Concrete Ship & Mulberry Blog

By 1940, during World War II, there were many threats to London that required urgent counter measures. 

As a consequence of magnetic mines, laid by the Kriegsmarine, and dropped from the air by the Germans, by 1940, a hundred Navy and merchant ships had been sunk in the Thames estuary. It also became clear that the Luftwaffe was also using the Thames to navigate their way to London, a fact that became acutely relevant during The Blitz of 7th September 1940 to 11th May 1941.

The Admiralty were pressed to find a solution that would prove effective in deterring the Luftwaffe and Kriegsmarine from their focus on the Thames Estuary, River Thames and London. 

Guy Maunsell, renowned British Ferro-Concrete Engineer, was asked for his ideas and his first response in October 1940 was: - ‘A Proposal to Establish Martello Towers For The Defence Of The Thames Estuary’. 

‘Martello Towers’ were inspired by a circular fort, built at Mortella Point in Corsica, in the C16th as part of the wider system of defence of Genoa. In the early C19th, ‘Martello Towers’ had been built all around Britain and the British Empire during the Napoleonic Wars to defend against French invasion. Many Martello Towers exist today around the world.  

Inspired by Martello Towers, the forts that Maunsell conceived would be of the nature of a citadel, built on top of a concrete pontoon. The key point was that the fort would not be built in situ, but rather rapidly constructed on the side of a river or harbour, floated off, towed into the desired position and then sunk. 

Whilst the Martello Towers idea did not gain traction, at a meeting at the Admiralty shortly afterwards, Maunsell met Captain John Hughes-Hallett and was asked to design Naval Sea Forts for the Thames Estuary. 

His designs were completed in November 1940 and presented to the Admiralty. As the proposal reached higher echelons, it nearly ended up in the same dustbin as had other visionary Maunsell proposals such as the ‘Submersible Observation Towers’. 

Maunsell was asked to a meeting on 20th January 1941 with the then Director of Naval Construction, Stanley Goodall, K.B.E (later Sir Stanley Goodall). A Naval Architect, his belief was that the proposed concrete and steel structures would be unstable and might capsize before being sunk into position. The idea was not approved.

A determined Guy Maunsell was not giving up this time. He conducted tests of the Naval Sea Fort with scale models in tanks, and also in the sea. He managed to elevate his proposal to the attention of the Controller of the Navy, Vice Admiral Bruce Fraser. Maunsell was asked to start construction immediately. 

By March 1941, Maunsell’s plan for ‘Naval Sea Forts’ had been agreed. 

Sir Alexander Gibb & Partners were appointed joint engineers and overseers for the construction project contract for four Naval Sea Forts. John Albert Posford M.I.C.E of Sir Alexander Gibb & Partners was appointed Resident Civil Engineer for the construction project.

In June 1941, Maunsell himself found a suitable site for the construction of the forts, a derelict dock and jetty at Red Lion Wharf, Northfleet, near Gravesend. Messrs Holloway Brothers Ltd were appointed contractors to prepare the site itself.

The Naval Sea Forts comprised of three discrete 'layers'.

Firstly, there was a base, which was a reinforced concrete 'pontoon', 168’ long, 88’ wide and 14’ deep. 

The base had chamfered corners which was a design feature to reduce scouring. Maunsell had surveyed some of the ships wrecked in the Thames Estuary to understand the impact of tidal scouring, discovering that when a ship was positioned in the direction of the tide, there was little scouring but if the wreck was side on to the tide, it would probably keel over and break up. Hence, the chamfered corners and a decision that the Naval Sea Forts would be sunk to 'lie with the tide'. 

The structure would float such that it could be towed out to the desired location and then sunk such that the base would rest on the seabed. 

Secondly, on top of the base were constructed two cylindrical reinforced concrete ‘legs’, each 60’ high, 24’ in diameter, and 3 ½ inches thick. 

Each concrete leg was divided into seven floors. Four floors were for crew quarters, the remainder for dining, operations, storage areas, generators, freshwater tanks and anti-aircraft munitions. 

Lastly, on top of the legs sat a steel platform that acted as a gun deck. Two 3.7” Anti-aircraft guns and two Bofors 40mm guns defended the fort.  Additionally, there was a central control centre building with radar, a central living area, galley, medical room and officers’ quarters buildings.

There was a steel framework at one end, that supported a landing jetty and crane, used to hoist supplies aboard.

The overall weight is estimated to have been 4,500 tons. 

Naval Hydrographers took on the task of finding suitable sites for the forts, in terms of sea depth and the nature of the seabed. 

Much of the knowledge that we have about the Maunsell Sea Forts was researched and published by a gentleman called Frank Turner who wrote a number of books and pamphlets about the work of Guy Maunsell, and in particular The Maunsell Sea Forts: The World War Two Naval Sea Forts of the Thames Estuary. He produced a video that you can watch on You Tube which goes into great detail about the design, build, launch, and grounding of the Naval Sea Forts.

