The Brooklyn class light cruisers were the US Navy's first attempt at a "light cruiser" built under the terms of the London Treaty of 1930. This treaty banned the construction of 8" gun heavy cruisers, on which the US Navy had previously focused its efforts. Probably the most noticeable feature of the class is the gun layout, an unprecedented fifteen 6" guns in five turrets, three forward and two aft.
As the US Navy studied the feasibility of a 10,000 ton cruiser, the Brooklyn design started off from where the New Orleans class heavy cruisers left off. But the advent of Japan's Mogami class cruisers, which featured fifteen 6" guns, led to the requirement that Brooklyn carry at least as many. The distinctive three turrets forward, two aft configuration was unique among US Cruisers, but the Brooklyns shared this configuration with the Japanese Mogamis. The general outline of the gun mounts as undoubtely copied from the Japanese Takao class heavy cruisers.
The ships, with its 6" main gun, ended up as a totally revised design, being the source of all subsequent US cruiser designs up to the Des Moines class. The class was unique in different respects, as they were the first flushdeck cruisers, which was necessary because the ships were to receive a hangar in the stern, belowdecks.
The London Naval Treaty (1930) had the immediate effect of triggering construction of the first seven Brooklyn Class light cruisers. These cruisers and the Brooklyn based design for the Wichita mark a defining moment in US cruiser design. Previous cruisers had evolved steadily under the limitations of the Washington Naval Treaty while the cruisers to follow would be unfettered by Treaty limitations. The Brooklyn Class undoubtedly would never have been built were it not for the influence of the London Treaty of 1930. The US Navy suddenly found itself in the difficult situation of being obligated to build 6-inch gun cruisers regardless of the military value of such ships. Absent the 1930 Treaty, there is little doubt that the US Navy would have preferred improved 8-inch gun cruisers.
The legacy of the London Treaty of 1930 stretched far beyond the Brooklyn Class. Both major wartime classes, the Clevelands and the Baltimores, share a direct lineage with the Brooklyn Class cruisers. This influence was principally channeled through the Wichita, an 8-inch Brooklyn Class variant. Consequently, the unrestricted wartime cruisers were heavily based on the Treaty-restricted Brooklyn Class ships.
Following the ratification of the London Naval Treaty, C&R began a study of possible 6-inch gun cruiser designs. They were to have a speed equal to the 8-inch gun cruisers with as near a range to the 8-inch gun cruisers as was possible. Acceptance of the US Navy eighteen ship limit stipulated in the 1930 agreement hinged on C&R estimates that showed a well designed light cruiser could strike an acceptable balance between guns and protection. C&R believed the ship could be made to stand up to heavy cruiser fire, while retaining adequate armament. This was based on the concept of "Immune Zone" protection, an idea which seems to have originated in the US Navy during or shortly after World War I. It was originally applied to capital ships.
The immune zone was a range band within which a ship's armor was intended to defeat enemy projectiles. For example, a certain cruiser might have armor designed to protect it against 8-inch shells from 15,000 yards to 22,000 yards. This means that for direct fire closer than 15,000 yards, an 8-inch shell was expected to have enough energy to penetrate the ship's side armor, but beyond 15,000 yards it did not. Beyond 15,000 yards, the shell would not have enough energy to penetrate the armor until the trajectory of the shell became so steep so that, as it plunged out of the sky, it had enough energy to penetrate the armor on the deck. At 21,000 yards, a shell may hit the deck, but the angle of impact and the armor on the armored deck were expected to be sufficient to prevent it from penetrating into the ship's vitals. Also playing into the calculation of required protection was the range at which accuracy of the fire controls made hits so rare that protection at certain ranges was not warranted.
The General Board desired a new cruiser with overmatched protection, a ship with protection against armament larger than her own. This was essential due to the number of 8-inch gun cruisers already in service with other navies which could handily defeat a 6-inch gun cruiser designed for protection against her own armament. The US Navy believed a 6-inch gun cruiser could be designed with acceptable armament while remaining protected from 8-inch shells from a heavy cruiser. Indeed, as previously mentioned, this was the catalyst which provided US Navy acceptance of a limit of eighteen heavy cruisers, with all additional restricted to 6-inch gun maximum armament.
The Brooklyn class was received favorably by the fleet. The rapid fire 6-inch guns received wide acclaim in the ship reports coming back to Navy Headquarters. Seakeeping was also reported to be excellent. However, it soon became apparent that in the name of weight saving, the structural integrity of the ship had been overly compromised. The Savannah ran over her anchor chain in a gale which sliced through her bow, causing severe damage. This led to a weakening of confidence in the survivability of the design among the fleet.
The design of the Brooklyn-class light cruisers was validated through wargaming. The result was very sturdy ships that survived many kamikaze attacks during WWII. All of these ships except Savannah and Honolulu found their way into foreign navies after the end of the War. The only ship of this class lost in combat was the Phoenix during the Falklands Conflict in 1982, she had been sold to Argentina and renamed General Belgrano.
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