As the relationship with the United States worsened day by day, the Imperial Navy expedited the implementation of the modification plan, along with the maintenance of newly built ships, and hastened the progress of these modifications. Facing the US Navy's attempt to obtain significant naval power through the Two-Ocean Navy Act, the Imperial Navy resorted to a desperate measure of the impoverished navy to narrow the gap.
The first to undergo modification were the four seaplane carriers: Chitose, Chiyoda, Nisshin, and Mizuho. Although these four ships were officially constructed under the limitations of the London Naval Treaty, being below 10.000 tons, they actually exceeded this limit upon completion. Naturally, they falsely reported their tonnage to be within the treaty limit to other nations. Furthermore, these four ships were originally designed with the assumption of conversion to aircraft carriers, making them somewhat inconvenient as seaplane carriers. In their conversion to aircraft carriers, Chitose, Chiyoda, Nisshin, and Mizuho adopted an enclosed bow to maximize the flight deck size, allowing them to accommodate high-speed and heavily loaded carrier-based aircraft. While the enclosed bow contributed to an increase in the internal volume of the ship, it also led to an increase in the windage area and negative effects on stability due to a top-heavy configuration. To mitigate these issues, the initially planned number of anti-aircraft guns was halved as a compensatory measure. In addition, during the aircraft carrier conversion, the propulsion systems of these four ships were replaced. Half of the engines and boilers were adapted from those used in aircraft carriers and cruisers, providing them with a combined output of 76.000 horsepower and enabling a speed of 30 knots.
As for the four ships of the Katori-class, originally intended as training cruisers, their construction was abruptly altered to complete them as escort cruisers. Historically, the Imperial Navy assigned outdated armored cruisers for the long-distance navigation training of officer candidates. However, due to concerns about their aging and outdated equipment, the Circle 3 Plan was initiated to build these four ships.
The Katori-class cruisers were designed to serve as training cruisers during peacetime and as escort cruisers, capable of leading escort fleets, during wartime. Fearing the deteriorating relations with the United States, the Imperial Navy high command decided to complete the Katori-class cruisers as escort cruisers instead of training cruisers, sacrificing the acquisition of state-of-the-art training vessels. These four Katori-class cruisers had their originally planned rear main guns and anti-aircraft guns removed, and additional hangars were added. The originally planned catapult was strengthened to two units, allowing them to operate four seaplanes, transforming them into aviation escort cruisers. Moreover, the torpedo launchers originally meant for the front and the machine guns on both sides, which were originally planned, were omitted. Instead, additional anti-aircraft guns were installed, significantly enhancing their anti-air firepower. These four Katori-class cruisers are expected to become the core force of the recently established Maritime Escort Force, playing a crucial role as surface vessels cooperating with seaplanes in anti-submarine warfare.
Memo
Chitose-class aircraft carrier (sister ships Chiyoda, Mizuho, Nisshin)
- Upon the carrier conversion:
-- Overall length 193,5 m, beam 21,5 m
-- Standard displacement 12.500 tons
-- Flight deck 192 m by 24 m
-- Four boilers, two shafts, 76.000 horsepower, 30 knots
-- Aircraft carried: 30
-- Four twin Type 89 12,7 cm anti-aircraft guns and 12 triple 25 mm autocannons
The Chitose-class, which emerged as seaplane carriers within the constraints of naval disarmament treaties, was, in reality, designed and constructed from the outset as aircraft carriers. The seaplane carrier style was a temporary form until the naval disarmament treaties were lifted, posing some operational challenges at the time. For the conversion to aircraft carriers, an enclosed bow was adopted to maximize the flight deck length. On the other hand, due to concerns about increased windage area and top-heaviness, the originally planned eight Type 89 12,7 cm twin AA guns were reduced to two twin mounts, totaling four guns.
Katori-class cruiser (sister ships Kashima, Kashii, Kashihara)
- Upon the carrier conversion:
-- Overall length 133,5 m, beam 17 m
-- Standard displacement 6.000 tons
-- 11.000 horsepower, 20 knots
-- Three twin 12,7 cm anti-aircraft guns
-- Four seaplanes and two catapults
Originally planned as training cruisers, the Katori-class was designed from the outset to be capable of carrying four seaplanes during wartime as aviation escort cruisers. Initially, the power was planned to be 8.000 horsepower for 18 knots due to budget constraints. However, to achieve a speed of 20 knots for effective pursuit alongside high-speed transports and against surfaced submarines, the main engines and boilers were upgraded to more powerful ones. The Katori-class cruisers, with their aviation capability, can perform anti-submarine patrols with onboard seaplanes and support landing operations. Despite their relatively modest specifications, these cruisers have earned high praise within the navy for their versatility and usability.