I. Seohui’s Underwater Endurance
Seohui, the second boat of Jangbogo-III Batch-II, combines air-independent propulsion (AIP) with lithium-ion batteries. Hanwha Ocean calls this a world first and says it delivers among the longest submerged endurance of any diesel submarine. That is Hanwha’s own characterization, as a Yonhap-affiliated report notes explicitly, and it has not been independently verified.
Current Status
- Seohui is due to be launched this autumn (within the year) and delivered to the Navy at the end of next year after launch and sea trials. The lead boat, Jang Yeong-sil, was launched in late October last year, and domestic content is 80%. Sources: Seoul Shinmun, Munhwa Ilbo, YTN. (Note: early-July reports gave 2028 for delivery, but the September media-tour reports all say the end of next year.)
- On September 15, the interior of a Batch-II boat was shown to the domestic press for the first time at Hanwha Ocean’s Geoje yard. The boat was moored at the quay with only about 20% of the hull above the waterline, and finishing work was under way throughout. The formal launch had not yet taken place. Source: Munhwa Ilbo
- The hull is about 89 m long versus about 84 m for Batch-I, and the number of vertical launch cells (usable for SLBMs) rises from 6 to 10. Source: Yonhap-affiliated report
- Combat system and weapons: Batch-II carries one penetrating attack periscope and one non-penetrating periscope, so it can search for targets even in an emergency. Batch-I carried only a non-penetrating periscope. The horizontal tubes can operate weapons such as the Bumsangeo heavy torpedo. Source: Munhwa Ilbo
1. Comparison with the Previous Generation (Batch-I) and Domestic Benchmarks
- Batch-I (Dosan Ahn Changho class): Batch-I, which uses AIP, is described as capable of roughly 20 days of submerged operation. Source: Naval Technology
- Seohui (Batch-II): It replaces lead-acid batteries with energy-dense lithium-ion batteries and keeps AIP.
- At MADEX 2021, Hanwha’s chief research engineer said submerged endurance would exceed 20 days. He cited a graph showing lithium-ion lasting 300% longer at full speed and 160% longer at cruising speed. Source: Naval News
- Reports after the Seohui media tour described operations of three weeks or more and a range of up to 7,000 nautical miles. Source: Dealsite
- The original text’s estimate of “4–5+ weeks of continuous submergence” has been deleted because no public data supports it. The publicly available figure is “three weeks or more.”
2. Comparison with Japan’s Taigei Class
- Sōryū and Taigei: The last two Sōryū-class boats (Ōryū and Tōryū) were the world’s first lithium-ion-powered submarines. Source: Wikipedia Taigei is the first class designed entirely around lithium-ion batteries, with the Stirling AIP dropped altogether. Source: 19FortyFive (This corrects the original’s claim that Taigei was the first with lithium batteries.)
- Different design philosophies: Taigei trades low-speed persistence for high speed, fast recharging, and stealthy mobility. Source: National Security Journal It is still diesel-electric and must recharge; lithium reduces but does not eliminate snorkeling vulnerability. Source: 19FortyFive
- Seohui’s approach: Hanwha’s logic is that AIP provides the ability to stay hidden for long periods, while lithium provides quick movement when needed. The original claim of “overwhelming superiority in submerged days over Japanese boats” is better stated as “advantageous by design, according to Hanwha,” since no supporting figures have been published.
II. Why Diesel and Not Nuclear?
The key point is that long submergence does not mean a submarine must be nuclear-powered. Diesel-electric/AIP boats like Seohui have their own advantages, but it must also be noted that South Korea is separately pursuing nuclear-powered submarines.
1. The Advantage of Nuclear Boats Is Near-Unlimited Propulsion Energy
- A reactor supplies power continuously, so nuclear submarines can operate submerged for months. The Sejong Institute says that, given crew stamina, food, and supplies, 60–90 days is a realistic operating period. Source: Sejong Institute
- The same piece notes that a diesel boat running submerged at full speed fully drains its battery and is effectively disabled.
- Seohui uses a composite chain: diesel engine, AIP fuel cell, lithium-ion battery, electric motor. It maximizes underwater endurance within the size and cost envelope of a conventional submarine. It is not designed for near-unlimited submergence like a nuclear boat.
2. Quietness Matters Around the Korean Peninsula
- AIP/battery boats can shut down the diesel and move quietly at very low speeds. Discussions of Japanese boats likewise cite the absence of continuously running reactor-related machinery as a natural advantage of conventional boats. Source: 19FortyFive
- However, modern nuclear boats are also very quiet, so “nuclear boats are always loud” cannot be asserted. “Conventional boats can have an edge in low-speed covert operations” is the safer formulation.
