The middle years of the 19th century witnessed the advent of radical changes adopted on the construction and arming of warships. In successive decades, the traditional wooden hull would be replaced by that of iron construction, and sail rigs, which initially remained in place as an auxiliary mean, would be eventually suppressed in favor of a reliable steam propulsion. This article narrates, in a concise yet comprehensive way, the different phases through which warships evolved during the second half of the 19th century, to become the basic design that would be continually upgraded until the decade of 1950, when the design of warships passed through another deep conceptual shift.

First generation: the broadside ironclad

The Crimean War of 1853-1856 demonstrated the extreme vulnerability of wooden ships against the new explosive projectiles of very increased destructive power. As a result of this experience, France, one of the belligerents, began, shortly after the end of the conflict, the conception and construction of the first oceanic warship that would be provided with an iron armor. In that time, the "sovereign of the seas" was the three-decked ship of the line, bulky and armed with a hundred of artillery pieces. These capital ships were then known as battleships, a simplified term derived from their official condition of "line of battle ships".

From 1850, these very characteristic ships (as they were always painted in black with white strips running along the gun decks) started to be fitted with steam engines attached to a rear screw propeller, as in modern ships. This meant an important tactical advantage, as the ships no longer would be fully dependant on wind for their combat maneuvers. However, the primitive steam engines used then were not yet very suitable for regular propulsion. It was at this point when a more pressing problem showed up, as the inherent weakness of a tall wooden hull against the modern artillery had been made evident.

Since the new French battleship would have a thick and hence heavy iron coat riveted to her wooden hull, this one no longer could have three or even two gun decks, as the ship would not be able to remain afloat. So, the battleship Gloire, launched in 1859, had only one gun deck and because of this she would be soon acknowledged as an "armored frigate", as were all the other similar ships that would be promptly built after her. This term, albeit generally accepted, is not entirely correct, as frigates were not as heavily armed as battleships, and it was irrelevant that these had only a fraction of the numerous artillery pieces found in ships of the line, as these would not be able to pierce through an iron armor.

French ironclad Gloire
The Gloire had a wooden hull, reinforced with 43 centimeters of timber and 12 centimeters of wrought iron. At that time, this layout could endure impacts from the most powerful artillery pieces of that time (the French 50-pounder and the British 68-pounder), effectuated at full charge and at a distance of only 20 meters. With a length of 77.8 meters, a beam of 17 meters, a draught of 8.4 meters and a standard displacement of 5630 tonnes, the Gloire was not as tall as a ship of the line but she was equally large and heavy. However, her original light barquentine rig of 1096 square meters was clearly insufficient for an effective propulsion, so it had to be later increased to a full rig of 2508 square meters. With a load of 665 tonnes of coal, the Gloire could keep her steam engine running during twelve days at an economic speed.

As the reader may imagine, the apparition of this ship suddenly rendered obsolete the unarmored ship of the line which had been the capital ship during two centuries, and every major navy in the world had no choice but to form its own fleet of ironclads (as the new battleships were called). In turn, the Gloire would be soon outclassed when the British ironclad HMS Warrior, first warship built with an integrally metallic hull, was launched during the last days of 1860.

With an overall length of 128 meters and a standard displacement of 9284 tonnes, the HMS Warrior was also the largest warship built until then, and fitted with a full rig of 4497 square meters she was more capable in the sea than the French counterpart. To the thirty-six 164-millimeter muzzle-loading rifled cannons of the Gloire, the HMS Warrior opposed twenty-six 68-pound muzzle-loading smoothbore cannons, ten 110-pound breech-loading rifled cannons and four 40-pound breech-loading rifled cannons. The Gloire would probably withstand an artillery barrage from the HMS Warrior, but France would have to keep improving its original idea to match its very industrious and ancestral adversary.

British ironclad HMS Warrior
In 1863 the French shipyards which had built the Gloire created a new battleship which looked much like the former but was rather technically superior. This ship had been ordered from Spain, where in that time iron naval construction was not available. The Numancia (as she was called after the famous town besieged by the Roman Empire) was the first warship built with an all-iron hull that served in the Spanish Navy. As a whole, the Numancia was probably equivalent to the HMS Warrior. While this one had a slightly superior firepower and better performance on propulsion power, the Numancia featured a more effective armor scheme. The armor had a thickness of 10-13 centimeters on the belt, backed up by 44 centimeters of teak wood. This was slightly thicker than the 11.4 centimeters and 41 centimeters on the HMS Warrior, but the most important aspect was the distribution of the armor, as explained in the next section of this article.

