The idea of sheathing ships with projectile-resisting metal undoubtedly existed before any attempt was made to put it into practice. It is reported that a Korean admiral used ironclads in the late 16th century. The first European proposal to do so was made by Sir William Congreve in England in 1805, but the first ironmaster to make the attempt was John Stevens of Hoboken, N. J., some 7 years later. Although other metals were considered, only iron, wrought or cast, seemed feasible as a protective covering; and of these, wrought iron showed itself to be superior. The first practical armor consisted of 4 or 5-inch wrought-iron plate, backed by 36 inches of solid wooden timber. The iron industry of the time, however, was not equal to the production of the necessary heavy forgings.

The Bessemer converter for the production of steel, followed a few years later by the Siemans-Martin open-hearth process, changed the course of development. The French, in 1876, produced a 22-inch mild steel plate (which is said to have been hammered to that dimension from a thickness of 7 feet) that resisted the fire of all guns then in use. It was the best armor produced up to that time, but its tendency to crack led to a return to research in built-up armor. The next advance resulted from the development of two rival processes. The first was the Wilson-Cammel compound plate which consisted of an open-hearth steel face cast on top of a hot wrought-iron back plate. The second was the Ellis-Brown process of cementing a steel face to an iron back by pouring molten Bessemer steel between them. In both cases, the plates were rolled after compounding. Actually, only a small advance had been made. All the efforts of the steel men and Naval designers had amounted to a mere 25 percent advantage: 10 inches of compound armor was only about the equal of 12 1/2 inches of iron. It was some progress, but not much.

In the early 20th Century armor was made of steel in which the face of the plates was hardened by a process of super-carbonization; the aim was to make this hard face more rigid than the point of the projectile and, at the same time, to have the plate as a whole too tough to be shattered by the impact. Two kinds of face-hardened armor were used in the U.S. Navy - the "Harvey" and the "Krupp." The Harvey process was used for plates having a thickness of five inches or less; the Krupp process for those having a greater thickness than five inches. Thin plates and hollow forgings, such as turret tops, doors, communication and ammunition tubes, are made of homogeneous nickel steel, oil or water-tempered and annealed, but not face-hardened. The bolts are of the best quality of nickel steel, oil or water-tempered and then annealed.

Hayward Harvey was proprietor of Harvey Screw and Bolt Company. In 1885 Harvey conceived the idea of making a bolt and nut of cast iron and then hardening or "steelifying" the surfaces so as to give them the necessary toughness. Harvey pursued his experiments, and soon succeeded in producing from ordinary low-grade Bessemer steel a steel equal in every respect to the finest crucible or cast steel. The first patents on this new product and process were granted to Mr. Harvey in 1888. He then decided to make some experiments looking to surface hardening armor; and the result was the Harvey process of surface carburization and hardening. In this process the plate is placed in a furnace , leaving the surface to be hardened uppermost. This is covered with carbonaceous material, which is rammed down upon it. The temperature of the furnace is then raised to about the temperature of melting cast iron and kept so for several days, until the required additional carburization - usually about 1 per cent - is effected. The plate is then removed, and, when cooled to a dull cherry red, is hardened by a water jet or immersion in running water. The trial of the first plate made by the Harvey process took place February 14, 1891, and was very successful. The popularity of the new armor was immediate.

Soon after the appearance of Harveyized armor, Krupp, Schneider, the Terni Works, and several other European makers began experiments along the same lines. The addition of nickel (about 3-1/4 percent), aided somewhat by the small percentage of manganese in the case of Krupp armor, gives great strength and toughness. The further addition of the chrome gives the metal more sensitiveness to temper, thereby permitting a greater depth of chill below the surface. It increases the affinity of the metal for carbon, which results in super-carbonization to a greater depth, and, with careful treatment, gives the plate a tough fibrous back which resists cracking. It is this quality which marked the superiority of Krupp armor.