There is also a film, produced by the consultant engineering firm, now known as Posford Duvivier, that is narrated by John Posford, partner of Guy Maunsell that provides an excellent explanation of the thinking, planning and execution of the Maunsell Naval Forts . It Is available on the BFI Player and you can watch it here

                         Conceptually, the idea behind the sea forts was to ‘disrupt’ the activities of the both the German air force and navy, essentially a first line of defense before the attackers reached the East Anglian coast, Essex coast, or the Thames estuary.

They needed to be positioned in places where the sea depth was optimal (the cylindrical legs were 60’ high) and the seabed was flat and sandy as the pontoon base would rest on the seabed. 

They did not float with the tide. Four positions were chosen, and the Naval sea forts took on the names of those positions.

‘H.M Fort Roughs’ – code name Uncle 1 -  was the first of the Maunsell Naval Sea Forts and was commissioned on 8th February 1942. Fort Roughs was intended to protect the East Anglian ports and it was to be positioned at Rough Sands, hence its name. Rough Sands is a sandbar located approximately 11 kilometres from the coast of Suffolk, and 13 kilometres (7 nautical miles) from the coast of Essex at 51.895194°N 1.480569°E.  

Fort Roughs was grounded at 16:45 on 11th February 1942, with over 100 men already positioned on the fort itself. What could possibly go wrong ?!!

The grounding proved to be a dramatic event. Whilst the grounding was meant to take place under the supervision of Guy Maunsell, a senior Naval officer chose to circumvent the grounding instructions, opening a sea cock on one side only and causing the platform to initially list at 30 – 35 degrees before ultimately righting itself and sinking to the seabed. 

John Albert Posford (1914-96), resident engineer at Red Lion Wharf, captured the event on film.  

You can view the dramatic grounding on the Maunsell Seaforts Appreciation Group on Facebook.

Subsequent groundings were managed by the contractors themselves, rather than the Admiralty, and no personnel were placed on the fort until the grounding was complete.

Having served her war purpose, Fort Roughs was decommissioned on 14th February 1945 and reduced to ‘care and maintenance. In 1956, it was abandoned.

Fort Roughs survives today and is better known as ‘Sealand’. 

On 24th December 1966, Roy Bates, who operated Radio Essex, and Ronan O’Rahilly of Radio Caroline, landed on Fort Roughs and occupied it. 

However, after disagreements, Roy Bates seized the tower as his own. When O'Rahilly attempted to place caretakers on board the fort on 27th June 1967, Roy Bates and family defended the fort with guns and petrol bombs and continued to occupy it. 

The ‘goings on’ were reported in the Kent Messenger & Gravesend Telegraph on 30th June 1967 with the headline ‘Pop Pirates Attack Gravesend Men – Guns and Bombs in Sea Fort Battle’

The incident caused the Royal Marines to be put ‘on alert’ and the British authorities ordered Roy Bates to surrender. Indeed, he and his son Michael were arrested and charged with various firearms offences. 

Ultimately, the the court dismissed the case, as it did not have jurisdiction over international affairs, and Roughs Tower lay beyond the territorial waters of Britain.

When the Royal Engineers blew up Fort Sunk Head in August 1967 - see below - reputedly, a navy craft swung close to Sealand after the explosion and shouted, ‘You’re next !’, or words to that affect.

The official Sealand website tells the story

Roy decided to declare this fortress island the independent state of “Sealand”, Claiming “Jus Gentium” (“Law of Nations”) over a part of the globe that was “Terra Nullius” (Nobody’s Land). On the 2nd of September 1967, accompanied by his wife Joan on her birthday, his son Michael (14), daughter Penelope (16) and several friends and followers, Roy declared the Principality of Sealand. The founding of this country was marked by the raising of a newly designed flag, and in an extremely romantic birthday gesture, the bestowing of a new title on his beloved wife, to be know from that moment on as “Princess Joan”.

There's a nice YouTube video that shows the life of the Bates family on Sealand in the 1960s.

The Principality of Sealand indeed has, amongst other things, a constitution, a flag, a national anthem, its own currency, One can even gain a title, the most prestigious being Duke and Duchess. Just visit the Sealand shop for options !

Fort Sunk Head was commissioned on 1st May 1942, situated approximately 18 kilometres from the British coast off Essex at 51.7773°N 1.50841°E.  It was grounded on 1st June 1942. 

Fort Sunk Head was decommissioned on 14th June 1945 and maintained until 1956, when it was abandoned. 

Trinity House briefly considered taking over this Fort to use as a lighthouse to mark the outer end of the Sunk Head Sands at the Barrow Channel, but concerns that any settling of the Fort on the seabed - making it list - would upset the lights, meant that the idea wasn’t pursued.

