The Maunsell Sea Forts were small fortified towers built in the Thames and Mersey estuaries during the Second World War to help defend the United Kingdom. They were named after their designer, Guy Maunsell. The forts were decommissioned in the late 1950s and later used for other activities. One became the Principality of Sealand; boats visit the remaining forts occasionally, and a consortium called Project Redsands is planning to conserve the fort situated at Redsand.
Maunsell sea forts, built in the Thames estuary and operated by the Royal Navy, were to deter and report German air raids following the Thames as a landmark, and attempts to lay mines by aircraft in this important shipping channel.
The design was a concrete construction; a pontoon barge on which stood two cylindrical towers on top of which was the gun platform mounting two 3.75-inch guns and two 40 mm Bofors guns. They were laid down in dry dock and assembled as complete units. They were then fitted out — the crews going on board at the same time for familiarisation — before being towed out and sunk onto their sand bank positions in 1942.
The naval fort design was the latest of several that Maunsell had devised in response to Admiralty inquiries. Early ideas had considered forts in the English Channel able to take on enemy vessels.
Maunsell Army Forts.
Maunsell also designed forts for anti-aircraft defence. These were larger installations comprising seven interconnected steel platforms, five carried guns arranged in a semicircle around the control centre and accommodation while the seventh, set further out than the gun towers, was the searchlight tower.
Three forts were placed in the Mersey and three in the Thames estuary, each of these AA forts carried four QF 3.75 inch guns and two Bofors 40 mm guns. During the war the forts shot down 22 aircraft and about 30 flying bombs. They were decommissioned by the MoD in the late 1950s.