The important decremental hardening process was introduced by Krupp shortly after the development of carburizing. The Krupp armor was processed by burying the plate, all but the face to be hardened, in clay, and exposing the face to a high, quick heat. This heat traveled from the face of the plate toward the back in an evenly descending plane, and when the critical heat for hardening had penetrated to from 30 to 40 percent of the thickness, the plate was removed to a spray pit and chilled by water played at first on the face alone and, a few moments later, on both sides of the plate together. This process may be applied to carburized or noncarburized armor as a final treatment.

The thickness of armor depends first upon the importance of the position it is designed to protect and next on the resisting power of the armor. In the early 20th Century, for the purpose of design, it was assumed that Kruppized armor will keep out projectiles of caliber equal to its thickness at fighting ranges. Plates above 5 inches in thickness consisted of cemented or supercarbonized steel plates, face-hardened by the Krupp process, comprising, on battleships, the main side-belt armor, turrets, barbettes, conning-tower, casemate, and intermediate-battery protection. (Plates not over 5 or less than 3 inches in thickness consisting of steel plates, face-hardened by the Harvey process, comprised the side-armor plates forward and aft, when the thickness tapers to 5 inches or less, fore-and-aft and athwartship bulkheads, etc. Thin plates, such as splinter bulkheads, protective-decks, protection for minor-caliber guns, turret-tops, sighting-hoods, signal-towers, ammunition- and conning-tower tubes are made of homogeneous nickel steel. Small, thick pieces, such as armor doors, strips above gun ports, etc., were made of Krupp non-cemented steel.

Protective- or armored-decks may properly he classed as armor and their function is so peculiar and important that it merits special attention. In all armored vessels, the value of in all armored vessels, the value of protective-decks is recognized. In designs up to, and including, the Maine class, it was the practice to work a deck at the level of the top of the side-armor for the length of the machinery and boiler spaces, carrying it to the bow and stern with sloping sides, the angle being usually 45 degrees, and sloping to below the water-line at the bow and sfern. In later designs, the slope is carried fore and aft, the angle of inclination remaining 45 degrees, touching the bottom of the armor-belt. In general, the thickness of the armored deck, both for battleships and armored cruisers, is from 2 to 4 inches on the slopes and from 1 to 2 inches on the flat.

Armored decks were very heavy, taking up a considerable proportion of the total weight of the armor. While the same weight applied to the side-armor would no doubt offer equal if not greater resistance to actual penetration, the fact that such decks will prevent fragments of shell that might perforate the side-armor, from reaching the vitals, coupled with the great structural strength it adds to the vessel, made armored decks an essential if not indispensable feature of warship design. A splinter-deck, about one inch thick, is sometimes worked in as the deck next above the protective-deck.

The evolution of design with reference to armor is brought out by a consideration of the types from the Oregon to the Connecticut class. The former has a very heavy water-line belt, 18 inches in thickness, extending along the machinery, magazine, and boiler spaces. At the ends of this are the barbettes, 17 inches thick, rising on the center line above the main deck. The ends of the ship were unarmored. The barbettes protect the turret turning-gear and ammunition supply. Mounted over the barbettes are the turrets for the 13-inch guns, with 15 inches of armor at the sides and back and 1 7-inch port plates. The casemate armor was 5 inches thick, but is of limited extent, and but poorly protects the two broadside 6-inch guns. The quadrilateral 8-inch turrets have 8J/2 inches of armor, the barbettes were 10 inches and the ammunition tubes 3 inches thick. Part of the secondary battery was protected by 2-inch armor, and thin gun-shields. There was a conning-tower, 10 inches thick, and communication tube. These vessels were designed before the Harvey process had been adopted for armor, though most of the armor was treated by that process.

The Iowa followed, the armor being distributed on the same general lines, but thinner in corresponding positions. In the Kearsarge and Kentucky there was a slight decrease in thickness to 16 inches and an extension of the water-line belt to the bow, tapering in thickness to 4 inches. In place of the quadrilateral 8-inch turrets of the Iowa, 8-inch turrets were superposed on the 13-inch turrets. This arrangement provoked much criticism. The barbette and turret-armor were slightly decreased in thickness and more importance given to the casemate-armor, 5 inches thick, which was made to cover all the space between the barbettes. The conning-tower, tubes, etc., remained practically the same.

