CL-40 Brooklyn - Design

A number of candidate designs were developed for the new class of light cruisers. The fundamental dilemma imposed by the naval treaties was determining the right balance between capability (i.e. displacement) and numbers of ships. The US fleet would have widely differing characteristics were all of the Treaty tonnage allowance used on a large number of 6,000 ton ships versus fewer but more capable 10,000 ton ships. Also competing for Treaty tonnage would be the possible flight-deck cruisers, limiting the number of true cruisers. Admiral Pratt, the CNO, wanted experimental flight-deck cruisers and large numbers of smaller cruisers. He believed that cruisers should be used as rangers with the Scouting Fleet or with the Battle Fleet and would be able to retire back to the protection of the main fleet once they had secured the necessary information. This view seems slightly odd in view of the later concern for using small cruisers as an anti-destroyer screen for the battle line, and in view of Pratt's feeling that aircraft had displaced cruisers as the primary scouts.

The studies that brought about the genesis of the Brooklyn Class were prepared in response to the Japanese Mogami Class light cruisers. The Japanese announced they would mount fifteen 6.1-inch guns (5 triple turrets) on a new cruiser that would make 37 knots at 8,500 standard tons. To accomplish this feat, Japanese designers resorted to the latest weight saving techniques, including light alloys in the superstructure and electric welding. On trials in 1935, the first two ships of the class had numerous problems. Firing the guns caused the welded seams to open, and the turrets frequently jammed because of deformation in the hull girder. Worse, the ships were dangerously unstable and some of the antiaircraft guns had to be removed. Later, they were bulged to regain a safe degree of stability, and full load displacement grew to 11,200 standard tons (beyond Treaty limits) with speed dropping to 35 knots.

It never seems to have occurred to anyone at the time what would happen if a ship was over the Treaty limit at delivery; the answer, it soon developed, was nothing. This "loophole" was exploited more or less dramatically by most of the navies of the interwar period. The Mogami class "cheated" in another way: their turret mountings were designed to accept 8-inch twin turrets in place of the triple 6.1-inch to get around the Treaty limits on heavy cruisers. This modification was performed before the war.

However incorrect the declared specifications were, the announcement had the immediate effect of galvanizing the US Navy and setting the new design acceptable armament at a minimum of fifteen 6-inch guns. Previous studies had shown a smaller number of guns, around twelve, would result in a better balanced ship. These results were discarded in light of the new threat, and protection was to be sacrificed for increased armament, not speed.

This was a reversal of policy from heavy protection and moderate gun power towards heavy gun power at the expense of protection. The US delegation at the 1930 London Naval Treaty had accepted the Treaty limitations on the assumption that a 6inch gun cruiser could be made to engage an 8-inch gun cruiser with some prospect of success, under the right conditions.

However, changes since 1930 had led to improved 8-inch gun cruiser designs to a point where guns, machinery, and magazines could be adequately protected within the 10,000 ton limit. Conversely, it was apparent that no fifteen gun light cruiser could be made with protection against attack by existing heavy cruisers. A twelve gun light cruiser design showed a very small, 1,600 yard immunity zone against 8-inch shells, with possible improvement with design refinement.

The designs submitted by Preliminary Design in response to the new direction made concerted steps towards reducing weight as much as possible to be placed back in as increased protection. Preliminary Design estimated that 280 tons could be moved towards protective measures by using longitudinal framing, and more general use of high-grade steels. This foreshadowed the more daring weight control program eventually incorporated during design. Weight reductions, however, were somewhat counterbalanced by the emergence of a new long 6-inch shell which threatened new designs with increased direct fire penetration power than previously assumed. Speed was held constant at 32.5 knots for all designs with armament and protection traded between various twelve, fifteen and sixteen gun schemes.

The final decision was solidified by a comparison between the twelve and fifteen gun US designs against the Mogami. The fifteen gun light cruiser was shown to be superior to the twelve gun light cruiser against the Japanese design, according to Naval War College analysis. Thus, the decision was made that the new Brooklyn Class cruisers would mount fifteen 6-inch guns.

The design of the Brooklyn cruisers differed sharply from existing US cruisers. They incorporated aft aviation features that were copied on all following heavy and light cruisers, excluding the Alaska. They also incorporated longitudinal framing as a weight saving measure. They were the first ships in the fleet to be designed with the new 6-inch gun firing semi-fixed ammunition, a precursor to the automatic dual-purpose weapons introduced in the late WWII Worcester and Des Moines Classes. Increased protection was evaluated by lowering speed to 30 knots, but this was rejected since the slower ships would be unable to operate in formation with existing cruisers.

The weight saving pessimism seen on the Portland and New Orleans designs gave way to an undue optimism causing weight saving measures to be compulsory to stay beneath the 10,000 ton limit as the ship was further refined. The forward belt was omitted and the acceptable hull stresses increased to further shave structural weight. The first seven Brooklyn cruisers (CL 40 – 43, CL 46 – 49) were built to this design.