The Japanese had the foresight to take a 60 percent stake in ten fast passenger and cargo liners all of a size that had had their plans altered to make them easily convertible to aircraft carriers. The first two Kashiwara Maru & Izumo Maru were laid down in 1938 as luxury passenger liners and purchased by the Navy in 1941. Conversion completed August 1942, the two ships joined the fleet.
Only three more of the merchant ships were able to be purchased and converted as the army had requisitioned the rest as fast troop transports. A category that was also of major concern to the Japanese. No good taking sea superiority in an area if you can't put boots on the ground to hold it. With mounting carrier losses, three more of the type were taken in hand in 1942 for conversion, joining the fleet in 1943-44 when the Japanese ended up with more aircraft carriers than pilots to serve on them. The Kamikaze operations of 1944 wasted so many pilots and aircraft there was nothing left to arm the carrier fleet.
These were the first Japanese carriers with a full bridge and funnel combination that was carried on with the following Taiho fleet carriers. The experience with enclosed and non-enclosed bows on aircraft carriers had proved to the Japanese, that the enclosed bow they had copied from the British and American designs of the early 20's was definitely the route to follow. The Americans went away from enclosed bows after the Lexington class to their detriment. It was not until post-war that the Americans decided all future ships should have enclosed bows and converted those that had open bows. The previous Zuiho class was converted from seaplane carriers without an enclosed bow and their sea keeping suffered in comparison with the Hiyo class.
