By 1940, London was the busiest port in the world and a target for World War II bombing raids. The Navy forts were completed in August 1942 as mid-estuary gun emplacements defending the obvious route to London up the River Thames. After the completion of the Navy forts, the Admiralty asked Maunsell to tackle the design of Army forts for the Mersey Estuary at the approaches to Liverpool and then in the Thames Estuary.
For the Thames, Maunsell used the design he had developed for the Mersey, though the towers were not to be as tall since the Thames Estuary is not as deep. The plan was to construct seven forts (49 towers). However, in the event, only three were built, at a cost of £724,000. HM Fort Nore (U5) was deployed between 20th May and 4th July 1943. HM Fort Red Sands
(U6, pictured above) between 23rd July and 3rd September. HM Fort Shivering Sands
(U7) was deployed between 18th September and 13th December. All were sited in water less than 30m deep, and are still in place, except Fort Nore, which was dismantled in 1959-60.
The seven towers of each fort had different functions. Five were gun towers, four with 94mm heavy anti-aircraft weapons and one with 40mm Bofors light anti-aircraft guns. One tower housed a searchlight and the last was the control tower. The gun towers are arranged in a circle around the control tower (equipped with radar), with the searchlight tower further away. This disposition was based on the successful layout of land fortifications. The outer towers were connected to the central tower by tubular steel walkways (now demolished). The searchlight tower was the power station for the whole fort, and was equipped with three 30kW diesel generators.
The modular tower design consisted of a two-storey steel accommodation pod supported on four slim raking reinforced concrete legs attached to a self-burying base structure on the sea bed. The Thames Estuary is too shallow for the pontoon design used for the Mersey forts. The guns, searchlights or other military equipment were mounted on the roofs of the pods.
As with the Navy forts, the Army forts were fabricated, fitted out and equipped on the south bank of the Thames at Red Lion Wharf, between Northfleet and Gravesend. Each tower took about eight weeks to build and weighed some 760 tonnes. Individual towers were then towed into position suspended between two specially modified barges, each taking eight hours to put in place. The Bofors tower was deployed first, so that it could offer anti-aircraft protection to the vessels and crews installing the other towers.
The 305 tonne base of each tower comprises four hollow concrete members arranged in a square formation with each member extending beyond the square ( (like a straightened hash symbol: #)). Each member is 25.9m long, 1.8m high and 2m wide at the bottom and 1.7m wide at the top. The junctions of the members are solid concrete.
The bases support the four raking columns of each fort. The cylindrical columns (or legs) are concrete reinforced with 32mm diameter steel bar. They are 19.8m high, with an external diameter of 90mm and wall thicknesses of 300mm. The columns were precast in three sections, joined by solid concrete. A cast-in-situ 4.3m square x 1.2m deep concrete cap, with a 1.8m diameter hole through it, joins the legs with the steel pod above, which is connected to two 13.1m long steel joists embedded in the cap.
The parallel walls of the octagonal pods are 11m apart and constructed in 6.3mm steel plate, with steel-framed windows. The walls of the living accommodation are insulated with hardboard and all floors have a 19mm layer of asphalt. Armoured parapets surround the armour-plated top deck (pod roof) and the magazine chambers. The armouring is made up of two steel plates with a layer of stones embedded in tarmacadam between.
In wartime, tours of duty for the 165-265 men aboard each fort lasted four weeks, followed by 10 days ashore. The Army and Navy forts in the Thames Estuary between them destroyed 22 enemy aircraft and some 30 flying bombs.
From their 1945 decommisioning up to 1956, the sea forts were cared for and maintained by the Army's Anti-Aircraft Fort Maintenance Detachment. New searchlights and improved radar devices were installed in 1952. The Army removed the guns and military equipment in 1956 using a crane brought from Chatham Dockyard. Full-time personnel left at that time. The access ladders and walkways were removed in the early 1970s.
On 1st March 1953, a Swedish ship, Baalbeck, collided with and destroyed two of Fort Nore's gun towers, killing four of the caretakers on board. In 1954 another collision by the Mairoula M led to the decision to dismantle the fort, which was obstructing shipping lanes. In 1959-60 the steel pods were sold for scrap, the concrete legs blown up and the bases lifted and towed to Alpha Wharf at Cliffe in Kent, where they can still be seen at low tide.
On 7th June 1963, the Ribersborg knocked down one of the gun towers of Shivering Sands in fog, isolating the searchlight tower, which was later used by the Port of London Authority to record wind speeds and tide levels (replaced by a data recording buoy in 1992). In 1964 the fort was taken over by David Sutch (aka Screaming Lord Sutch), who ran his pirate radio station there between May and September before it was purchased by Reg Calvert for £5,000. The station was renamed Radio City and broadcast until 8th February 1967, though Calvert was shot by Major Smedley in an altercation over ownership of the fort. In 2005 artist Stephen Turner spent 36 days alone in one of the gun towers.
Fort Red Sands has been home to several radio stations, first Radio Invicta from June 1964. The owner and two staff died when their boat sank on a trip to shore in December that year. Radio King took over, broadcasting from March to 22nd September 1965. On 23rd September the station became Radio 390 and continued until it was outlawed in July 1967. A development group occupied the fort until 1969, though plans to convert it into an hotel were never completed.
The forts are not open to the public.