Royal Navy Unveils XV Excalibur, Europe’s Largest Uncrewed Sub

On May 15, 2025, the Royal Navy celebrated a historic milestone in underwater warfare with the official unveiling of XV Excalibur, the largest extra-large uncrewed submarine ever developed by any European navy. The event took place at the South Yard of HMNB Devonport, marking a major leap forward in the UK’s autonomous maritime capabilities.

Measuring 12 metres long, 2.2 metres in diameter, and weighing 19 tonnes, this experimental vessel signals the Royal Navy’s ambitious push toward integrating uncrewed underwater systems into its future operational fleet.

Project CETUS and the Birth of XV Excalibur

XV Excalibur is the flagship product of Project CETUS, an experimental program launched in 2022 with the goal of exploring and advancing uncrewed underwater vehicle (UUV) technology. The project reflects the Royal Navy’s drive to stay at the forefront of innovation in undersea warfare amid growing global competition and evolving maritime threats.

Project CETUS is a collaborative effort, bringing together the Royal Navy’s autonomy and innovation teams, Defence Equipment and Support (DE&S), and industrial partner MSubs, a Plymouth-based company specialising in underwater vehicles.

Constructed in Plymouth and first launched in February 2025 at Turnchapel Wharf, Excalibur has since undergone extensive harbour and sea trials at Devonport. These tests have assessed the vessel’s seaworthiness, endurance, sensor integration, and system reliability.

While not designed for operational deployment just yet, the trials have proven invaluable in gathering data to refine the technology and better understand the operational challenges uncrewed submarines face in real maritime environments.

The submarine’s name, Excalibur, carries historical significance. It pays homage to a British submarine commissioned in 1947 that experimented with high-test peroxide (HTP) propulsion.

Although the HTP concept was eventually dropped due to the volatile nature of the chemical fuel, the name Excalibur represents innovation and bold experimentation—qualities that continue to drive the Royal Navy’s current technological ambitions.

A New Era for Undersea Warfare and Naval Innovation

The unveiling ceremony attracted approximately 200 guests, including senior naval officers like Rear Admiral James Parkin, Navy Director Develop, representatives from AUKUS partner nations, and Royal Navy cadets and recruits.

Rear Admiral Parkin underscored the importance of this event as a “significant moment” for the Royal Navy and UK Armed Forces. He emphasized that XV Excalibur is more than just a vessel; it is a critical stepping stone toward developing a hybrid fleet combining crewed and uncrewed underwater systems.

The Royal Navy’s focus on uncrewed systems comes at a time when undersea warfare and the protection of vital maritime infrastructure, such as undersea cables and pipelines, are growing in strategic importance.

Autonomous platforms like Excalibur can operate for extended periods without risking human life, covering larger areas with advanced sensor suites and conducting surveillance, reconnaissance, and other vital tasks in contested or denied environments.

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Excalibur’s design is modular and scalable, allowing it to carry a wider range of payloads and sensors than smaller autonomous underwater vehicles (AUVs). This extra-large uncrewed underwater vehicle (XLUUV) boasts significantly greater endurance, making it suitable for long-duration missions such as seabed warfare, sensor deployment, and intelligence gathering in restricted-access zones where crewed submarines may face operational limitations.

The submarine will be assigned to the Fleet Experimentation Squadron under the newly established Disruptive Capabilities and Technologies Office. It will work alongside the surface vessel XV Patrick Blackett, acting as a testbed for innovative maritime technologies. These efforts focus heavily on intelligence, surveillance and reconnaissance (ISR), sensor technology, and payload integration—vital areas as the Royal Navy evolves to meet 21st-century threats.

Building a Hybrid Fleet for Future Undersea Operations

Commodore Marcus Rose, Deputy Director of Underwater Battlespace Capability, highlighted that the knowledge gained from Project CETUS and Excalibur’s trials will build on lessons from previous initiatives like the Mine Hunting Capability program.

These insights will help the Royal Navy craft a more comprehensive doctrine for the combined use of crewed and uncrewed underwater systems, enhancing flexibility and operational reach.

Rose praised the strong cooperation between the Royal Navy and its industrial partners, noting that such close collaboration is essential for successfully developing and deploying cutting-edge maritime technologies. He also pointed out that Excalibur is a tangible example of how innovation and tradition come together in the Royal Navy, combining the historic values of naval exploration with the demands of modern warfare.

The integration of XLUUVs like Excalibur into the fleet represents a strategic pivot toward mass autonomous capabilities, where uncrewed systems complement traditional submarines and surface vessels. This hybrid approach aims to increase operational efficiency, reduce risks to personnel, and maintain a technological edge in contested maritime domains.

As the Royal Navy continues to refine its autonomous underwater capabilities, Excalibur will play a central role in shaping future British naval doctrine and strategy. The vessel’s ongoing trials and experimentation will provide critical insights into how large uncrewed submarines can be effectively deployed alongside crewed platforms to protect national interests and maintain superiority beneath the waves.