ARMOR.
Material. In 1861, the British iron-plate committee fired with 68-pounders at many varieties of iron, cast-steel and puddled-steel plates, and combinations of hard and soft metals. The steel was too brittle, and crumbled, and the targets were injured in proportion to their hardness. An obvious conclusion from all subsequent firing at thick iron plates was, that, to avoid cracking on the one hand, and punching on the other, wrought-iron armor should resemble copper more than steel, except that it should be elastic, although not necessarily of the highest tensile strength. Copper, however, proved much too soft. The experiments of Mr. E.A. Stevens of Hoboken, with thick plates, confirm this conclusion. But for laminated armor, (several thicknesses of thin plates,) harder and stronger iron offers greater resistance to shot, and steel crumbles less than when it is thicker. The value of hard surfaces on inclined armor will be alluded to.
Solid and Laminated Armor compared. Backing. European experimenters set out upon the principle that the resistance of plates is nearly as the square of their thickness,—for example, that two 2-inch plates are but half as strong as one 4-inch plate; and the English, at least, have never subjected it to more than one valuable test. During the last year, a 6-inch target, composed of 5/8-inch boiler-plates, with a 1-1/2-inch plate in front, and held together by alternate rivets and screws 8 inches apart, was completely punched; and a 10-inch target, similarly constructed, was greatly bulged and broken at the back by the 68-pounder (8 inch) smooth-bore especially, and the 100-pounder rifle at 200 yards,—guns that do not greatly injure the best solid 4-1/2-inch plates at the same range. On the contrary, a 124-pounder (10 inch) round-shot, having about the same penetrating power, as calculated by the ordinary rule, fired by Mr. Stevens in 1854, but slightly indented, and did not break at the back, a 6-5/8-inch target similarly composed. All the experiments of Mr. Stevens go to show the superiority of laminated armor. Within a few months, official American experiments have confirmed this theory, although the practice in the construction of ships is divided. The Roanoke's plates are solid; those of the Monitor class are laminated. Solid plates, generally 4-1/2 inches thick and backed by 18 inches of teak, are exclusively used in Europe. Now the resistance of plates to punching in a machine is directly as the sheared area, that is to say, as the depth and the diameter of the hole. But, the argument is, in this case, and in the case of laminated armor, the hole is cylindrical, while in the case of a thick armor-plate it is conical,—about the size of the shot, in front, and very much larger in the rear,—so that the sheared or fractured area is much greater. Again, forged plates, although made with innumerable welds from scrap which cannot be homogeneous, are, as compared with rolled plates made with few welds from equally good material, notoriously stronger, because the laminae composing the latter are not thoroughly welded to each other, and they are therefore a series of thin plates. On the whole, the facts are not complete enough to warrant a conclusion. It is probable that the heavy English machinery produces better-worked thick plates than have been tested in America, and that American iron, which is well worked in the thin plate used for laminated armor, is better than English iron; while the comparatively high velocities of shot used in England are more trying to thin plates, and the comparatively heavy shot in America prove most destructive to solid plates. So that there is as yet no common ground of comparison. The cost of laminated armor is less than half that of solid plates. Thin plates, breaking joints, and bolted to or through the backing, form a continuous girder and add vastly to the strength of a vessel, while solid blocks add no such strength, but are a source of strain and weakness. In the experiments mentioned, there was no wooden backing behind the armor. It is hardly possible,—in fact, it is nowhere urged,—that elastic wooden backing prevents injury to the armor in any considerable degree. Indeed, the English experiments of 1861 prove that a rigid backing of masonry—in other words, more armor—increases the endurance of the plates struck. Elastic backing, however, deadens the blow upon the structure behind it, and catches the iron splinters; it is, therefore, indispensable in ships.
Vertical and Inclined Armor. In England, in 1860, a target composed of 4-1/2-inch plates backed with wood and set at 38° from the horizon was injured about one-half as much by round 68-pounder shot as vertical plates of the same thickness would have been. In 1861, a 3-1/4 plate at 45° was more injured by elongated 100-pounder shot than a 4-1/2 vertical plate, both plates having the same backing and the weights of iron being equal for the same vertical height. When set at practicable angles, inclined armor does not glance flat-fronted projectiles. Its greater cost, and especially the waste of room it occasions in a ship, are practically considered in England to be fatal objections. The result of Mr. Stevens's experiments is, substantially, that a given thickness of iron, measured on the line of fire, offers about equal resistance to shot, whether it is vertical or inclined. Flat-fronted or punch shot will be glanced by armor set at about 12° from the horizon. A hard surface on the armor increases this effect; and to this end, experiments with Franklinite are in progress. The inconvenience of inclined armor, especially in sea-going vessels, although its weight is better situated than that of vertical armor, is likely to limit its use generally.
Fastening Armor. A series of thin plates not only strengthen the whole vessel, but fasten each other. All methods of giving continuity to thick plates, such as tonguing and grooving, besides being very costly, have proved too weak to stand shot, and are generally abandoned. The fastenings must therefore be stronger, as each plate depends solely on its own; and the resistance of plates must be decreased, either by more or larger bolt-holes. The working of the thick plates of the European vessels Warrior and La Gloire, in a sea-way, is an acknowledged defect. There are various practicable plans of fastening bolts to the backs of plates, and of holding plates between angle-irons, to avoid boring them through. It is believed that plates will ultimately be welded. Boiler-joints have been welded rapidly and uniformly by means of light furnaces moving along the joint, blowing a jet of flame upon it, and closely followed by hammers to close it up. The surfaces do not oxidize when enveloped in flame, and the weld is likely to be as strong as the solid plate. Large plates prove stronger than small plates of equally good material. English 4-1/2-inch armor-plates are generally 3-1/2 feet wide and to 24 feet long. American 4-1/2-inch plates are from 2 to 3 feet wide and rarely exceed 12 feet in length. Armor composed of light bars, like that of the Galena, is very defective, as each bar, deriving little strength from adjacent, offers only the resistance of its own small section. The cheapness of such armor, however, and the facility with which it can be attached, may compensate for the greater amount required, when weight is not objectionable. The 14-inch and 10-inch targets, constructed, without backing, on this principle, and tested in England in 1859 and 1860, were little damaged by 68-and 100-pounders.
The necessary thickness of armor is simply a question of powder, and will be further referred to under the heads of Ordnance and Naval Architecture.