Provisions of the 1922 Washington Treaty also had freed large U.S., British, French, and Japanese hulls for conversion into carriers. The United States and France converted two battle cruisers and a battleship, respectively, into the flush-deck carriers Lexington, Saratoga, and Béarn. British and Japanese concepts emphasizing rapid aircraft launching led both navies to develop designs incorporating multiple flight deck levels to permit several aircraft to fly off simultaneously. Britain rebuilt the Furious (which had served as a fleet carrier since 1917 in two earlier guises) with a three-quarter-length flush deck and a forward flying-off deck at a lower level, and it similarly converted two near-sister ships, the Courageous and the Glorious. Japan took this idea still further and configured a battleship and a battle cruiser, the Kaga and the Akagi, as carriers with two forward flying-off decks beneath the main deck. Both navies learned through experience that efficient deck-handling procedures were more effective in increasing launch rates. Japan subsequently rebuilt its two carriers with conventional flush decks and greatly enlarged air groups, but the British ships still served unaltered in the front line at the outbreak of war.
Operational experience with these large converted carriers had a profound influence on subsequent carrier doctrine and designs. Their speed allowed them to operate with the battle fleet, and their size and aircraft capacity gave commanders invaluable opportunities to appreciate the importance of efficient deck-handling procedures, rapid launch and recovery, and concentrated mass attacks. They also served as development platforms for crucial operational equipment, including effective arresting gear using transverse wires, safety crash barriers, hydraulic catapults, and fast elevators to move aircraft between the hangar and the flight deck.
During the 1930s, Japan and the United States added new carriers to their fleets. Although constrained by provisions of the 1922 Washington Treaty, both navies evolved effective designs that became the basis for later construction. Their first treaty vessels, the Japanese Ryujo and the U.S. Navy's Ranger, were not entirely satisfactory but formed the bases for the two ships of the Soryu-class and the three-vessel Yorktown-class, respectively. They were ships that combined large flight decks, substantial air groups of 60–80 aircraft, strong defensive armament (for the period), high speed, and long range in vessels suitable for extended oceanic operations.
Britain was a latecomer to new-carrier construction in the 1930s. The Ark Royal, commissioned in 1939, incorporated internal hangars, an enclosed bow, and a flight deck that was also the vessel's principal strength deck—all features that characterized subsequent British carrier designs—and embarked a similar size air group to those of its American and Japanese contemporaries.
The large fleet carriers commissioned by Britain, Japan, and the United States during World War II derived from their earlier 1930s designs. Japan commissioned two ships of the enlarged Shokaku-class in 1941 with greater offensive and defensive capabilities, followed by the Taiho, a variant incorporating an armored flight deck (although at the cost of a reduced air group). In 1942–1943, Japan laid down the six-ship Unryu-class, which was derived directly from the Soryu, although only two of these vessels entered service. The United States standardized on the Essex-class, an expansion of the Yorktown-class. No fewer than 32 units were ordered, of which 24 were completed to serve as the backbone of U.S. carrier forces from 1943. They combined a powerful offensive air group of as many as 100 aircraft, substantially augmented defensive armament, long range, and high speed in hulls the size of which conferred great adaptability to changing operational requirements.
The six British wartime carriers of the Illustrious type introduced armor protection for both flight decks and hangar sides. Incorporating this feature into the basic Ark Royal design produced vessels that proved very effective in the confined waters of the Mediterranean and in the face of kamikaze attack, but it also incurred severe penalties. Air-group capacity was slashed substantially (the original design accommodated only 36 aircraft; modified to carry 54, it still fell short of the Ark Royal's embarked 72 machines), hangars were cramped, and it proved very difficult and expensive to upgrade these ships postwar.
Both the U.S. Navy and the British Royal Navy developed a third generation of carrier designs from their wartime experience. These emphasized the importance of large air groups, efficient layout for fast aircraft operation, and strong defensive features—both passive in the form of armor at hangar and flight-deck level and active by means of very large batteries of automatic antiaircraft guns. None of these carriers served during World War II. The U.S. Navy commissioned the three ships of the Midway-class just after the war, but the Royal Navy's Malta-class was canceled, although two vessels of the intermediate Audacious-class entered service postwar as the Ark Royal and Eagle.
Both Britain and the United States studied small austere carrier designs before World War II, but only the Royal Navy seriously considered vessels for trade protection (the U.S. Navy's XCV projects envisaged second-line fleet duties). In 1935–1936, the British Naval Staff agreed on sufficiently firm requirements to earmark five specific merchant vessels for conversion should war break out. Nevertheless, no action was taken until December 1940, when work began to create Britain's first escort carrier, the Audacity, commissioned in June 1941.
U.S. Navy planning for austere mercantile conversions began in October 1940, resulting in the completion of the Long Island, its first escort carrier, also in June 1941. The Long Island was converted from a completed diesel C-3 cargo ship, the Mormacmail, but 45 subsequent conversions used partially completed hulls and steam turbines rather than the mechanically unreliable diesel plants featured in the first five U.S.-built escort carriers. More than half of these vessels went to Britain under Lend-Lease, and all 50 were in service before the end of 1943.
The United States also converted four fleet tankers into escort carriers. These larger twin-shaft turbine vessels were very successful, but a general shortage of tanker hulls prevented further conversions. Nevertheless, they formed the basis for the U.S. Navy's first purpose-designed escort carriers, the 19 Commencement Bay–class vessels. These were the only escort carriers to continue to operate postwar, since their size and speed suited them for the larger antisubmarine warfare aircraft then entering service.
