Hakugei-class submarines are cutting-edge modular nuclear submarines developed in Japan in order to combat "Fleet of Fog". There are three known submarines of the class, all named after the ship class, but only Hakugei (3) has been seen seaworthy.
Not content with being pushed out of the seas by the Fleet of Fog, Japan began the development of various weapons intended specifically to defeat Fog warships. The Hakugei-class submarines were developed as part of this project, designed as nuclear-powered submarines equipped with the latest technologies available. Hakugei-class submarines varied in size depending on what modules they were equipped with; the Hakugei (3) was large enough to dock I-401 while equipped with a cruising module that increased its beam length to nearly 80 meters. These modules could be safely ejected in the field.
Hakugei-class submarines have several modes of underwater movement. In addition to standard propellers, there are also micromotion tiles that allow the submarine to remain hidden even from the sensors of Fleet of Fog vessels using micromotion tiles. However, the tiles are rather fragile and strong shocks can render them unusable. Finally, Hakugei-class submarines are equipped with supercavitation propulsion that allows the submarine to reach the speed of at least 110 knots while underwater, surpassing the speeds of even Fog warships. However, the submarine can carry only enough rocket fuel for approximately 30 seconds of sustained supercavitation propulsion.
Hakugei-class submarines, while using an assault module, have eight frontal torpedo tubes and a number of anti-missile defense guns in the side ports. There are also underwater mine dispensers hidden along the submarine's body - these are also used as a part of anti-missile defense system. It is possible that these submarines have other weapons, but those were never shown. It is also unknown, what weapons and in what quantities the Hakugei-class submarine has while using cruising module.
Hakugei (3) Edit
The only Hakugei-class submarine shown is Hakugei (3), under the command of Captain Komaki. Hakugei (3) was immediately sortied after completion in response to the attack on Yokosuka port, where she was docked, by battleships Haruna and Kirishima.
Hakugei (3)'s performance proved that micromotion tiles gave excellent stealth performance against even Fleet of Fog warships: Haruna and Kirishima were only able to detect Hakugei (3) the submarine sent a radiogram. Hakugei (3) used her supercavitation propulsion in order to distract Haruna and Kirishima, allowing I-401 to escape into open waters. However, it soon left Hakugei (3) alone against two Fog battleships, with insufficient protection against either.
Captain Komaki ordered the use of sound-cluster torpedoes, but they were quickly destroyed by Kirishima. The same attack badly damaged Hakugei (3)'s micromotion tiles and temporary disabled a lot of her internal systems. However, Hakugei's anti-missile defense guns kept it safe for long enough for I-401 to return onto the battlefield and draw Haruna's and Kirishima's attention.
Later during the battle, Hakugei (3) used a Corrosive Warhead Torpedo, given to them by I-401 before the battle began, to destroy Haruna and Kirishima while the battleships were using their joint-mode Super-Graviton Cannon. Their success marked the first time in history that a human warship sunk a Fleet of Fog warship, even though a Fog weapon was used to deliver the final blow. While Hakugei (3) had sustained significant damage during the battle, the command and the reactor modules survived with only minor damage. The badly damaged assault module was ejected.
Hakugei (3) later docked with I-401 and the crews discussed the trans-Pacific voyage to deliver the vibration warhead. Gunzou had the vibration warhead transferred to the Hakugei (3) so that Komaki's crew can deliver it to the United States. Afterwards, I-401 broke away from Hakugei (3) in order to rescue Takao from U-2501.
While waiting for I-401 to rendezvous with them again, Hakugei (3) witnessed an in-fighting between Repulse and a destroyer accompanying her against a group of the "Fleet of Fog" vessels led by Prince of Wales. After debating with his crew, Captain Komaki decided that they should assist the fleeing heavy cruiser, firing a Sound-Cluster torpedo to provide Repulse with a window of opportunity to slip away from her pursuers and switching to silent running to escape from Prince of Wales' fleet. Repulse later docked with Hakugei (3) and thanked the crew for their help.