Taiwan's Hai Kun submarine heads for deep-dive tests in late October, the last hurdle before delivery

Taiwan's first locally built submarine, the prototype Hai Kun (SS-711), is expected to begin deep-diving tests in late October, military sources told the Liberty Times on Tuesday, 13 October 2026. They described the trial as the last major hurdle before the boat can be handed over to the navy.

The submarine left port in Kaohsiung at about 8am Taipei time on Tuesday for its 24th sea outing and 18th submerged test, its first trip out since National Day, the Liberty Times and Formosa TV reported. Formosa TV said the late-October phase will pair deep diving with an overnight endurance test as the navy and shipbuilder CSBC push to deliver the boat by the end of the year.

According to the military sources, the dividing line between shallow and deep diving sits at about 100 metres. Below that depth the boat has to work its way down toward its design limit, where far greater water pressure tests every weld and valve, and colder water strains air conditioning, life support and battery endurance. Earlier trials, including surface runs, a torpedo firing and shallow dives, mainly checked watertightness and handling, the sources said.

The navy plans to escort the deep dives with a rescue ship and several surface vessels to keep the risk down, the sources said. If the deep-water trials go smoothly, the Hai Kun could be delivered before the end of the year once acceptance checks and remaining fixes are complete.

Questions over the schedule

Tuesday's outing came as the programme faces repeated questions over its slipping timetable. Chi Tung-yun, who heads the Taiwan Security Development Association and spoke to reporters at the quayside, said only a handful of countries can build submarines on their own and that a first boat is a prototype that has to be debugged step by step. He argued that sea trials should be paced by safety rather than by the calendar.

Chi compared integrating the boat's systems to rolling out enterprise software, saying more subsystems mean longer debugging and that shipyards often add engineers close to delivery to trace software faults. He said handover normally follows an agreement between builder and buyer on which open issues are acceptable. Formosa TV noted that a former navy adviser had earlier said a fault in the platform management system left the side-scan sonar unable to link to the combat system.

A precedent from the Hai Lung

Military sources also pointed to the in-service submarine Hai Lung (793) as a precedent. Its combat system upgrade was completed on 15 July 2024, but the fixes found during later integration and testing took until 7 July 2026, about two years, to finish, the Liberty Times reported.

Its sister boat Hai Hu (794) is due to finish its own combat system upgrade, carried out alongside a major overhaul, by 31 December this year, and work is on schedule, the sources said. They argued the Hai Lung experience shows that continued tuning of the Hai Kun is normal practice for a brand-new design rather than a sign that the project has stalled.

The Hai Kun was named and launched at CSBC's Kaohsiung yard on 28 September 2023. Taiwan sees submarines as a key part of its asymmetric defence against China, and supporters of the programme say that without them the navy is only half a navy.