Mitsubishi Heavy Industries held the naming and launch ceremony for the Japan Maritime Self-Defense Force submarine Shogei at its Kobe Shipyard on October 8, 2026. Shogei is the seventh boat of the Taigei class, the successor to the Soryu class. Outfitting work will continue, and delivery to the Ministry of Defense is scheduled within fiscal 2027. According to Mitsubishi Heavy Industries' announcement, the launch marks a milestone in construction of the hull. Behind it are the adoption of lithium-ion batteries and improvements to the diesel engines made over successive boats of the class. For a conventionally powered submarine, storing large amounts of electricity and shortening the time spent near the surface to recharge are closely linked design challenges.
The Maritime Self-Defense Force lists the Taigei class at a standard displacement of about 3,000 tons, a length of 84.0 meters, a beam of 9.1 meters and a crew of about 70. It uses single-shaft diesel-electric propulsion. Comparing these principal specifications with the engines and hull structures of boats built so far reveals technical improvements that hull size alone does not show.
Lithium-ion batteries and the air needed to recharge
While submerged, Taigei-class boats run their electric propulsion motors on battery power. Mitsubishi Heavy Industries describes the fifth boat, Chogei, delivered in March 2026, as a conventionally powered submarine that dives on power from lithium-ion batteries. Diesel engines drive generators, converting fuel energy into electricity to charge the batteries, and that electricity turns the propulsion motor.
Running a diesel engine requires air, however. To recharge, a submarine can surface, or rise close to the surface and raise a tube called a snorkel to draw in air. In other words, a submarine's batteries cannot be charged indefinitely while it remains deep. It dives on stored power and returns to a depth where it can take in air when it needs a charge. This constraint weighs heavily on how conventional submarines operate.
That is why battery performance matters. It affects not only how much power is available while submerged but also how long charging takes. The 2021 Defense White Paper says the Taigei class aims to shorten snorkel charging time through a propulsion system using lithium-ion batteries. The aim of the design was not simply to change the type of battery but also to shorten the time spent operating near the surface to recharge.
While snorkeling, a submarine must stay connected to the surface to draw in air. Shortening that time makes it easier to spend more time operating at depth. Stealth is an important element for submarines, and charging time is one indicator of that capability. The white paper, however, gives no specific figures, such as how many minutes charging time has been cut or how many hours the submarine can stay submerged at what speed.
Adopting lithium-ion batteries in submarines followed many years of research and development. In a February 2017 announcement, GS Yuasa said it had worked on lithium-ion batteries for submarines since the 1990s. It also announced plans to start mass production that March at a dedicated factory built in Kusatsu, Shiga Prefecture. Rather than installing consumer batteries as they are, the company built a dedicated development and production system to meet the quality and performance that submarines require.
Lithium-ion batteries did not first appear on the Taigei class either. The Maritime Self-Defense Force presents Oryu as a lithium-ion-battery submarine, and Kawasaki Heavy Industries announced their adoption on Toryu, the 12th Soryu-class boat delivered in 2021. Shogei's launch is another milestone showing that construction of submarines using this technology continues.
Same 3,000-ton class, but newer diesel engines
The second and fourth Taigei-class boats, Hakugei and Raigei, both use lithium-ion batteries. Documents Kawasaki Heavy Industries published at delivery, however, show that the diesel engine models they carry differ.
The table below compares the principal specifications of Hakugei, delivered on March 20, 2023, and Raigei, delivered on March 6, 2025.
| Published principal specifications | Hakugei (2nd boat) | Raigei (4th boat) |
|---|---|---|
| Delivery date | March 20, 2023 | March 6, 2025 |
| Length | 84.0 m | 84.0 m |
| Beam | 9.1 m | 9.1 m |
| Depth | 10.4 m | 10.4 m |
| Standard displacement | 3,000 tons | 3,000 tons |
| Diesel engines | Kawasaki 12V 25/25SB ×2 | Kawasaki 12V 25/31 ×2 |
| Propulsion motor / shaft | 1 motor, 1 shaft | 1 motor, 1 shaft |
Hakugei and Raigei share the same length, beam, depth and standard displacement, but the diesel engines changed from the 12V 25/25SB to the 12V 25/31.
This comparison rests on the design specifications published at each boat's delivery, not on performance measured under identical conditions. Even so, it shows that engines were improved while boats of the same 3,000-ton class were being built.
Kawasaki Heavy Industries says the engines on Raigei offer improved capability over the previous type. The new engines have also been announced for Chogei, which Mitsubishi Heavy Industries delivered in 2026. Lithium-ion batteries are common to boats of the class, but the generating engines have not stayed the same.
