Although more than a century has elapsed since its beginnings, the conditions which gave rise to the ironclad warship were the same which later were to bring about the development of the dreadnought, the aircraft carrier, and the nuclear submarine: the responses of a burgeoning industrial technology to the demands of particular strategic requirements.
The ironclads, in fact, were among the very first products of the modern age of technology, along with the railroad and the telegraph, and they were unquestionably the first of the melancholy procession of modern tools of war. As such, they were a whole era ahead of any weapons in use ashore, save the revolver. The armies of the Civil War really differed relatively little from the armies of Marlborough, or even Cromwell, but even the crudest of the navies' armored vessels would have seemed familiar to a seaman of today.
The first contribution to the development of armored warships was most certainly the marine steam engine, which had been in use since about 1815 and, by the beginning of the Civil War, had reached a relatively high state of development, although it was used at that time only as a primary means of propulsion for riverboats and tugs. While essential to the creation of ironclads, the steam engine in no way dictated or forced their adoption; nor, for that matter, did the second necessary factor: the availability of large amounts of relatively cheap wrought iron ( brought about by the rise of the railroads).
The decisive factor in their birth was the development of the heavy naval gun, and especially of the heavy shell gun.
The War of 1812 had demonstrated, to the United States Navy at least, that a few heavy, long-range guns were more effective than many smaller lighter guns. While the great frigates such as the Constitution generally carried more guns than their opponents, it was remarked that it was usually their heavy chase guns which were decisive. Similar experiences during the same period led French and British naval architects to the same conclusions, and in the 182-'s and '30's the trend in warship design began to turn toward smaller, faster vessels mounting only a few heavy powerful guns.
An early application of the pivot gun—an American gunboat of 1S04
The pivot gun, which could be rotated to fire in any direction, began to come into general use. This arrangement, which had been used for several centuries on Mediterranean galleys, as well as on minor combatant ships in the Swedish Navy in the 18th century and in both the British and the American navies at the beginning of the 19th, was generally adopted for the armament of the new steam-powered sloops and frigates.
The interest in more effective naval guns led simultaneously to the development of the large calibre rifle and the shell gun. Like the pivot gun, the principle of both was fairly well known. Explosive shells had been used in mortars for many years, and the muzzle-loading rifle, as students of the American Revolution well know, was already in use as a small calibre weapon. Improved methods of producing and forming wrought iron now made large calibre rifles feasible.
The shell gun, for which the French General Paixhans must be credited, spelled doom for the wooden ship. Hitherto, the sides of even the heaviest ships had to contend only with solid shot, which simply attempted to punch a hole through the wood. Now the physical properties of wood are such that it is well suited to withstand this kind of shock, coming as it does across the grain, and since wood is naturally buoyant it is always possible, in theory at least, to make the sides thick enough to absorb the impact of virtually any size solid shot. The effect of an explosive shell, however, is something else again. The shell strikes the wooden wall, burrows part way into the wood, and then, almost entirely confined, explodes under almost ideal conditions. And, since its force is exerted in all directions, it splinters the wood (and most likely sets it afire). The difference can be compared to that between chopping a log and splitting it.
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