The island configuration for carriers was present in carrier design from the 1920s. HMS Argus had a dummy island fitted to test the concept in 1918. Both HMS Hermes and, at least initially, the IJN's Hosho had islands. The island was beneficial for conning the ship, coordinating air-operations, and gunnery direction. Another benefit of the island configuration, recognized by the American and British first and later the Japanese, was that trunking up exhaust gasses through the island minimized its impact upon flight operations.

The early interwar recourse to flush-deck carrier designs was largely because of interrelated problems with weight and flight operations. The IJN removed Hosho's island after landing experiments showed that its masts presented a visual obstruction and wake turbulence that made it difficult to land. The island's real and perceived- remember this was when navies were still figuring out the science of air operation- negative impact on aircraft takeoff and landing meant there was some pushback from naval aviators and designers against islands. After their experience with both Lexington and Saratoga's island configuration, BuAer officials testified that the island's occupation of vital deck space and wind disturbances increased the chances of accidents and USS Ranger was initially to be a flush-deck design. Ranger did have an island, but one quite small and offset away from the flight deck. The island also posed a weight and size penalty and this became an increasing issue in the era of the Washington and London naval treaties. This was most evident in the design of Ryujo, which tried to cram as much of an airwing on as small a hull necessitated a flush deck and the installation of a gyrostabilizer. Not only would an island superstructure add more weight to an already overloaded design, but it would also hinder flight operations on narrow decks.

As it became increasingly clear that the naval arms restrictions were a dead letter in the mid-1930s, islands became a major component of carrier design for the three carrier navies. As aviators became used to carrier operations, the island's fixed position was used as a point of reference during landing. The increasing importance of fire control for AA suites also underscored the need for an island to house these mountings.

The weight and size penalties of the islands were still an issue for escort carriers and conversions. The first British CVE's pioneered the use of small offset islands for navigation and coordinating flight operations. From the Attacker class onwards, island design became standardized. The IJN's CVEs and seaplane-carrier conversions kept the flush-decker design partly because of weight, but also because earlier experience with Hosho and other flush-deck carriers like Akagi and Kaga in their original configurations meant that IJN doctrine would stress directing carrier operations from a platform besides the flight deck. The IJN's novel system of landing lights for carrier landings also made landings on flush-deck carriers easier. Yet, the IJN's conventional carriers also conformed to the international trend towards islands, although Japanese islands tended to be smaller than either their British or American counterparts.

Sources

Friedman, Norman. US Aircraft Carriers: An Illustrated Design History. Annapolis, Md: Naval Institute Press, 1983.

Hobbs, David. British Aircraft Carriers: Design, Development and Service Histories. Annapolis, MD: Naval Institute Press, 2013.

Peattie, Mark R. Sunburst: The Rise of the Japanese Naval Air Power, 1909-1941. Annapolis, Md: Naval Institute Press, 2001.