Charging time depends not only on how much power the batteries can accept but also on how much power the generators can supply and on charge control. Even with ample battery capacity, charging cannot be fast if the power supply is insufficient.
It therefore makes sense to evaluate battery performance and diesel engine improvements together. The fact that the engine model changed, however, does not allow us to calculate by what percentage generating output rose or how much charging time was shortened.
Mitsubishi Heavy Industries' announcement of Shogei's launch does not give the boat's engine model or battery capacity. The comparison above shows improvements made over the course of Taigei-class construction and is not grounds for asserting Shogei's specific performance.
A floating-deck structure for quieter running, and a new sonar
The Taigei class also incorporates measures to limit the transmission of vibration generated inside the boat through the hull into the sea.
The "floating deck structure" described in the 2021 Defense White Paper supports the deck so that it floats relative to the pressure hull, absorbing vibration produced inside. It reduces noise radiated outside the boat and is also described as cushioning external shocks to protect crew and equipment.
Propelling a submarine with an electric motor does not by itself guarantee quiet running. If vibration generated inside reaches the hull, it can radiate into the surrounding water as sound. The floating deck structure is meant to curb that transmission.
While the battery-and-motor propulsion system contributes to quietness on the power side, the floating deck structure limits vibration transmitted through the hull. The two work differently to support the submarine's stealth.
The sonar used to understand the surroundings has also been improved. Sonar detects nearby objects and ships using sound traveling through water.
The white paper says the Taigei class adopts a new sonar system with improved detection capability over earlier boats. Mitsubishi Heavy Industries likewise says Chogei uses a sonar with better detection capability than the Soryu class and a hull structure with improved quietness.
A submarine's own noise can also interfere with hearing sounds from its surroundings. Reducing its own noise and improving its ability to pick up others' sounds are therefore closely related.
Published information, however, does not allow actual detection ranges or noise reductions to be stated in specific figures. What can be confirmed is that improvements aimed at better performance have been made to the propulsion system, hull structure and detection equipment.
Crew living conditions have changed as well. Mitsubishi Heavy Industries says Chogei adopts facilities for female service members, such as partitions to secure sleeping quarters for women and curtains in the passage to the shower room.
The limited space inside a submarine must hold not only equipment and weapons but also the living facilities crew need for long periods of duty. Habitability is also an important element of submarine design.
VLS-equipped submarines and nuclear propulsion under consideration
Future submarine development also involves questions about armament and propulsion.
The defense buildup program published in the 2026 Defense White Paper sets out a policy of developing a submarine equipped with a vertical launching system (VLS) that can carry stand-off missiles.
Stand-off missiles are missiles used to attack from outside the range of an adversary's threat.
This is not, however, an announcement that Shogei, which has just been launched, will carry a VLS. The main armament the Maritime Self-Defense Force lists for the Taigei class is torpedo tubes, which must be distinguished from the future VLS-equipped submarine.
Nuclear propulsion is also still being examined as a possible future option.
At a press conference on September 11, 2026, Defense Minister Shinjiro Koizumi said nuclear-powered submarines are not being ruled out as an option but that no conclusion has been decided in advance.
The generally cited advantages of nuclear propulsion include the ability to stay submerged for long periods and operate for extended periods. Challenges include the cost of the nuclear plant, larger hulls, maintenance and the specialized expertise needed to operate it.
Easing the charging constraint on today's conventional submarines and considering other propulsion such as nuclear power both relate to the question of what missions submarines should be given.
A longer submerged endurance alone does not make a better submarine, though. Quietness, maintenance burden and the personnel and technical base needed for operations must also be considered. Improvements to lithium-ion batteries alone also cannot settle whether nuclear propulsion should be adopted.
Shogei will now undergo outfitting of its propulsion, electrical systems and armament, and is scheduled for delivery to the Ministry of Defense within fiscal 2027, that is, by the end of March 2028.
Launch is an important milestone in hull construction, but the process of installing and adjusting the various systems and completing the boat's overall functions remains before it is finished as a submarine.
The Taigei class has accumulated a range of improvements: shorter charging times through lithium-ion batteries, updated diesel engines, vibration countermeasures through the floating deck structure, and better detection through a new sonar.
Shogei's launch means a new submarine using this technology has moved closer to completion. Attention now turns to whether it is delivered on schedule after outfitting, and to how consideration of future VLS-equipped submarines and new propulsion methods proceeds.