

Pickling. It is found during the process of manufacturing steel plates that there is formed on the surface a black oxide or scale, called mill scale, which clings very tenaciously to the surface. If, however, the plate is damaged, the scale will peel off and take with it the paint, leaving the bare steel underneath liable to corrosion. This scale also is electro-negative to the steel, so that if moisture gets in under the scale, corrosion of the steel will go on very rapidly. It is, therefore, most important to remove all traces of the mill scale before the steel is painted. In Admiralty work it is the practice to immerse the plates on edge for a few hours in a bath containing dilute hydrochloric acid (1 part acid to 19 parts water). This loosens the scale, and the plates when removed are brushed with wire. brushes and washed with a hose to remove the scale. The portions of the structure thus pickled are those liable to come into contact with sea-water or water in the bilges. These are the plates of the outer and inner bottoms, lower plates of bulkheads, plates of freshwater tanks, and the lower plates of frames.

Rivet Steel and Rivets, Tests of.-The efficiency of the structure of a ship must ultimately depend on the quality of the rivets used to connect the various portions of the hull together. The following is a summary of the tests made to ensure having rivets of first-class quality:

The steel bars from which the rivets are made must stand a tensile test of between 26 and 30 tons per square inch, with an elongation of not less than 25 per cent. in 8 in. For the bending test, the bar, either cold or heated as for mild steel, must stand bending double over a block having the same diameter as the bar. These tests are seen to be rather more severe than those for mild steel.

The rivets themselves, when made, must cool slowly, and samples taken at random must stand the following tests :

(i.) A cold test as (a), Fig. 7.

(ii.) A hot test as (b), Fig. 7.

(iii.) A hot test as (c), Fig. 7, the head being flattened out to two and a half times the diameter of the rivet without cracking.

(iv.) The shank is nicked and bent over to show the structure of the steel.

1 I.e. If an electric current is set up between the scale and the steel, the steel will corrode, like the zinc does in the zinc-copper cell.

Rivets for use with high tensile steel are of special quality, so as to utilize the greater strength of the steel. These rivets are made with three projections on the head, as P, Fig. 9, to distinguish them from ordinary rivets.

Steel Castings, Tests of.-There are many parts of a ship's structure which formerly were made of iron forgings; these can now be more conveniently made of cast steel, and this material is always used in steel ships for the stem, sternpost, rudder-frames, shaft brackets, etc. The following are the tests laid down to ensure satisfactory castings:

On each casting three projections are cast, to provide, when cut off and planed, test pieces 1 in. square. One of these must stand a tensile test of 26 tons per square inch, with an elongation of 10 per cent. in 8 in. One must stand bending to an angle of 45° over an edge having a radius of 1 in. The other

test piece is available if the results of either of the others are disputed. It will be noticed that these tests are less severe than for mild steel.

Although the material of the casting may satisfactorily stand the above tests, it is most necessary to find out, if possible, whether any blow holes exist in the body of the casting. In order to thoroughly shake up the casting, it is either (a) raised to an angle of about 60°, or (b) lifted bodily to a height of about 12 ft. and dropped on hard ground. The latter would be adopted for small castings. The casting also is suspended in chains and hammered all over with a heavy sledge to find out if it rings true; the surface is also carefully examined for defects.

Phosphor Bronze Castings, Tests of.-When a vessel is sheathed with wood and copper, it is not possible to use cast

steel for the stem, sternpost, etc., because of the galvanic action that takes place between copper and steel, if metallic connection is established between them. For such ships, therefore, the stem, sternpost, etc., are made of the copper alloy, phosphor bronze. This consists of 90 per cent. copper with 10 per cent. of phosphor tin (containing 5 per cent. of phosphorus). This material should stand a tensile test of 17 tons per square inch, with an elongation of 15 per cent. in 6 in. This is seen to be considerably lower than for cast steel. A chemical analysis of some drillings is made to see how near the material is to the composition specified.

Sections of Steel.-One great advantage attending the use of

iron or steel for shipbuilding purposes is the possibility of obtaining the material in convenient and efficient forms. Thus, to form a beam in a wooden ship, it would be necessary to take a log of timber of solid rectangular section. We have seen in the previous chapter that in such a section the material is not disposed to the best advantage to resist bending. A beam of iron or steel can be rolled so that it has a broad upper flange and a bulb or flange at the lower part, both of which are so situated as to be most effective against bending. Examples are seen in Fig. 8.

The various sections used in Admiralty shipbuilding are shown in Fig. 8.

The angle bar (a) is used to connect plates together; as a stiffener to a plate; for beams, frames, etc.

The tee bar (b) is used as a stiffener to important bulkheads as (n) and inside masts.

The channel bar (ƒ) is used as a stiffener in certain cases.

The zed bar (e) is largely used for transverse framing, to avoid the use of two angles riveted back to back. It is also used as a bulkhead stiffener.

The I or H bar (g) is used for the main stiffening of important bulkheads.

The angle bulb (c) is used for deck beams.

The tee bulb (d) is used for deck beams.

A flange (k) is frequently put on the edge of a plate to serve the purpose of an angle bar for stiffness or connection.

The half round (1) is usually hollow, and is used as a moulding round the ship.

The segmental bar (m) is used as a finish round the top of hatch coamings, etc.

The sections shown in Fig. 8 are drawn to scale, and show the exact form of sections now used. It is worth noting that the flanges of the zed, channel, and I bars are more substantial than the webs. These sections thus illustrate very clearly the principles touched upon in Chapter I. in connection with beams.

Rivets, Forms of, etc.-Fig. 9 shows the usual form of rivets and riveted connections employed in Admiralty shipbuilding.

A is the most common form of rivet, called a pan head rivet, from the shape of its head. It will be noticed that the rivet is formed with a conical neck. All rivets -in. diameter and above are thus formed, because the hole formed in the plate by punching has a slight taper, and it is most necessary that the rivet should completely fill the hole (see D, etc., Fig. 9).

D, E, F, Fig. 9, show various points associated with the pan head. D is the countersunk point necessary when the surface has to be flush, as for the skin plating. The hole formed by the punch has to be made conical by the countersinking drill. E is the point adopted for most of the internal work. No countersinking is done, and the point is hammered up full as shown. F is the point adopted where a finished appearance is desirable; this point is called a snap point.

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B is a snap head rivet, and it is associated with a snap point, as G, when the riveting is done by the hydraulic riveter.

In some few cases it is necessary to have a flush surface on both sides, and in this case a countersunk head rivet is used, as C, and the point is also countersunk, as H.

Certain parts of the structure cannot be riveted by the ordinary means, as, for example, where outer bottom plating is connected to a stem casting. In such cases tap rivets are used, as L, M, and N (Fig. 9). In L, the most usual form, the head is countersunk, and the square projection is chipped off after screwing up. Where it is necessary to have the work portable, a recess is fitted in the head,

as N. Where a flush surface is not necessary, as for some internal work, the head is made hexagonal, as M.

In parts of the ship behind armour, the place would be dangerous in action, because when the armour is struck the rivet heads are likely to fly off. In such places, where men are likely to be, the inside of the framing is closed in with a steel lining (see Fig. 13, on the main deck). This lining is secured to the framing, etc., by screw rivets, as K, Fig. 9. Tap rivets, as L, M, or N, would be undesirable, because of the likelihood of the rivet breaking and the head flying out if the armour was struck.

Laps, Butts, etc.-When two plates are connected together,

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