In the Imperial Navy, while there was a rush to build new ships after the arms reduction treaty, renovations of existing ships were also underway. Battleships like the Fusō and Nagato classes were all built during the Taishō era, and efforts were made to prolong their service life by improving their attack power, defense capabilities, and speed. In addition, cruisers that couldn't receive sufficient armament and defense due to displacement limitations in the disarmament treaty were also under consideration for renovations, similar to battleships. Furthermore, in response to new threats posed by rapidly developing submarines and aircraft enabling three-dimensional maneuverability, even smaller vessels like destroyers were undergoing improvements, including the upgrade of sonar systems, the introduction of listening devices, and the reinforcement of anti-aircraft weaponry.
- Speed increase due to the engine replacement of the Nagato-class battleships.
- Replacement and engine upgrade of the main guns from 36 cm to 41 cm for the Fusō-class battleships.
- Strengthening of anti-aircraft weaponry for the Sōryū-class aircraft carriers.
- Replacement of the main guns from 20 cm to 23 cm for the Myōkō-class heavy cruisers.
- Replacement of the main guns from 20 cm to 23 cm for the Takao-class heavy cruisers.
- Replacement of the main guns from 15,5 cm to 23 cm for the Mogami-class light cruisers.
- Equipment upgrade for outdated light cruisers like the Tenryū and Tama, as well as outdated destroyers.
Among these, the Nagato and Mutsu had their main engines and boilers replaced. This resulted in a significant increase in output from 80.000 horsepower to 144.000 horsepower, coupled with a reduction in propulsion resistance due to hull extension work, making them fast battleships capable of 29 knots.
The four ships with 36 cm twin turrets, Fusō, Yamashiro, Ise, and Hyūga, were secretly replaced with 41 cm guns. Additionally, by replacing the engines with new ones generating 136.000 horsepower, these six battleships transformed into powerful, high-speed warships boasting 29 knots. Furthermore, all six battleships removed their 14 cm to 15 cm secondary guns and added high-angle guns and machine guns to enhance their anti-aircraft capabilities.
For the large cruisers, the Myōkō and Takao classes, eight heavy cruisers, and the light cruiser Mogami with fifteen 15,5 cm guns, all main guns were replaced with 23 cm guns. Although the number of turrets decreased from 10 or 15 to 8, these cruisers became undoubtedly the world's strongest at that time, with a 23 cm gun that had almost four times the weight of the traditional heavy cruiser's shell. Additionally, the upgrade included the addition of high-angle guns and machine guns to enhance anti-aircraft capabilities, significantly improving their ability to counter the rapidly evolving aircraft.
Destroyers, specifically those of the Mutsuki-class and earlier obsolete models, had all their main guns removed. Furthermore, all torpedo launchers, except one for self-defense, were dismantled. The vacant spaces were then utilized for the installation of anti-aircraft guns and machine guns. Additionally, the anti-submarine equipment was updated to the latest technology to enhance both anti-air and anti-sub capabilities. The Fubuki-class destroyers had all their main guns replaced with Type 89 12,7 cm twin anti-aircraft guns, upgrading their previously weak anti-air capabilities. However, to accommodate the weight of the new anti-aircraft guns, machine guns, and fire control devices, all spare torpedoes were removed. Nevertheless, the torpedo launchers were upgraded to be capable of operating oxygen torpedoes, maintaining torpedo capabilities surpassing those of other nations' destroyers.
Submarines underwent improvements not only in the addition of oxygen torpedo launch capabilities but also in the adoption of radar, new sonar systems, and German-made advanced periscopes. Rigorous soundproofing measures and battery enhancements significantly improved detection capabilities and stealth, though these specifications might not be readily apparent.
Concerning the future mainstay aircraft carriers of the Imperial Navy, four Sōryū-class ships that escaped the constraints of the disarmament treaty had their anti-aircraft weaponry reinforced. The anti-aircraft guns were upgraded from the traditional eight 8 cm single-mount anti-aircraft guns to sixteen 12,7 cm twin anti-aircraft guns. Sponsons were added to the sides for machine guns and their firing mechanisms, and twenty 25 mm triple-mount autocannons were additionally installed.
Moreover, each of these vessels saw the introduction of corrosion-resistant, waterproof, robust, and reliable communication equipment, as well as emergency command devices such as firefighting facilities and the latest drainage systems to minimize damage. While not easily quantifiable, these enhancements undoubtedly elevated the overall performance of the ships. Additionally, the vessels were equipped with well-furnished workshops and medical facilities, and considerable attention was given to improving the living conditions, which could significantly impact the well-being of the crew.
Furthermore, similar upgrades were planned for land-based facilities, including enhanced communication capabilities and the installation of radar even in smaller bases.