Next came the Illinois class, including the Alabama and Wisconsin. The principal features of the armor design of these vessels was the abandonment of the 8-inch turrets, a more extensive use of the casemate armor, which was made 5^/2 inches thick to protect the 6-inch battery, the introduction of splinter-bulkheads, and what is very important, the adoption of the inclined port-plate for the 13-inch turret-guns.

The Maine class followed. The general design of these vessels was similar to that of the Illinois ; 6-inch armor was used for the casemates, and Krupp armor being at this time adopted, the armor protection was considered to be very much superior to the Illinois class. In the Virginia class there is a return to the 8-inch turrets, two being superposed on the 12-inch like the Kearsarge and Kentucky and two being placed in broadside. However, the inclined port-plates of the former which permits of a minimum port opening, and at the same time are inclined at a sufficient angle to deflect projectiles, give very much better protection, though the thickness is only 6 inches against 8 inches for the Kearsarge. The belt was made 11 inches, but was carried all the way aft, tapering to 4 inches at the stern as well as at the bow. The casemate was still further extended, being carried forward and aft with athwartship bulkheads so as to include the 12-inch barbettes.

In the Connecticut and Louisiana, there was a reduction in thickness to 11 inches in the water-line belt, the 8-inch turrets are disposed quadrilaterally, the upper casemate armor is made 7 inches thick to protect the 7-inch guns. The later vessels follow the general design of the Connecticut and Louisiana. The belt, however, is reduced to 9 inches in the Kansas class.

The decremental face-hardening remained the general process by which protective armor was produced through World War II, though further refinement of the method constantly went forward. Carburized face-hardened plating is known as Class A armor. Its use is protection of the vertical surfaces around the more vital parts of heavily armored ships - the sides, the turrets, the barbettes, etc. The angle of obliquity is measured between the axis of the projectile and the normal to the plate at the point of impact. The impact of a projectile against such surfaces would necessarily be at a very small angle of obliquity and as such would have to be withstood by a very hard face to resist the initial impact, plus great backing strength to absorb the shock. Class A armor must defeat a projectile by stopping it, by breaking it up, or by rupturing the explosive cavity (thus reducing its effectiveness even though it penetrates the plate). Such armor must be of considerable weight, and naval design admits of the use of only a limited quantity of it. Enclosing the hull of a ship with heavy armor not only does not add to the strength of the craft, but actually diminishes it, for the great mass, affixed to the framing members and other strong points, complicates the stresses. For that matter, all armor represents dead weight, and naval designers must balance the requirements of essential protection against dead weight. Class A armor can be machined only with difficulty, and cannot be fitted snugly against the skin of a ship. The accepted method is to suspend it from the strong points of the hull by means of extended watertight bolts which allow about 2 inches clearance between the armor and the hull, and then to fill the space with concrete.

Class B armor is designed for the protection of horizontal surfaces, and otherwise, where the anticipated angle of obliquity is great, is physically quite different, although chemically about the same. Here, instead of boldly meeting force with resistance, advantage can be taken of the tendency of a projectile to ricochet. This glancing rebound is best achieved when the impact of the projectile is met with a plate that gives slightly, thus spreading the force over a wider area. Moreover, the curvature of the depression induced by the impact tends to pick up the curvature of the ogive, further inducing the projectile to rebound harmlessly away by increasing the angle of obliquity immediately after the instant of contact. Homogeneous armor can be used in this application. Homogeneous armor is not face-hardened. In thicknesses 3 inches and less, it is called "STS" (Special Treatment Steel). In thicknesses greater than 3 inches, it is termed class B armor. It can be integrated with the structure of the ship, but it complicates the problem of weight, since it is heavy and lies high in the ship's structure, thereby shifting materially what would otherwise be the ship's normal center of gravity.

Light armor is a rough designation for any armor less than 2 inches thick. It is for the most part constructed like heavier armor, though some is compounded armor consisting of a hard face fused to a tough back. Its prospective usefulness as a protection for aircraft personnel and engines makes this one of the most important fields of ballistic research. Weight, however, is again a limiting factor. Nonferrous armor (of aluminum alloy) can be used to protect against fragments. And plastic armor may be used for personnel.

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