HMS Stirling Castle begins sea trials as UK’s first mothership for autonomous mine warfare.

HMS Stirling Castle departed Birkenhead on October 23, 2025, beginning her first deployment as a British Royal Navy vessel after her transfer from the Royal Fleet Auxiliary. The 6,000-tonne ship will operate from Portsmouth as the Navy’s lead platform for testing and supporting autonomous mine countermeasures systems.

On October 23, 2025, the British Royal Navy announced that the HMS Stirling Castle had begun its sea trials, following her formal transfer from the Royal Fleet Auxiliary to the Royal Navy in July 2025. The vessel left West Float, Birkenhead, to continue systems testing and crew training before operational certification later this year. The ship will serve under the Mine and Threat Exploitation Group as the first Royal Navy unit dedicated to supporting autonomous mine countermeasure operations.
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The HMS Stirling Castle is the Royal Navy’s first mine countermeasures mothership acquired to act as a floating base for autonomous technology dedicated to minehunting, with initial operations focused primarily in UK waters. (Picture source: British Navy)

The British Navy announced that the HMS Stirling Castle departed West Float, Birkenhead, on October 23, 2025, marking her first voyage as a Royal Navy warship following her transfer from the Royal Fleet Auxiliary (RFA) four months earlier. The ship will be based at His Majesty’s Naval Base Portsmouth and is the Royal Navy’s first mine countermeasures mothership specifically acquired to serve as a floating base for autonomous technology. This sailing marks the beginning of a series of sea trials and training activities before the vessel’s formal operational certification later this year. HMS Stirling Castle’s commissioning represents a step in the United Kingdom’s transition from traditional minehunting methods toward autonomous systems designed to reduce crew risk and enhance operational coverage in British waters. The 96.8-metre vessel, weighing around 6,000 tonnes, now flies the White Ensign and begins preparing for her first front-line duties under Royal Navy command.

The crew of 55, under the command of Commander Phillip Harper, completed several months of maintenance, system checks, and sea safety training to bring the ship back into operational condition after her lay-up. According to her commanding officer, refitting and readying Stirling Castle within four months required intensive work from all personnel involved, combining RFA and Royal Navy teams to ensure full compliance with naval operational standards. This rapid return to service involved testing of power generation, propulsion control, and dynamic positioning systems, as well as safety and damage control drills. The ship will next undergo additional trials and assessments at sea before beginning operational sea training to prepare both crew and systems for sustained deployment. Once these steps are complete, the ship will proceed toward front-line service with the Royal Navy’s Mine and Threat Exploitation Group (MTXG), the primary formation responsible for autonomous mine countermeasures and subsurface threat assessment.

The vessel was originally constructed as the offshore support ship MV Island Crown by Vard Brăila in Romania in 2013 and operated commercially under Norwegian ownership in offshore wind, oil, and gas projects. The Ministry of Defence purchased the vessel in February 2023 for approximately £40 million, after which it was transferred to HM Naval Base Devonport for conversion into a platform dedicated to Mine Countermeasures Maritime Autonomous Systems (MCM MAS). Conversion included modification of the working deck, integration of workshops, and installation of a heavy-duty crane with a safe working load of 10 tonnes at a 34-metre radius and 5 tonnes at 40 metres, enabling the launch and recovery of unmanned surface and underwater vehicles. The Stirling Castle’s 4 × Bergen C25:33L-6 diesel engines, twin Kongsberg azimuth thrusters, and three bow thrusters provide 6,000 tonnes of displacement and precise station-keeping for deployment operations. The ship also features a helipad and is equipped to carry mission containers and portable command centres to coordinate mine warfare systems.

Between 2023 and mid-2025, Stirling Castle operated under the Royal Fleet Auxiliary, helping the Navy evaluate the integration of autonomous minehunting platforms. She conducted initial trials with RNMB Apollo, Hydra, and Hazard under the MHC Block 1 programme, which aims to develop and validate modular autonomous mine countermeasure systems. However, the vessel later faced operational interruptions caused by a defective crane and an RFA-wide manpower shortage that left her inactive at Cammell Laird shipyard through late 2024. These limitations prompted the decision to transfer the ship to the Royal Navy in May 2025 to ensure her continued use as an autonomous systems platform. The transfer, completed officially in July 2025, allowed the ship to re-enter service under direct Royal Navy command, ensuring full utilisation of her technological capabilities and personnel complement.

HMS Stirling Castle’s commissioning also reflects broader changes in Royal Navy mine warfare policy following the gradual retirement of the Sandown-class minehunters, the last of which will leave service by the end of 2025. The new approach replaces traditional, crewed minehunters with larger, commercially derived motherships operating autonomous vehicles capable of conducting mine detection, classification, and disposal at range. Stirling Castle serves as the prototype vessel for this system-of-systems approach and provides a model for the next generation of Mine Hunting Capability (MHC) platforms. The Royal Navy’s MHC Block 2 programme envisions three additional purpose-built Offshore Support Vessels to be delivered in the early 2030s, offering enhanced deck capacity, integrated launch and recovery systems, and increased resilience in higher sea states. Lessons learned from Stirling Castle’s operation will directly inform their final configuration, particularly regarding modular payloads, mission container integration, and autonomous system interoperability.

The Stirling Castle’s design as a VARD UT 776 CD vessel, featuring advanced dynamic positioning and stability systems, makes her suitable for the precise deployment of uncrewed assets in coastal and offshore environments. Her working deck spans approximately half the ship’s length and supports mission modules for unmanned vehicle storage, maintenance, and control. Adjacent workshops allow crews to prepare and repair Mine Countermeasures Maritime Autonomous Systems, while the dynamic positioning system stabilises the vessel for recovery operations in varying weather conditions. Although primarily intended for mine warfare, the Stirling Castle could also support hydrographic survey, seabed protection, and subsea infrastructure monitoring missions, reflecting the expanding operational scope of Royal Navy auxiliary-type vessels in the undersea domain. The ship currently retains her blue and white commercial livery but will be repainted in grey to conform to naval standards during her next scheduled refit.

The name Stirling Castle continues a naval heritage dating back to 1679, when the first ship of the name, a 70-gun third-rate, entered service as part of Samuel Pepys’s Thirty Ships Programme. Five earlier ships carried the same name through the 17th to 19th centuries, participating in Mediterranean, Caribbean, and North American campaigns. The modern HMS Stirling Castle is the sixth vessel to bear the title and the first since 1811. Her affiliation with the City of Stirling was renewed in 2025 during visits by Commander Harper to strengthen civic ties. Local officials, including Lord Provost Elaine Watterson, highlighted the ship’s symbolic connection to Scotland and its role in national maritime security. The crew will represent the city in commemorative events such as Remembrance Day and Armed Forces Day, linking centuries of naval history to the Royal Navy’s adoption of autonomous mine warfare technologies aimed at enhancing the safety and effectiveness of future maritime operations.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.