IN Brief:
- Seo Hui is South Korea’s second 3,600-tonne Jang Bogo-III Batch-II submarine, with delivery scheduled for 2028.
- The vessel combines revised sonar and combat systems, lithium-ion energy storage, and air-independent propulsion.
- Seventy types of onboard equipment are domestically manufactured, supporting future maintenance and submarine export programmes.
Hanwha Ocean has launched Seo Hui, the second 3,600-tonne submarine in South Korea’s Jang Bogo-III Batch-II programme, at its Okpo shipyard on Geoje Island. The 3 November launch moves the vessel from construction towards sea trials, with delivery to the Republic of Korea Navy scheduled for 2028.
Seo Hui, designated SS-088, follows Jang Yeong-sil, the first Batch-II submarine, which was launched in October 2025. The programme builds on the three 3,000-tonne Batch-I vessels already in service, but the latest design has a larger hull and revised internal systems intended to increase submerged endurance, detection capability and available weapons capacity.
At 89 metres long, Seo Hui accommodates upgraded combat management and sonar equipment alongside an expanded vertical launch arrangement. The weapons system can support submarine-launched ballistic and cruise missiles, while the vessel also carries conventional submarine armament. Exact missile loads depend on the fitted configuration and are not established by the launch announcement.
The sonar and combat-management upgrades must operate as an integrated detection chain. Acoustic sensors supply signals that need filtering, classification and correlation before they can become tracks in the combat system. The disclosed changes cover both parts of that chain, although DAPA has not given detection ranges, processing performance or the extent of any changes to the operators’ displays.
The propulsion installation pairs diesel-electric machinery with domestically developed lithium-ion batteries and an air-independent propulsion system. Batteries provide stored electrical energy for submerged operation, while air-independent propulsion allows the submarine to generate power without relying on the normal atmospheric intake required by a diesel engine. Together, the arrangements give commanders more flexibility over when to operate machinery and when to remain submerged.
Lithium-ion technology also changes the engineering requirements around battery management. Cells must be monitored for temperature and electrical behaviour, and the installation needs safeguards against faults that could propagate within an enclosed compartment. South Korea’s Defense Acquisition Program Administration says the locally developed battery system underwent multiple safety reviews and evaluation at a land-based test installation before being incorporated into the vessel.
After land-based safety evaluation, the battery installation must still prove itself within the submarine’s electrical and cooling systems. Its output feeds the vessel through power converters and distribution equipment, while propulsion demand and auxiliary loads change during manoeuvres. The shipboard testing programme will need to assess these interfaces under the conditions that cannot be reproduced fully by testing batteries in isolation.
Acoustic performance has received attention alongside propulsion. Noise and vibration reduction measures are intended to lower the submarine’s detectable signature, particularly when machinery and auxiliary systems are operating. The resulting acoustic performance will depend on the interaction of equipment mountings, hull design, propulsors and operating conditions, rather than on any single component.
Seo Hui also incorporates an auxiliary propulsion arrangement intended to retain manoeuvring capability if a fault affects the main propulsion system. A conventional optical periscope provides a direct observation route when electro-optical equipment cannot be used. Both features address survivability and redundancy, adding alternative means of performing essential functions after an equipment failure.
Seo Hui incorporates 70 types of domestically manufactured equipment, 12 more than the earlier submarine generation. The additions include its battery installation and reflect the expansion of South Korean suppliers able to manufacture and qualify complex naval components. That wider supplier base is expected to support both initial construction and subsequent technical maintenance.
The equipment localisation programme extends into the maintenance phase. Locally available spares, drawings and technical support can simplify repairs and modifications during refits, particularly when a submarine remains in service for decades. South Korean suppliers will still need to maintain qualified production processes and access to specialised parts, even where final assembly takes place domestically.
That industrial base also contributes to overseas submarine bids. Potential customers assess training, maintenance, repair, overhaul and upgrade arrangements alongside the initial hull and combat systems. The localisation figures demonstrate the breadth of South Korea’s supply network, but they should not be read as evidence that every critical component or intellectual property right is domestic.
Hanwha Ocean received its construction contract for Seo Hui in 2021, making the launch a visible milestone in a development and manufacturing programme spanning several years. Construction progress does not mean the vessel is ready for operational service: harbour checks, equipment integration tests, machinery assessments and sea trials remain necessary before acceptance.
The scheduled 2028 delivery follows a period of harbour trials, equipment integration and testing at sea. Machinery and electrical trials will examine the enlarged boat as an assembled system, while its revised sonar and combat-management installations require acceptance testing of their own. Those results will determine readiness for naval handover and provide practical evidence for future Batch-II construction and export proposals.