- Seohui’s concept can be summarized as less “a submarine that roams the open ocean for months” and more “a submarine that hides near the peninsula for a long time and moves at the decisive moment.”
3. Why the AIP + Lithium-Ion Combination Matters (Analytical Comparison)
| Scenario | Nuclear submarine | Seohui |
|---|---|---|
| Long-range ocean operations | Very favorable | Limited |
| Extended submerged stay | Very favorable | Greatly improved vs. Batch-I |
| Low-speed covert operations | Very favorable | Very favorable |
| Burst of high speed | Very favorable | Greatly improved by lithium batteries |
| Compact size | Unfavorable | Favorable |
| Construction/operating system | Very complex | Relatively simple |
| Reactor infrastructure | Required | Not required |
| Littoral/narrow-water operations | Advantages may be limited | Well suited |
4. Isn’t a Nuclear Boat Simply Better? The Debate Is Ongoing
- The case for nuclear: Diesel boats face structural limits in long-term tracking of North Korean or Chinese submarines. If an SLBM-carrying submarine leaves a North Korean port, it must be tracked underwater from the moment it sorties, which calls for high-speed, long-duration mobility. Source: Sejong Institute
- The counter-argument: Former senior presidential secretary Chun Yung-woo argued that new batteries and larger battery capacity can extend diesel submergence to a month or more without much difficulty. He said the nuclear budget could instead fund densely deployed small diesel submarines and unmanned underwater vehicles. Source: Pressian Seohui’s multi-week endurance strengthens the conventional side of this debate.
- The government’s choice: On May 26, 2026, the government announced the “Jangbogo-N” basic plan to build a nuclear-powered submarine domestically with Korean technology and launch it by the mid-2030s. Source: Kyunghyang Shinmun The goal is operational deployment in the late 2030s. Source: Seoul Shinmun It will use low-enriched uranium, and the government pledged not to develop or possess nuclear weapons. Source: News1/Nate
- Why diesel until now: Jangbogo-III was a program confirmed back in 2007. Source: Wikipedia (Korean) US atomic energy law prohibits transferring nuclear material abroad, so Seoul agreed to pursue a separate agreement to obtain an exemption for submarine fuel. Source: SBS/Nate Seohui is thus less an alternative chosen instead of a nuclear boat than the product of constraints and choices together.
- Current situation: US-Korea negotiations on securing nuclear fuel have been stalled for three months, and no date has been set for a second round. Source: Hankyung A Foreign Ministry official said the US raised no objection to domestic construction. Source: Kyunghyang Shinmun
5. Nuclear Weapons and Nuclear Propulsion Are Different Matters
A nuclear-powered submarine is a matter of the power source; a nuclear-armed submarine is a matter of payload. Nuclear propulsion does not mean carrying nuclear missiles, as US attack submarines (SSNs) show. Conversely, Seohui is conventionally propelled but has ten vertical launch cells capable of operating SLBMs. In short, Seohui is not a nuclear submarine, but it is a large conventional attack submarine with significant strategic strike capability.
6. The Next Step
The original’s question of whether Batch-III would adopt nuclear propulsion has been overtaken by the government’s formalization of a separate “Jangbogo-N” program. This review could not officially confirm how that program relates to Jangbogo-III Batch-III. Broadly, Seohui represents the maximization of conventional submarine technology, while a future Korean nuclear boat would be a paradigm shift in submerged endurance.
III. Doesn’t It Suit European Waters? Could It Help Exports?
The direction is reasonable, but with results already in for Poland and Canada, the framing should shift from “tailwind” to “possibilities and challenges.”
1. Where It Fits European Operating Environments
Many countries around the Baltic, North Sea, and Mediterranean do not need ocean-going nuclear boats. The logic Hanwha advances is that KSS-III, designed for Korea’s shallow coastal waters, can be attractive under the conditions of “shallow seas + narrow waters + long covert operations.”
2. Poland’s Orka Program: Korea Lost
- In November 2025, Poland selected Saab’s A26 over five competing bids, including Hanwha Ocean’s. Source: Breaking Defense
- On June 29, 2026, a contract worth about SEK 47 billion for three boats was signed, with the final boat due in 2038. Source: Großwald The weapons package and interim boat are covered by Naval News.
- What held up signing right before the contract was industrial cooperation, not performance. Poland demanded a joint venture with its own shipyard for maintenance, plus technology transfer. Source: Großwald The original’s emphasis on local MRO and technology transfer as a key competitive factor was accurate, but Korea did not win that contest.
