In the Imperial Navy, there was a period when there was a growing demand, mainly from the Naval General Staff, for the development and deployment of 61 cm torpedoes due to the anticipated increase in size and improvement in underwater defense of future main fleet vessels, considering the inadequacy of the conventional 53 cm torpedoes. However, since the torpedoes for submarines remained at 53 cm, even if 61 cm torpedoes were developed, it would result in the coexistence of two different sizes of torpedoes, 53 cm, and 61 cm, in the navy. Furthermore, the development of torpedoes for aircraft, which would likely be around 45 cm, was also urgently needed. If these were also deployed, the Japanese Navy would end up having three different sizes of torpedoes. However, this would be challenging in terms of production, supply, and cost.
On the other hand, with the advancement of aviation, there were many voices suggesting that torpedoes equipped on aircraft should be enlarged, and efforts should be made to standardize the sizes of torpedoes for surface vessels, submarines, and aircraft. Torpedoes require more complicated maintenance compared to shells, and that was a reason to push for standardization as much as possible. Indeed, standardization was advantageous in terms of maintenance and cost. Moreover, it was argued that the power of torpedoes could be increased through improvements in explosives without relying on size enlargement. In fact, the performance of explosives, including their handling, was steadily evolving.
In the case of equipping 61 cm torpedoes, the number of torpedoes carried per ship would naturally decrease compared to the 53 cm torpedoes. While the weight of the 53 cm torpedoes was around 1,6 to 1,7 tons, the 61 cm torpedoes were expected to weigh around 2,5 tons, an increase of fifty percent. Naturally, the number of torpedoes that could be launched would decrease, and consequently, the expected number of hits would also decrease accordingly. Unless there were sophisticated guidance devices for torpedoes, which were not available at that time, the situation would not change. Due to these reasons, there were heated debates among stakeholders about the development of the exceptionally large 61 cm torpedoes, but ultimately, the project was abandoned. The disadvantages in terms of manufacturing and cost were too significant for the financially constrained navy to overlook.
As a result, during the planning stage of the Mutsuki-class destroyers, there was a proposal to equip them with two sets of triple 61 cm torpedo launchers, but in the end, they were equipped with two sets of quadruple 53 cm torpedo launchers.
Similarly, the later Fubuki-class destroyers were also equipped with quadruple 53 cm torpedo launchers. However, in this case, there were three sets, giving the Fubuki-class an unprecedented torpedo firepower for destroyers of that time, with 12 torpedo tubes along the ship's centerline. Additionally, the quadruple 53 cm torpedo launchers were lighter than the triple 61 cm torpedo launchers, contributing to the reduction of top-heavy weight.
On the other hand, a policy was decided not to equip torpedo launchers on the future 10.000-ton cruisers to be constructed. The 10.000-ton cruisers were designed with awareness of the Washington Naval Treaty. Since the treaty did not impose restrictions on vessels below 10.000 tons, the Imperial Navy took advantage of this and proceeded with the development of these cruisers. In a sense, the 10.000-ton cruisers could be considered as the unconventional offspring born out of the treaty. However, despite having a displacement less than half of a battleship, these cruisers were quite large with a length extending to 200 meters, making them significant targets. With their size, the probability of being hit also increased significantly.
Naturally, voices questioning the wisdom of equipping torpedoes on such vessels arose. It was argued that no officer or crew member would want to die due to the self-destruction of torpedoes, especially considering that the Imperial Navy already recognized the danger of torpedo explosions during gunfire. Additionally, concerns were raised about the cramped living spaces resulting from the installation of torpedo launchers. The prevailing opinion was that there was no need to force the inclusion of large torpedoes with a high risk of explosion if it meant compromising the living conditions of the crew in a harsh environment. In other words, the message was clear: for gun cruisers, the emphasis should be on winning with artillery.
By the way, although the Fubuki-class destroyers were actually close to 2.000 tons, it was officially announced as 1.680 tons to avoid provoking other nations. Regarding this measure, it seemed that insiders were secretly anxious about whether other countries would uncover this deception. Surprisingly, however, no inquiries or objections regarding the displacement of the Fubuki-class were ever received from any country. Emboldened by this, the Imperial Navy adopted a habit of consistently underreporting displacement and ship types to foreign nations. In retrospect, this practice was a very bad habit, but since rival nations, including the United States, were engaging in similar tactics, it was somewhat of a stalemate.