South Korea launches Seo Hui, its second 3,600-ton submarine built to carry ballistic missiles

South Korea's navy and its arms procurement agency launched the submarine Seo Hui (SS-088) at Hanwha Ocean's Okpo shipyard in Geoje on the afternoon of Tuesday 3 November, according to a joint release from the Republic of Korea Navy and the Defense Acquisition Program Administration that was translated by Naval News.

The 3,600-ton boat is the second of the Jang Bogo-III Batch-II class, also known as KSS-III Batch-II, after the lead submarine Jang Yeong-sil (SS-087), which was launched in October last year. The Batch-II design is larger than the three 3,000-ton Batch-I boats already in service, Dosan Ahn Changho, Ahn Mu and Shin Chae-ho, according to the release and Korean press reports.

What the new boat adds

The Korea Times reported that the Seo Hui is 89 metres long and carries an upgraded combat management system, improved sonar and vertical launch cells able to fire both submarine-launched ballistic missiles and cruise missiles. The Korea Herald, carrying a Yonhap report, said the expanded vertical launch system is the main step up from the earlier boats, alongside a stronger sensor suite.

Propulsion pairs a diesel-electric plant with air-independent propulsion and high-capacity lithium-ion batteries developed in South Korea. Compared with lead-acid batteries, the lithium-ion system lets the boat stay submerged longer and run at high speed for more time. The procurement agency told the Korea Times that the batteries went through several layers of safety design checks and evaluation at a land-based test facility.

The submarine uses 70 types of locally developed equipment, 12 more than the Batch-I boats, according to the Korea Times. The joint release said the high level of domestic content is meant to make follow-on logistics support easier and to strengthen the base for South Korean defence exports.

The ceremony and the timeline

About 200 guests attended, with National Assembly Speaker Cho Jeong-sik as guest of honour, along with officials from the defence ministry, the navy, the procurement agency, the legislature and defence industry. Following naval custom, the speaker's wife cut the launching line before a bottle was broken against the hull, the release said.

The boat is named after Seo Hui, a Goryeo Dynasty statesman best known for negotiating an end to a Khitan invasion in 993 and securing territory in the northwest without a battle. The navy said the name reflects the submarine's role as a key asset for defending the country's waters.

Construction followed a contract signed in 2021, a steel-cutting ceremony the same year and keel laying in 2024. The Seo Hui will now go through sea trials and operational tests, with delivery to the navy scheduled for 2028, according to the Korea Times and the Korea Herald.

Why it matters

A senior official from the procurement agency's nuclear-powered submarine project group told the Korea Times that the launch shows the strength of South Korea's own submarine design and building and should support both naval readiness and submarine exports. A navy planning chief described the boat as a national strategic asset for responding to security threats through underwater operations.

The Korea Herald report said the submarine is expected to serve as a key naval deterrent as North Korea expands its own fleet, including its new 5,000-ton Choe Hyon and Kang Kon destroyers.