The 50 Casablanca-class ships, however, formed the bulk of the U.S. Navy escort carrier force, even though they were outside the mainstream of U.S. Navy design. All came from the Kaiser Vancouver yard and were commissioned within one year starting in July 1943. Their design was by Gibbs and Cox, and their construction was under the auspices of the Maritime Commission. Shortages of both turbines and diesels forced the use of reciprocating machinery, but the ships were faster and more maneuverable than the original C-3 conversions, had longer flight decks, and had larger hangars than even the Sangamon-class converted tankers.
Other than the Audacity, Britain completed only five escort carriers of its own, all conversions from mercantile hulls. They were similar to contemporary American C-3 conversions, although generally somewhat larger. Thirty-eight of these, transferred under Lend-Lease, formed the core of the Royal Navy's escort carrier force throughout the war.
Escort carriers, initially conceived as platforms providing air cover for convoys, soon expanded their activities into a wide variety of tasks. In the U.S. Navy, escort carriers formed the core of specialized antisubmarine hunter-killer groups, provided close air support for landings, served as replenishment carriers and aircraft transports, and operated as training flight decks. In addition, during 1942 the Sangamons took on fleet carrier assignments to compensate for shortages of first-line vessels.
The Royal Navy employed its escort carriers in much the same way. Its own shortage of large carriers, however, and its operational responsibilities within more confined waters led it to assign escort carriers additional frontline duties. The small carriers operated in strike roles either within a larger force or as autonomous units in the East Indies, the Aegean, and off the Norwegian coast, including in the attacks on the German battleship Tirpitz. Escort carriers also provided night-fighter coverage for the British Pacific Fleet.
To circumvent 1922 Washington Treaty quantitative limitations, Japan designed several fast naval auxiliaries and passenger liners for quick conversion into carriers. Beginning in 1940, conversions from five auxiliaries and three liners joined the Combined Fleet as frontline light fleet carriers. Japan also completed several mercantile conversions similar in capability to the British and American escort carriers. However, unlike the Allied vessels, these were designed and usually were deployed as integral components of Japan's main carrier force. In addition, Japan converted one Yamato-class battleship hull, the Shinano, into a huge carrier that never entered operational service, and it commenced conversion of an incomplete cruiser as a light fleet carrier.
The United States, too, deployed converted warships—the nine Independence-class light fleet carriers based on Cleveland-class cruiser hulls formed an integral part of the fast carrier force from early 1943. Although conceived as first-line units, their design owed much to plans for the escort carriers, and their operational limitations made them suitable only for emergency service.
Britain also appreciated the need for smaller, less sophisticated carriers that could enter service more quickly, but it chose to construct new vessels rather than convert existing hulls. The design was similar to that of the larger fleet carriers, but the carrier was unarmored. Britain also deliberately conformed to mercantile rather than naval standards, since the Admiralty contemplated selling these vessels for conversion into passenger liners or fast cargo ships after the war, an interesting reversal of procedures! Four of this Colossus-class of light fleet carriers served with the British Pacific Fleet late in 1945, and they joined six sister ships to form the core of British carrier power into the later 1950s, since they proved very economical to operate.
France's converted carrier Béarn remained its only example throughout the war, serving mainly as an aircraft transport because of its low speed. France began building a pair of new carriers, the Joffre and the Painlevé, just before war began, but the fall of France in 1940 terminated construction. The final design incorporated a flight deck offset to port to minimize superstructure intrusion, a feature that has reappeared in several designs in recent years.
Before and during the war, Germany undertook some carrier construction. Its prewar design, the Graf Zeppelin, reached an advanced stage of construction by 1940, but subsequent reductions in priority, design changes, and disputes among the Kriegsmarine, the Luftwaffe, and the Reichs Luft Ministerium (Reich Air Ministry) over provision of aircraft and aircrew combined to prevent carrier completion before the war's end. A similar fate befell several conversion projects from merchant vessels and warships.
Italy evinced little interest in aircraft carriers before the war, subscribing to the position that geography would permit shore-based aircraft to provide entirely sufficient air cover and offensive strike potential for its fleet. Wartime experience led to a change in this view, and the Italian navy began two conversions from mercantile hulls to create the fleet's first carriers. The Aquila was a sophisticated nearly total reconstruction of the liner Roma that was virtually complete when Italy surrendered in 1943. The Italians sabotaged the Aquila to prevent its use by Germany, and the ship subsequently was seriously damaged by Allied bombing and an attack using "chariots" (manned torpedoes) at Genoa. The hulk was scrapped after the war. Conversion of the liner Augustus into the Sparviero, a more austere vessel similar to Allied escort carriers, began in 1941, but she, too, was never completed.
Air power at sea came of age during World War II. The combination of unprecedented striking power (both in volume of ordnance and range of delivery), mobility, and flexibility of use transformed the aircraft carrier into the world's major fleets' new capital ship, a position it retains today. Paul E. Fontenoy
Chesnau, Roger. Aircraft Carriers of the World, 1914 to the Present: An Illustrated Encyclopedia. 2d ed. London: Arms and Armour Press, 1992.; Friedman, Norman. U.S. Aircraft Carriers: An Illustrated Design History. Annapolis, MD: Naval Institute Press, 1983.; Friedman, Norman. British Carrier Aviation: The Evolution of the Ships and Their Aircraft. Annapolis, MD: Naval Institute Press, 1988.; Jentschura, Hansgeorg, Dieter Jung, and Peter Mickel. Warships of the Imperial Japanese Navy, 1869–1945. London: Arms and Armour Press, 1977.
Paul E. Fontenoy