Excalibur rises: UK names first extra-large uncrewed submarine

The Royal Navy named its first uncrewed submarine Excalibur at HMNB Devonport on May 15, 2025.

Officially classified as an extra-large uncrewed underwater vessel or XLUUV, the submarine got its name after the Arthurian legend, and an experimental high-speed submarine trialled by the Royal Navy in the 1950s and ’60s.

The 12-meter experimental vessel, displacing 19 tonnes and measuring two meters across, is the culmination of the three-year-long Project Cetus and the largest uncrewed underwater vessel trialed by the navy to date. The crewless craft was built by Plymouth-based MSubs, which specializes in automated submersibles.

The XLUUV has undergone harbor and sea acceptance trials (HATs and SATs) in Devonport Naval Base.

Over the next two years, Excalibur will carry out extensive sea trials, helping to accelerate the Royal Navy’s use of advanced technologies. It will help develop a better understanding of the unique challenges that come with operating uncrewed vessels of this size, with the aim of future vessels working alongside crewed platforms.

The trials and testing will contribute significantly to the Royal Navy’s constant efforts to remain ahead of potential foes beneath the waves, safeguarding key infrastructure, protecting British and allied ships and submarines, and gathering intelligence. 

“The naming of Excalibur is a significant milestone for the Royal Navy and the upcoming sea trials will allow us to rapidly develop our understanding of operating uncrewed vessels of this size underwater,” Commodore Marcus Rose, deputy director Underwater Battlespace Capability, said.

“The lessons learnt from this exciting programme will build on our experience from existing programmes, such as the Mine Hunting Capability programme, to inform more extensive use of these technologies in a mixed force of crewed and uncrewed systems.”

Excalibur will join the recently created Fleet Experimentation Squadron. Sitting under the Disruptive Capabilities and Technologies Office, it will join surface ship XV Patrick Blackett, the Royal Navy’s experimental vessel, which already sits within the squadron.

It will serve as the testbed for modern maritime technology, both exploiting the natural Intelligence surveillance and reconnaissance (ISR) and the stealth aspects of the XLUUV.

As a demonstrator, the vessel will not perform operational duties but will shape future concepts, according to the Royal Navy.

“In our journey towards autonomy and mass, it is vital we learn by doing, and Excalibur will be our mechanism for understanding the complexity and challenges of operating a future team of crewed and uncrewed vessel and systems underwater,” Rear Admiral James Parkin, Navy Director Develop, concluded.

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