Hai Kun is the prototype boat in Taiwan’s Indigenous Defense Submarine program. Sea acceptance testing began in mid-2025, with trials structured in stages (surface navigation, then shallow dives, then deeper dives).

While Taiwan’s shipbuilder CSBC and the navy have described the process as normal defect-finding and tuning, multiple outlets and officials have confirmed delays and acknowledged that key systems still required additional testing before full diving trials could proceed.

Steering/Hydraulics Failure

A Taipei Times report said Hai Kun allegedly suffered a total failure of stern hydraulic systems during a sea acceptance trial on June 26, forcing sailors to manually operate the X-rudder to return to port; the report also said accompanying tugboats helped avoid collisions due to rudder malfunction.

Taiwan’s Navy Command Headquarters responded that trials are designed to identify and fix issues, that the sub has redundant operating modes (automatic/semi-auto/manual), and that the June 26 handling was within procedures and safety limits; it also said the issue was addressed after returning to port and did not recur.

“Technical issues”

A Republic of China Navy source told Janes in September 2025 that the sea acceptance trials had encountered “technical issues,” adding they were solvable but required time to implement; Janes also reported concern about the vessel’s ability to conduct a fully submerged test at that stage.

In parallel, CNA/Focus Taiwan reported CSBC said it was “impossible” to finish sea acceptance trials by September given progress, and the Navy said the submarine was moored and needed to clear a safety assessment before surfaced and submerged tests could continue.

In October 2025, the Taipei Times reported the navy said two major system groups still required testing before the submarine met preconditions for diving tests:

• Main engine and power management system
• Integrated Platform Management System (IPMS)

The navy stated the main engine/power management testing required more time due to coordination among multiple contractors, and that IPMS hardware had been tested while software modifications and validations were ongoing (with extra software engineers brought in).

Pipe Leaks

A South China Morning Post report (May 2025) cited former navy captain and project adviser Kuo Hsi alleging repeated piping system failures led to compartment flooding, prompting foreign testing teams to suspend work.

The same report cited Kuo claiming fewer than 400 of 4,000+ IPMS checkpoints had been tested at that time.

These claims were presented as allegations made in public commentary rather than confirmed findings.

Power/Voltage Surge

A June 2025 Taipei Times report noted earlier reporting that harbor testing had been hampered by technical issues including damage to components allegedly caused by voltage surges from an onshore power supply used during testing in February; the same piece also referenced Kuo Hsi alleging leakage problems, while noting the navy declined to comment on his remarks.

Rumored Hull Deformation

After early sea trials, online speculation circulated about an “uneven” section of Hai Kun’s bow.

CNA/Focus Taiwan reported CSBC refuted the “hull deformation” rumor, stating the protrusion on the port side of the bow was the housing for the passive ranging sonar system (a sonar dome), not structural deformation.

Schedule Slip

Reuters described Hai Kun’s June 2025 maiden sea trial as a milestone but noted the program had already been hit by delays.

Reuters also reported Taiwan’s plan to build eight submarines, aim to deploy at least two by 2027, and that Hai Kun cost T$49.36 billion (US$1.67 billion) with a Lockheed Martin combat system and U.S.-made Mark 48 torpedoes.

Domestic reporting also tied delays to budget and oversight pressure: Taipei Times reported the legislature froze 50% / NT$1 billion of submarine funding pending successful sea acceptance tests and a report to lawmakers.