Meet the Shinano.
Key point: Tokyo was desperate for new carriers and converted the Shinano into a carrier. However, the Shinano was poorly built and had flaws that would doom it.
Over the last hundred years, the navies of the world have constructed, operated, and taken to war hundreds of aircraft carriers. Some carriers have been truly outstanding designs, while many more were simply adequate and lost to history. One ship that achieved fame not out of greatness but sheer incompetence was the Imperial Japanese Navy (IJN) carrier Shinano. Originally constructed as a battleship, she was redesigned to support the air war in the Pacific before being sunk, with considerable irony, by a submarine before she could even see battle.
In May 1940 the Yokosuka Naval Yard laid down the third hull of the Yamato-class battleships. The largest battleships ever built, the Yamato-class featured nine eighteen-inch guns and were considerably larger and more powerful—on paper anyway—than even the U.S. Navy’s Iowa-class battleships. The Yamato and her sister ship Musashi were completed as designed, but work on the third ship, Shinano, halted shortly after the outbreak of hostilities with the Allied powers—principally the United Kingdom and Holland.
By June 1942 Shinano was complete up to her main deck but Japan no longer had use for battleships. A series of reversals at sea, particularly the Battle of Midway, had dealt a serious blow to Japanese carrier aviation. (The Battle of Midway alone saw the loss of four Japanese fleet carriers.) At the same time, it was becoming increasingly clear that aircraft carriers had eclipsed the battleship as the dominant weapon at sea. Japan needed more aircraft carriers, and fast.