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Knock John Fort

Official Name

HM Fort Knock John

Code Name

Uncle Four (U4)


51.33.72 North

1.09.83 East

8.5 nautical miles (10.06 miles) from Foulness, Essex,16 nautical miles (18.41 miles) from Southend.


Grounded on 1st August 1942

WW2 Military use

Navy Sea Fort

Post WW2 Military use

Decommissioned on 14th June 1945, evacuated 25th June 1945 and reduced to care and maintenance until May 1956 when it was abandoned.

Stations Housed

Radio Essex


Radio City had plans to use the Knock John Fort to house another radio station they were planning to open. They sent a small team to Knock John  with a transmitter, a small generator and various other items of broadcasting equipment in mid 1965.

However, Roy Bates also had plans to use the Fort as the base for his Radio Essex, and the Radio City crew were  forcibly ‘evicted’ from Fort late in 1965. Radio Essex/BBMS broadcast from Knock John Fort from 27th October 1965 until 25th December 1966.

When BBMS left Knock John Fort in December 1966 they took all broadcasting and other equipment to Roughs Tower, off Felixstowe, which Roy Bates developed as the Principality of Sealand.

Knock John Fort Knock John Fort Knock John Fort being grounded 1942

Top: Knock John Fort when being used by Radio Essex

Above: an aerial view of the Fort in the early days of Radio Essex before the radar tower was cut down

Left: Knock John Fort being grounded 1st August 1942 and shortly after grounding had been completed, 1942

Sea Structures

Thanks to Martin van der Ven for allowing us to use some additional information from the Broadcasting Fleet section of the Offshore Radio Guide  in this Gallery

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