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Navy Sea Forts, Thames Estuary
Thames Estuary, east of Shoeburyness, Essex, UK
Navy Sea Forts, Thames Estuary
associated engineer
Guy Anson Maunsell
Sir Alexander Gibb & Partners
date  5th June 1941 - August 1942
UK era  Modern  |  category  Defence Structure  |  reference  TR193895
photo  © 2006 Hywel Williams
Four sea forts were built in the Thames Estuary for the British Navy as wartime gun emplacements to defend the approaches to London during World War II. Innovatively designed by Guy Maunsell to be erected quickly and inexpensively, they were well-protected structures. Two of the forts are still in position, though they are, of course, decommissioned.
By 1940 London was the busiest port in the world and suffering from massive night bombing raids that came to be known as the Blitz. It was clear that the city needed more protection against air and sea attacks. Magnetic mines were also a problem and a hazard to shipping. The Thames Estuary has been protected from enemy action since Tudor times, when Henry VIII built riverside blockhouses. In the 19th century, Martello towers protected Britain, and Maunsell developed this concept further for his estuarine World War II forts.
Commander E.C. Shankland, Port of London Harbour Master, sought Maunsell’s advice and his solution was a small defensive fort that rested on the sea bed but rose above water level, with a flooded-pontoon foundation and citadel superstructure. The design was submitted to the Admiralty in November 1940. After protracted arguments, during which Maunsell offered to demonstrate the 2m long model he had constructed, Navy Controller Vice-Admiral Fraser asked him to proceed on 6th March 1941. Maunsell and Sir Alexander Gibb & Partners were joint engineers for the project.
The original plan for five forts was amended to four — HM Forts Roughs Tower, Sunk Head Tower, Tongue Sands and Knock John (pictured above). They were fabricated, fitted out and equipped on the south bank of the River Thames at Red Lion Wharf, between Northfleet and Gravesend, then towed into position by tugs. The method of placing the forts by sinking a massive concrete pontoon shell straight onto the seabed had not been tried before, but was so successful that it later formed the basis for the design of the Mulberry Harbours, used for the D-Day landings on Normandy.
Maunsell used three docks for construction — one dry dock for building the pontoons, one tidal dock for the towers and deck, and a deep water dock for mooring the finished forts during fitting out. Each fort was constructed and placed in less than two months.
The reinforced concrete pontoon base is 51.2m long, 26.8m wide and 4.3m deep. This supports hollow twin tower legs with an external diameter of 7.3m and wall thickness of 300mm, cast in three 6.1m high sections. The deck is a reinforced concrete slab with a parapet surround, gun emplacements and radar station above offices. The total weight of the structure is some 4,500 tonnes.
Each fort carried two Vickers 94mm heavy anti-aircraft guns, two 40mm Bofors light anti-aircraft guns and (then new technology) radar equipment to give early warning of enemy approach. A 25kW diesel generator provided power for lighting, heating and ventilation. There are four floors of accommodation for officers and ratings inside each leg with up to three floors of storerooms below. Every man served six weeks aboard (longer in bad weather) followed by 10 days leave and four days of training before returning to the fort.
The first, Roughs Tower (U1), had its full complement of 120 naval personnel aboard when it was sunk in 11m of water on 11th February 1942. The base was flooded from one side only and hit the sea bed at an angle of 35 degrees before righting itself. The other three forts were sunk into position without incident — Sunk Head Tower (U2) on 1st June, Tongue Sands (U3) on 27th June and Knock John (U4) on 1st August 1942. The design of these three forts was modified to include a reinforced concrete buffer that crumpled on impact with the sea bed. It takes 15-16 minutes for the fort to settle into position after the sea cocks open.
Knock John was active against enemy aircraft in March and April 1943 and February 1944. The Navy forts, along with the estuarine Army forts (also designed by Maunsell) built the following year, between them destroyed 22 enemy aircraft and some 30 flying bombs during the war. Tongue Sands also destroyed one German E-boat.
In gales on 5th December 1947, part of Tongue Sands fell into the sea and the fort tilted 15 degrees. The crew was rescued and the fort remained empty until collapsing into the sea during a storm in February 1996, leaving just 5.5m of the west leg. One of its guns can be seen at New Tavern Fort Gardens, Gravesend.
In November 1965 pirate station Radio Essex (later Britain’s Better Music Station) began broadcasts from Knock John, but court action and lack of funds contributed to the station's closure on 25th December 1966.
The day before the closure, Roy Bates from Southend took over Roughs Tower, which, unlike the other three forts, is in international waters (outside Britain's 4.8km territorial limit). All the Radio Essex equipment was taken aboard. Bates' family claimed squatters' rights and in September 1967 set up an independent state called Sealand, even printing their own money and postage stamps. It is now run by Roy's son Michael. It was damaged by a fire in June 2006.
Another pirate radio station — Tower Radio — was attempted on Sunk Head Tower but failed to generate enough funding. Squatters in 1966 and rumours of smuggling operations led the Royal Engineers to blow up the fort on 21st August 1967, leaving 6m of each tower leg. A ship carrying liquid gas later collided with the legs, resulting in damage to her hull.
The sea forts were cared for and maintained by the Thames Estuary Special Defence Unit from decommissioning in 1945 until they were abandoned by the Navy in 1958. Full-time personnel left in 1956 — the isolation and cramped living conditions on board had increased the incidence of mental illness among the crew. The Bofors guns and radar were removed and soon the forts were scavenged for their copper and brass fittings. The Vickers guns were taken away by RAF helicopter in June 1992. Entrances were welded shut and the access ladders removed (date unknown).
Resident engineer: John A Posford
Contractor: Holloway Brothers (London) Ltd
Research: ECPK
"Thames Forts" by Victor Smith, Port of London Magazine, London, 1992
"Maunsell: The Firm and its Founder" by Nigel Watson and Frank Turner,
AECOM Technology Corporation, 2005

Navy Sea Forts, Thames Estuary