French/Spanish ironclad Numancia
As in previous French ironclads, the rig was of lesser importance than in British counterparts. The Numancia required 1100 tonnes of coal to keep her steam engine running during twelve days at an economic speed, as she was somewhat larger than the Gloire, having a length of 96.1 meters, a beam of 17.3 meters, a draught of 7.9 meters and a standard displacement of 7500 tonnes. Still so, the inner space was not as generous as it was in the HMS Warrior, and before the delivery of the ship six cannons were removed, as there were complaints about the foremost pieces being hindered by the anchor chains and the four aftermost pieces taking too much space on the dining area. So, the Numancia was finally armed with thirty-four 68-pound muzzle-loading smoothbore cannons.

French/Spanish ironclad Numancia
The habitability in the Numancia was, in general terms, better than in other contemporary ships. While in the engine rooms of other ships the workers had to endure temperatures of up to 70 Celsius degrees, in the Numancia these did not exceed 42 degrees. As the Spanish Navy often operated in areas of warm climate, the Numancia was the first French ship built with forced ventilation. Besides, the space per person granted by this ship (4.54 cubic meters) was considerably larger than in other contemporary ships. Ventilation and accommodation was favored by a longer hull and taller living spaces. The infirmary, located on the forecastle, was well ventilated and could accommodate up to 18 beds.

Basic concepts

Now that we have seen some prominent examples of broadside ironclads, it will be interesting to deepen into some technical aspects before advancing into the later generation of ironclads. The fundamental characteristic of ironclads is their armor, and the most fundamental element of protection is the citadel. This element is so fundamental that it has been present in any armored ship, from the earliest ironclads to the last battleships built during World War Two. The citadel is a box-like structure made of armored plates whose purpose is to provide protection to the most vital elements of a warship: the armament and the machinery.

As the citadel was a particularly heavy element on a warship its size had to be downgraded as much as possible. This meant that the engines and boilers should occupy the lesser space possible and that the cabins and rooms occupied by crewmen and materials were left unprotected against heavy calibers. The citadel was formed by four walls: the two side belts and the two transversal bulkheads which connected the belts at both ends. Protection against heavy calibers did not exist on the upper part, even if there was a considerable chance of impact on the deck, which increased proportionally with combat range.

Armor scheme of ironclads
On the Gloire, the plates that formed the belts, of 12 centimeters in thickness, were fastened to the wooden hull, of 43 centimeters in thickness, through numerous and very long bolts which added a significant weight to the ship; in this case, the armor alone weighed about 900 tonnes. On an all-iron hull, like that of the HMS Warrior or that of the Numancia, the constructive method was similar, but in this case the wooden backing occupied the space between the armor plates and the iron structure of the hull. The wooden backing acted as a shock absortion element, preventing the strong vibrations caused by a direct impact on the armor from reaching the structural elements of the hull, which could have negative if not disastrous consequences.

In French naval construction, the side armor extended from the upper deck to about two meters below the waterline, and ran along the whole hull. This is how the Gloire and the Numancia were built. It seems that the hull of the HMS Warrior was actually excessively long and not less than a third part of it was left unprotected. In fact, only twenty-six of the forty cannons onboard were inside the citadel. But what has been pointed as the worst deficiency was that the steering room was outside the citadel as well. And unlike the French ironclads, the HMS Warrior preserved the elegance of capital ships from the age of sail; reluctant to leave apart the comfort that they had traditionally enjoyed, the British officers had at their disposal a wide gallery which helped nothing to improve protection.

Naval artillery was made heavier and more powerful, developing faster than armor, and the iron plates of the Gloire and the HMS Warrior no longer could withstand the projectiles. Because of this the armor of the new ironclads was reinforced and concentrated amidships together with an artillery of larger caliber, to avoid an excessive weight. In some warships the armament was protected from axial fire by two transversal bulkheads; this arrangement was known as casemate. Many former ships of the line were rebuilt with armor and bow spur, in an attempt to adapt them to the most uncertain times that the history of warships has witnessed. Technical progress was being very fast and a warship could be obsolete already at the moment of being launched.

In the center batteries or casemates cannons such as the 254-millimeter Armstrong depicted below were installed. The firing chamber of these pieces was extraordinarily reinforced, rendering them very heavy and cumbersome to manage. There was a rack and rail mechanism which allowed a limited rotation of the carriage, and the ammunition, now of ogival shape to improve precision, had to be fed with the help of a hoist. Broadside ironclads were equipped with simpler pieces of older design, but they could receive armament of this type during upgrades on their operative life.

Armstrong 254-millimeter cannon
Center battery ironclads were a transition type between the broadside and turreted types, and so they enjoyed only an ephimeral existence. On the first artillery turrets were installed muzzle-loading cannons as well, which meant that the tubes could barely protrude outside the turret. The adoption of breech-loading cannons allowed the utilization of longer barrels, which greatly improved the range and piercing power of the artillery, along with the adoption of rifled bores.

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