3. The Argument That “Being Diesel” Can Help Exports
Nuclear submarines offer overwhelming performance, but buyers face heavy political, technical, safety, and infrastructure burdens from reactors. KSS-III offers a large boat with long submerged operations, AIP, lithium-ion batteries, and heavy armament without nuclear propulsion, so it can be a realistic option for countries that find nuclear submarines politically or legally hard to acquire. This is the original’s analysis, however, and as the Canada case below shows, it did not by itself decide the contract.
4. Positioning as a “Large Conventional Submarine”
- Batch-I displaces about 3,700 t-class submerged; Batch-II is longer, keeps AIP, swaps to lithium-ion, and expands to 10 vertical launch cells. Source: Wikipedia
- The original argues there is a gap between small European coastal submarines and large US/UK-style nuclear boats, in which a “large, long-range conventional submarine” could be a product. Hanwha also calls Batch-II a strategic platform aimed at overseas markets. Sources: Yonhap-affiliated report, Munhwa Ilbo
5. Canada’s Program: Korea Was Not Selected as Preferred Bidder
- Prime Minister Carney announced Germany’s TKMS as the final choice on July 6, 2026. Source: OhmyNews However, Canada retains the right to open negotiations with Hanwha Ocean if final talks with TKMS collapse.
- The Canadian government judged both models to meet requirements, and the decision was made on economic benefits. Source: Newdaily Hanwha offered over $70 billion in trade and investment and 25,000 jobs a year, but TKMS reportedly proposed a far larger economic impact. Source: EKN
- Industry analysts attribute the outcome to NATO alliance considerations and a “buy European” trend. Source: Dealsite Hanwha itself acknowledged it could not overcome “the NATO alliance wall.” Source: The Fact
- The Dosan Ahn Changho deployment: The boat left Jinhae on March 25 and reached Esquimalt on May 23 via Guam and Hawaii. Source: Naval News The distance was about 14,000 km per USNI News, though some outlets reported up to 15,000 km. Since it is the Batch-I lead boat, the voyage shows that vessel’s sailing reliability, not the lithium-plus-AIP endurance of Batch-II. Source: UPI
6. What Seohui Means for Exports
- KSS-III Batch-II is not a diesel boat imitating a nuclear submarine; it is a high-performance large conventional submarine for countries that do not need nuclear boats.
- Hanwha’s product structure combines AIP, lithium-ion batteries, a large hull, long range, VLS, and Korea’s shipbuilding capacity.
- However, this has not yet translated into orders in Europe or Canada, and which design is ultimately selected is a separate question.
7. Position in the European Conventional Submarine Market, and Alliance/Political Coordination
What is especially interesting is what position KSS-III could occupy in the European conventional submarine market, not only in Poland and Canada but also in Germany, the Netherlands, Sweden, and Norway. Looking at this country by country makes it much clearer why a Korean diesel submarine could be a surprisingly strong competitor in the European market.
Supplement: Alliance and Political Coordination
However, KSS-III’s position in this market will not be determined by performance alone. It also requires political coordination: understanding the alliance and industrial structures each country has already built and finding points of cooperation within them.
- Germany and Norway: These two countries are already inside a joint framework. Norway decided in 2017 on extensive naval cooperation with Germany and TKMS, and the 212CD is a jointly procured program. A joint German-Norwegian building program office was set up in Kiel, and the plan is to base the boats together. Norway signed for two more boats in January 2026, bringing its total to six, and Germany ordered four additional boats in December 2024. Room for Korea to enter as a sole supplier appears small; cooperation in specific areas such as weapons, sensors, and MRO looks more realistic.
- The Netherlands: It operates within a Franco-Dutch cooperation framework. The €5.6 billion Orka program, launched in September 2024, is derived from the Barracuda family, and a 2024 industrial cooperation agreement requires high domestic content in key procurements. The Dutch have emphasized joint ventures with local shipyards since the candidate-selection stage.
- Sweden: Sweden is the developer of the A26. Poland’s selection was viewed positively in Sweden as securing additional A26 volume. Source: Breaking Defense Backed by the UK, Sweden launched a security partnership with Poland, the Baltic Sea Pact, alongside the order. The submarine was sold as part of a security partnership, not merely as an arms transaction.
As the Canada and Poland cases show, outcomes turned on alliance and industrial cooperation factors more than on performance. To gain a foothold in Europe, Korea would therefore need to coordinate, alongside its technical offer, a government-to-government security and industrial cooperation package, local MRO and technology transfer, and compatibility with existing European systems. (This conclusion is analysis drawn from the cases above, not factual reporting.)
Peter Yang