Taiwan’s push to field an indigenous diesel-electric attack submarine is less about achieving parity beneath the waves and more about changing the calculus of any potential maritime coercion against the island. Taipei’s Indigenous Defense Submarine program, embodied in the Hai Kun prototype, has moved from ceremonial launch toward operational testing and refinement — a trajectory that, if sustained, will incrementally harden Taiwan’s asymmetric deterrent and complicate Beijing’s options for coercion or blockade.
By early 2025 Taiwan’s shipbuilder and the navy were publicly preparing Hai Kun for acceptance trials and a transition to sea trials, after a period of dockside tuning and systems integration. CSBC and Taiwanese reporting described the vessel’s movement between docks and completion of harbor-level checks as the program approached the more demanding stages of sea acceptance.
Technical descriptions available in the public domain portray Hai Kun as a conventional diesel-electric design sized for littoral and near-sea operations, with reporting that places submerged displacement in the roughly 2,500 to 3,000 ton class and a weapons fit intended to include heavyweight torpedoes and submarine-launched anti-ship missiles. Taipei’s acquisition and domestic integration strategy emphasizes proven Western combat systems and weapons where possible while keeping construction and systems integration onshore. The program is framed inside a phased production plan often described as “2+3+2,” with a near-term ambition of expanding the IDS fleet beyond the prototype.
Why invest in an IDS fleet when the People’s Liberation Army Navy already fields far larger numbers of submarines? The answer is strategic, not technological. Taiwan cannot outbuild the PLAN. It can, however, raise the costs and risks of channeling a coercive naval campaign. Well-handled diesel-electric submarines are particularly useful for sea denial in constrained geographies. They can hold choke points, threaten amphibious shipping, interdict logistics, and force an adversary to divert significant ASW resources just to achieve a measure of freedom of maneuver. In short, they force complexity and delay into an aggressor’s plans, which is precisely what deterrence by denial requires.
Operationally, an IDS capability amplifies several defensive levers that Taiwan has been developing in recent years: distributed anti-ship missiles, mobile rocket and coastal defenses, improved coastal surveillance, and a resilient communications and space architecture. Submarines complement these by turning the undersea dimension into an asymmetric terrain. Even a small number of quiet diesel-electric boats operating in the deeper waters east of Taiwan or in key chokepoints such as the Bashi Channel can impose disproportional uncertainty on a PLAN commander evaluating timelines for a blockade or invasion. That uncertainty is a strategic asset in peacetime deterrence and a tactical multiplier in wartime containment.
But the promise of sea denial is contingent on three connected capabilities that Taiwan will need to sustain simultaneously. First, survivable logistics and sustainment for submarines: building the hull is the opening act; creating secure maintenance, spare parts pipelines, trained crews, and depot-level repair capacity is the long play. Second, sensor and networking investments to maximize the operational reach of each boat: better passive acoustic arrays, maritime patrol aircraft and helicopters, shipborne sonar, and shore-based sonar nets allow submarines and friendly ASW assets to cooperate rather than operate in isolation. Third, political and budgetary continuity: Taiwan’s submarine program has faced fiscal tempering in the legislature that could slow serial production if sea trials and delivery milestones do not reassure skeptical lawmakers.
These constraints are manageable, but only with steady policy choices. For Taipei, that means protecting the IDS budget through demonstrable milestones and transparent reporting on trial outcomes so that lawmaker concerns are addressed with facts rather than politics. It also means deepening procurement and industrial relationships with partners for key subsystems and sustainment support while retaining critical integration work domestically. Allies can help with sensor kits, training, and sustainment frameworks that do not require direct transfer of nuclear propulsion technology but materially improve the operational availability and lethality of conventional boats.
For Beijing the arrival of a credible IDS fleet alters some risk calculations. Where once PLAN planners could assume relative undersea superiority around Taiwan, diesel boats increase the probability of surprise interdiction and attrition. That does not remove PLAN advantages in numbers and reach, but it increases the friction, time, and political costs of operations designed to isolate or seize Taiwan. Sea denial is not an impenetrable shield; it is a cost-imposition strategy that aims to make the price of aggression intolerable or at least uncertain.
Policymakers in Taipei and partner capitals should treat Hai Kun not as a single trophy but as the opening move in a decade-long program. Success will be judged less by a maiden trial than by the institutionalization of submarine operations: routine deployments, integrated ASW tasking, logistics pipelines, and the serial construction of follow-on boats. If Taiwan can translate a prototype into a resilient deterrent architecture, the island’s ability to practise effective sea denial will be its most consequential contribution to stability in the first island chain.
In the coming months the most important indicators to watch are whether harbor and sea acceptance tests proceed to completion on schedule, whether the Legislative Yuan restores funding for serial production, and whether Taiwan and its partners accelerate investments in ASW sensors and sustainment. Each milestone will either reinforce the strategic logic of denial or expose vulnerabilities that adversaries can exploit. The Hai Kun is important. The policy, industrial, and alliance choices that follow will determine whether it becomes decisive.