Taiwan orders two improved Hai Kun-class submarines for stronger patrol and combat performances.

As reported by the Liberty Times on June 14, 2025, Taiwan has signed a construction contract with CSBC Corporation for two new submarines under the Indigenous Defense Submarine (IDS) program. These two units are referred to as "small variant" models and are derived from the prototype platform ROCS Hai Kun (SS-711). The contract is part of a phased construction scheme known as “2+3+2,” covering two improved early-production boats, three standard production units, and two optimized submarines.
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The Hai Kun’s hull is based on the Zwaardvis-class submarines previously acquired from the Netherlands, using a teardrop design with an X-shaped stern rudder configuration. (Picture source: Facebook/IDF 經國號)

Although the contract is signed, the budget for these two boats remains frozen by the Legislative Yuan. The legislative freeze, estimated at NT$1 billion, will only be lifted once the Hai Kun completes its sea trials and the Ministry of National Defense submits a comprehensive performance report to lawmakers. As a result, while CSBC can proceed with preparatory steps, it cannot begin physical construction work until the trial results are formally reviewed and approved.

The decision to delay the release of funds reflects Taiwan’s requirement for evidence that the Hai Kun meets operational standards before continuing with serial production. The two contracted small variant submarines are expected to incorporate improvements based on difficulties encountered during the development and harbor testing of the prototype, including internal layout adjustments and system integration modifications. According to statements from officials involved in the program, these design changes are based on lessons learned during the construction and harbor acceptance trials (HAT) of the Hai Kun. Although the small variants are intended to enter production after the prototype, construction activities at CSBC’s Kaohsiung shipyard are already being prepared to ensure continuity. Equipment procurement, material preparation, and workforce alignment are being organized so that production can begin immediately after legislative authorization is granted.

The Hai Kun began its Sea Acceptance Test (SAT) phase on June 17, 2025, in waters off Kaohsiung. This first sea trial involved surface propulsion checks and general verification of ship control and maneuvering systems. The trial lasted about one hour and marked the first time the Hai Kun operated outside the harbor under its own power. The sea trials follow a harbor testing period of over eight months, during which the integration of propulsion, navigation, and combat systems was verified while moored. According to military sources, the SAT process will involve more than 30 distinct evaluation items, covering submerged operation, weapon system activation, acoustic trials, and live-fire tests. Taiwan’s Ministry of National Defense aims to complete the SAT cycle by September, with delivery of the submarine planned for November. Successful completion of the SAT and acceptance by the navy are prerequisites for the budget release for the two contracted submarines.

The Hai Kun submarine is equipped with systems sourced from several countries, including sonar, periscopes, electronic warfare equipment, and a combat management suite. The integration of these foreign-sourced systems into a domestically designed platform was reported to be one of the main challenges during harbor trials. The prototype’s development involved modifications to internal arrangements to improve system layout and operational functionality. These adjustments were reportedly implemented progressively throughout the testing phase, including during harbor-based evaluations. The system refinements and feedback from the prototype phase are intended to inform the construction of the two follow-on submarines, improving technical consistency and avoiding repetition of earlier integration issues. The Ministry of National Defense expects the transition from prototype to serial production to benefit from this accumulated experience.

The Hai Kun’s hull is based on the Zwaardvis-class submarines previously acquired from the Netherlands, using a teardrop design with an X-shaped stern rudder configuration. The vessel has a submerged displacement estimated between 2,500 and 3,000 tons and a length of approximately 70 meters. It is powered by a diesel-electric propulsion system with a single-shaft layout. It remains unconfirmed whether lithium-ion batteries are used. The submarine carries six 533 mm torpedo tubes capable of launching Mk 48 Mod 6 heavyweight torpedoes and UGM-84L Harpoon submarine-launched anti-ship missiles. Taiwan reportedly plans to acquire more than 100 Harpoon missiles to equip the IDS fleet. The submarine is also designed to deploy naval mines and is fitted with decoy launchers to reduce vulnerability to enemy torpedoes. The Hai Kun is the first submarine built domestically by Taiwan and includes features not previously integrated into the older Hai Lung-class, such as internal decoy systems.

The long-term objective of the IDS program is to field eight new submarines, to be added to the two existing Hai Lung-class boats, resulting in a total submarine fleet of ten. The Hai Lung-class vessels, built in the Netherlands in the 1980s, are expected to remain in service and operate alongside the new IDS platforms. The development of a domestic submarine construction capability is intended to reduce Taiwan’s reliance on foreign platforms and ensure continuity in undersea deterrence capability. The progress of the IDS program is closely monitored by the Legislative Yuan, which has imposed strict oversight mechanisms before approving further stages of procurement. The upcoming report on the Hai Kun’s SAT performance will play a decisive role in unlocking the next steps of the program, including physical construction of the two small variants.

The construction readiness phase at CSBC includes aligning tooling equipment, reviewing parts inventory, and coordinating skilled labor allocation in anticipation of the budget being unblocked. While no physical construction work on the new submarines has yet begun, CSBC’s production infrastructure remains active in support of the program’s timeline. If the Hai Kun completes all sea trials as planned and the Ministry of National Defense report is accepted, work on the two new submarines may begin before the end of 2025. This timeline remains dependent on legislative decisions, SAT outcomes, and internal government review procedures. The IDS program is a central part of Taiwan’s defense planning and continues to progress in line with a phased development and construction schedule, combining foreign-sourced technology with domestic production efforts.