Royal Navy Submarine Achieves First Underwater Milestone

Newest Royal Navy Submarine Achieves First Underwater Milestone at Barrow

Maritime October 21, 2025

BAE Systems’ Barrow-in-Furness site has marked a major achievement in the UK’s submarine programme as the latest Royal Navy Astute-class submarine, HMS Agamemnon, successfully completed its first submersion.

The 7,400-tonne, 97-metre-long nuclear-powered attack submarine underwent a successful ‘trim dive’ – a critical phase of testing to prove the vessel’s stability, buoyancy and watertight integrity – within Devonshire Dock in Barrow, Cumbria. The three-day trial represents one of the final stages before HMS Agamemnon departs for sea trials and joins her sister boats at HM Naval Base Clyde in Scotland. The milestone comes shortly after King Charles III formally commissioned the submarine into Royal Navy service, underscoring continued investment in sovereign defence capability and industrial resilience across the UK’s submarine enterprise.

Pete Tumelty, Astute Programme Director at BAE Systems Submarines, said: “This trim dive is the culmination of months of hard work. I’d like to thank all teams involved for their commitment and professionalism. We’re incredibly proud of the contribution we’re making to the nation’s security and Barrow’s long and distinguished heritage as the home of UK submarine design and build.“

Commander David Crosby, Commanding Officer of HMS Agamemnon, added: “The trim and basin dive is a key step in the commissioning of HMS Agamemnon. This period enables us to set the boat’s internal weight, prove her water-tight integrity, test sensors and put some of our systems through their paces ahead of sailing for the first time. It takes a great deal of planning and preparation to achieve this key step and all involved should be very proud of the part they have played.”

Strengthening the Defence Industrial Base

HMS Agamemnon is the sixth of seven Astute-class submarines being built by BAE Systems under a long-running programme that has sustained and expanded the UK’s highly specialised submarine industrial base. Alongside the Astute build, BAE Systems is also leading on construction of four Dreadnought-class submarines, which will replace the current Vanguard-class fleet responsible for maintaining the Continuous At Sea Deterrent (CASD).

These vessels form the backbone of the UK’s Defence Nuclear Enterprise, providing critical capabilities that underpin national security and deterrence strategy. Henry Musgrave, Head of the Astute Delivery Team at the Submarine Delivery Agency (SDA), said: “The successful completion of HMS Agamemnon’s trim dive marks a pivotal milestone in our mission to safely deliver available and capable submarines to the Royal Navy. This achievement reflects the exceptional collaboration between the SDA and our partners across the Defence Nuclear Enterprise, demonstrating our unwavering commitment to supporting the UK’s nuclear deterrent as a national endeavour.“

Investment and Growth in UK Capability

As part of ongoing investment in the UK’s submarine design and build capabilities, BAE Systems’ Submarines workforce has grown from 10,700 in 2023 to 15,000 today, with plans to reach 17,000 in the coming years. This expansion supports not only the Astute and Dreadnought programmes but also the design work for the future AUKUS-class nuclear-powered attack submarines, a trilateral endeavour between the UK, the US and Australia.

The achievement at Barrow underscores the strategic importance of the UK defence supply chain, which spans thousands of specialist engineers, manufacturers and SMEs contributing components and expertise to the nation’s most complex defence assets.

If you would like to join our community and read more articles like this, please click here.

Astute BAE Systems Royal Navy submarine