May 15, 2025. Under the grey skies of Britain’s largest naval base, the Royal Navy unveiled a silent predator with no crew aboard — the extra-large uncrewed underwater vehicle (XLUUV) named XV Excalibur. Measuring 12 meters long and weighing 19 tonnes, it is the most advanced uncrewed submarine ever tested by the Royal Navy, and the largest of its kind in Europe.
This isn’t just a new platform. It’s the most tangible signal yet of the UK’s strategic shift toward robotic warfare beneath the waves — a realm growing more contested by the month. For the Royal Navy, Excalibur is a glimpse into the future: a fleet where manned and unmanned systems operate side by side, navigating complex threats, gathering intelligence, and protecting infrastructure critical to the nation’s security.
Excalibur is the flagship product of Project CETUS, launched in 2022 as a bold experiment in autonomy and innovation. CETUS was a joint initiative between the Royal Navy, Defence Equipment and Support (DE&S), and industry partners, specifically MSubs Ltd, a company based in Plymouth with a deep pedigree in building underwater vehicles.
Built at Turnchapel Wharf and launched in early 2025, the vehicle has since undergone extensive sea and harbour trials at the Devonport base. The project has moved with remarkable speed, considering the technical complexity and strategic significance. In just three years, a concept on paper became a vessel in the water.
Unlike previous unmanned systems limited by endurance or payload, Excalibur pushes the envelope. It can travel farther, carry more, and operate longer than any previous autonomous underwater vehicle (AUV) in the Royal Navy’s arsenal. Its modularity is what sets it apart — it can be fitted with different sensors and payloads depending on the mission.
Rear Admiral James Parkin, Director Develop of the Royal Navy, described Excalibur’s unveiling as “a pivotal moment” in the service’s journey toward autonomous capability. “This isn’t just about technology,” he said. “It’s about rethinking how we fight beneath the waves.”
The naming ceremony was held at HMNB Devonport’s South Yard, attended by over 200 guests, including military leaders, foreign representatives, cadets, and industry figures. The name Excalibur pays homage to a Royal Navy submarine launched in 1947, itself once an experimental vessel testing high-test peroxide propulsion — an idea later abandoned for safety reasons. That legacy of trial and innovation continues.
In line with naval tradition, a bottle of Plymouth Gin was smashed against the hull by Honorary Captain Peaches Golding OBE CStJ RNR, the Lord Lieutenant of Bristol, in a symbolic blessing of the vessel. Among those watching were delegates from AUKUS partner nations, a subtle but unmistakable reminder of the trilateral partnership’s growing emphasis on advanced underwater warfare.
“This is where heritage meets the future,” said Commodore Marcus Rose, Deputy Director of Underwater Battlespace Capability. “We’re taking risks — calculated, informed risks — to learn what works, and to keep Britain competitive in this domain.”
Despite the high-tech design and dramatic unveiling, Excalibur is not a combat-ready weapon. At least, not yet.
Instead, it will operate under the Royal Navy’s Fleet Experimentation Squadron, an initiative under the Disruptive Capabilities and Technologies Office. The goal? Experimentation at scale. Alongside the cutting-edge surface test ship XV Patrick Blackett, Excalibur will serve as a mobile lab — helping engineers, analysts, and operators understand how to integrate large uncrewed systems into real-world naval operations.
It will be used to test a wide range of technologies: advanced sonar arrays, ISR payloads (intelligence, surveillance, reconnaissance), seabed warfare systems, and modular mission packages. Some tests will be run in permissive environments; others will simulate contested zones where communications may be jammed or GPS denied.
This focus on testing rather than immediate deployment is crucial. “We’re not rushing this into battle,” Rear Admiral Parkin stressed. “We’re learning fast, but we’re learning smart.”
Excalibur’s development comes at a time of mounting global tension beneath the seas. From the sabotage of the Nord Stream pipelines in the Baltic Sea to increased submarine activity in the Indo-Pacific, undersea warfare has returned to the strategic foreground.
The seabed is now a battleground — a place where data cables, gas pipelines, and national secrets lie exposed and vulnerable. Traditional submarines still play a critical role, but uncrewed systems offer new possibilities: silent observation, persistent presence, and a lower risk to personnel.
That’s where XLUUVs like Excalibur come in. By removing the need for human operators aboard the vessel, navies can deploy these systems into high-risk zones without putting lives in danger. Their smaller size and lower cost compared to full-sized submarines also make them ideal candidates for “mass” deployment — saturating areas with sensors or decoys to confuse the enemy.
With this in mind, Excalibur is more than an experiment. It’s a signal to allies and adversaries alike: Britain intends to stay ahead in the emerging arms race under the waves.
The Royal Navy’s vision for the future is a hybrid fleet — one in which crewed submarines, surface ships, and autonomous platforms work together seamlessly. That future is still taking shape, but Excalibur is one of its cornerstones.
Rear Admiral Parkin described the vessel as a “bridge,” helping the Navy learn how to command and control uncrewed systems, how to maintain them at sea, and how to fuse their data into operational planning. “There are things you only learn when you put a system in the water,” he said. “Excalibur gives us that.”
This isn’t the first step into autonomy for the Royal Navy. Projects like the Mine Hunting Capability programme and various drone boat trials have laid the groundwork. But Excalibur goes further — in scale, complexity, and ambition.
Commodore Rose explained that much of the learning will also feed into allied interoperability. “If we want to operate alongside the US and Australia under frameworks like AUKUS, we need systems that can talk to each other, work together, and respond to threats in concert. Excalibur is part of that journey.”
Key to Excalibur’s success is its origin — built not in a sprawling government yard, but in the private sector by MSubs Ltd, a small but specialized British firm with a track record of delivering cutting-edge underwater vehicles.
MSubs worked closely with Navy engineers and DE&S procurement teams to translate the Navy’s ambitions into a tangible vessel. The company’s founder, Brett Phaneuf, has long advocated for autonomous submarines as the future of naval warfare. “The sea is vast and unforgiving,” he once said. “Machines don’t get tired, don’t need oxygen, and don’t panic.”
The vessel’s modular design reflects that thinking. It can be quickly adapted for different roles — from long-duration surveillance to deploying seabed sensors or operating in denied environments. That adaptability is what makes Excalibur so valuable as a platform for testing and iteration.
Following the naming ceremony, Excalibur will enter an extended phase of operational experimentation. It will be tested in a wide range of sea states, under different conditions, and with various payloads. The Navy will assess everything from endurance to data quality to communication latency.
There’s no fixed timeline for when — or even if — the XLUUV concept will be deployed in combat operations. But the data gathered from Excalibur will shape that decision. “We’re playing a long game,” said Rear Admiral Parkin. “But we’ve made a big first move.”
