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.
There were four naval forts:
- Rough Sands (HM Fort Roughs) (U1)
- Sunk Head (U2)
- Tongue Sands (U3)
- Knock John (U4)
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.
Knock John sea fort