The coasts of Kent and Essex Counties, England, overlook the Thames Estuary, the only sea route to London. Throughout World War II it was constantly endangered by German minelayers, U-boats, and the Luftwaffe. From 1939 until 1942 the British navy patrolled the area; then a series of seven sea forts was built to permanently guard the river mouth. They were an innovative architectural and engineering achievement. The reinforced concrete and steel structures were entirely prefabricated in a Gravesend dry dock, floated to their locations, sunk, and anchored on the bottom of the sea, up to 9 miles (14 kilometers) off the coast. Although not as large as the now almost commonplace offshore oil and gas platforms around the world, the sea forts predated them by about five years, and the six so-called “Texas Towers” that form part of the U.S. lighthouse system by almost twenty.
Two kinds of forts, one for the navy and another for the army, were designed by the civil engineer Guy Maunsell. Even when war was little more than a threat, he submitted several proposals for seaward defenses, but it was not until October 1940—over a year after the outbreak of war—that the Admiralty commissioned him to design a prototype sea fortress. His initial costly proposal, for a 2,900-ton (2,640-tonne) pontoon supporting a gun battery, was shelved by the government. But when France fell, the Admiralty was moved to action and asked Maunsell to produce five sea forts for the Royal Navy.
The naval sea forts were essentially steel gun platforms with two 6-inch (150-millimeter) cannon and a Bofors antiaircraft gun. The huge structures were assembled by Holloway Brothers at the Red Lion Wharf, Gravesend, towed downriver by three tugs, and sunk by flooding their hollow pontoon base. Two were positioned in the estuary off the Essex coast and two off the Kent coast. Each fortress had a crew of about 100, who lived, provisioned for more than a month, in the two 26-foot-diameter (8-meter), 7-story concrete “legs” that supported the main platform, with its guns, radar, and control tower. The first was sited at The Roughs in February 1942. Sunk Head followed on 1 June, and Tongue Sands was completed about a fortnight later. Knock John was ready for action on 1 August. The fifth was never built.
The army sea forts, also designed by Maunsell, were England’s response to German air attacks on the strategic Liverpool docks via the undefended Mersey Estuary. It was decided to build five in the Mersey mouth and seven in the Thames Estuary. Each self-contained fort had living quarters for twenty-four men and comprised seven steel platforms supported on four 160-foot (49-meter) concrete legs. Four were gun towers with 3.7-inch (95-millimeter) cannon; a fifth was armed with a Bofors gun; the sixth was a searchlight tower; and the last was for radar. They were linked high above the sea by tubular steel catwalks that also carried power and fuel lines between the platforms. Their disposition was based upon the proven layout of shore gun batteries. In the event, only three were built on each side of England. Those in the Thames Estuary, constructed by the engineers who built the navy forts, were towed downriver in pairs and lowered by winches at strategic sites: The Great Nore, Shivering Sands, and Red Sand—all rather closer inshore than the navy counterparts. The pontoon bases used in the earlier structures would have been unsuitable in shallower water, where tidal currents constantly shifted the seabed; instead, Maunsell designed a self-burying footing that firmly anchored each tower in place. Construction began in August 1942, and the last tower was completed sixteen months later. At each site, the Bofors platform was erected first to defend the construction crews as they assembled the rest of the fort.
There is now no way to measure the passive deterrent effect of the Maunsell forts, but during their short active life they accounted for the destruction of twenty-two enemy aircraft and about thirty flying bombs. Because the Ministry of Defence believed that a combination of bad weather and tidal action would quickly destroy them after the war, no thought was taken for their disposal. For a few years after 1945 the naval forts were serviced by the Thames Estuary Special Defence unit, and two were temporarily adapted as lightships. Difficulty of access in storms led to that being discontinued; in fact, Tongue Sands was wrecked in bad weather in 1966. Only Knock John and The Roughs survive. After May 1964 the former, together with Red Sand and Shivering Sands army forts, was occupied at various times and for various periods by pirate radio stations, until the last was shut down under the Offshore Broadcasting Act in July 1967. The Roughs continues to have an eccentric postwar history.
It lies slightly north of the Thames Estuary off Harwich, and in September 1967, when it was still outside British territorial waters, a former British army officer named Paddy Roy Bates formally (and it must be said legally) annexed it as the Principality of Sealand, going aboard as the “prince” with his family. In the late 1990s a consortium of U.S. Internet entrepreneurs set up the world’s first offshore data haven there, offering prospective clients security for their computer operations, free from the interference of legislation.
The army forts also went into decline. For a short while, under the control of the Anti-Aircraft Fort Maintenance Detachment, they were furnished with improved searchlights and radar installations. Any perceived crisis past, the army stripped all guns and equipment from them in 1956. The Red Sand fort, off the Isle of Sheppey, was abandoned that same year. The Great Nore fort was dismantled in 1958 after being struck by a ship and officially declared a hazard to shipping. In 1959 another vessel collided with the Shivering Sands fort, bringing down one of the towers. Despite their short-lived roles as radio stations, the survivors are now derelict. Their robustness means that their skeletons will stand in the North Sea for some years to come, gaunt confirmation of the proverb “Necessity is the mother of invention.” Guy Maunsell did not survive his great sea forts, dying in 1961 after establishing an international civil-engineering partnership, which continues.
Turner, Frank R. 1995. The Maunsell Sea Forts. Gravesend, UK: F. R. Turner.