In the mid 1960s, the era of Pirate Radio, Sunk Head Fort was occupied and used by Tower Radio / Radio Tower between October 1965 and April 1966. 

When the project ran out of money, they abandoned Sunk Head leaving behind a virtually new generator and all the broadcast equipment.

Unlike some of the other forts, Sunk Head was well outside territorial waters,  and when the Marine Etc Broadcasting (Offences) Act came into effect in August 1967, the Government was anxious to ensure that it wouldn’t be taken over again by an offshore broadcaster. 

On 18 August 1967, Sunk Head was boarded by a contingent of the 24th Field Squadron of Royal Engineers from Maidstone from the tug Collie, commanded by Major David Ives. The fort was weakened by acetylene cutting torches and 3,200 pounds of explosives were set. 

On 21st August 1967, Fort Sunk Head was spectacularly blown up, leaving just 20 feet of the leg stumps remaining. Those stumps have subsequently collapsed.

Also known as ‘Uncle 3’, Fort Tongue Sands is situated approximately 10.2 kilometres from the Kent coast  off Margate at 51.492915°N 1.36662°E. 

Uncle 3 was grounded on 27th June 1942 and was decommissioned on 14th February 1945.

Fort Tongue Sands was damaged by gales on 5th December 1947, and as it shook violently, with pieces falling off, its crew had to be rescued. It was believed that the fort had settled badly when grounded, although divers concluded that the foundations were solid.

A later storm caused the fort to take on a 15 degree list, scouring caused the fort to distort, the platform detached from the legs, holes appeared in the legs themselves and they flooded.

Fort Tongue Sands collapsed into the sea on 21st/22nd February 1996. Leaving just a single stump of one leg visible.

Probably, Fort Tongue Sands suffered from scouring of her pontoon base due to tidal conditions.

Knock John Fort is situated approximately 16.1 kilometres from the coast off Essex at 51.562277°N 1.162059°E and was grounded on 1st August 1942. 

It was decommissioned on 14th June 1945 and evacuated on 25th June 1945. The platform was maintained until May 1956 when it was abandoned. 

In 1964, inspired by the success of Radio Caroline and Radio Sutch, Roy Bates (of Sealand fame) decided to investigate the possibility of starting his own radio station based on one of the abandoned Maunsell wartime forts around the Thames estuary. With a number of the Maunsell Sea forts already occupied, Bates decided to establish his ‘pirate’ radio station, Radio Essex’ at Fort Knock John.  

There was competition for Fort Knock John and by the time Bates had put together the equipment and finance, Reg Calverty of Radio City had already landed some of his radio equipment on the fort. A violent struggle followed, with Roy Bates emerging victorious, and he then went about setting up his radio station and started broadcasting.

As is well documented, the British Government was not prepared to allow ‘pirate radio stations’ to operate with impunity and on 27th July 1966, the first reading of the Marine etc. Broadcasting (Offences) Bill was introduced into the House of Commons. On 29th September 1966 Roy Bates  was served with a summons to be heard at Rochford Magistrates Court alleging the illegal use of a transmitter at Knock John Fort, contravening the Wireless Telegraphy Act of 1949. 

On 6th October 1966, before the case had been heard in court,  Bates changed the name and format of Radio Essex to BBMS (Britain’s Better Music Station). The outcome of the case was that Rochford Magistrates held that the Knock John Fort was within British territorial waters and imposed a fine of £100 on Bates. 

They however refused a Post Office request for confiscation of the transmission equipment and payment of costs. Roy Bates lodged an appeal against his conviction, and BBMS stayed on the air, but not for long. Advertising revenues dried up, his money ran out, and BBMS closed on Christmas Day 1966. The transmitting equipment on Fort Knock John was dismantled, and Roy Bates moved operations to Fort Roughs which was clearly outside of British territorial waters. 

Today, Fort Knock John still survives, over 81 years after it was grounded. One of its legs is ‘distorted’, slightly off perpendicular, but considering that it was abandoned 67 years ago and is probably at least half a century older than any expected service life. It is thought that scouring is the cause of this.

Testimony to the excellence in design and robust concrete construction of the Maunsell Naval Sea Forts is that two survive to this day. 

They are an attraction by virtue of their history and their curious nature and indeed, there is a growing interest in their history as a result of the efforts of Facebook Group 'The Maunsell Seaforts Appreciation Group', founded by Margaret Flo McEwan. 

Various companies offer sea trips to see the Naval Sea Forts at close quarters, such as X-Pilot out of Queenborough

In a subsequent Blog, we will be covering the Maunsell Army Sea Forts which were another curious concrete construction, designed by Guy Maunsell.

With thanks to Margaret Flo McEwan https://www.facebook.com/groups/1839860506119353/ is a great source of photographs and knowledge about the Maunsell Sea Forts.

A primary source of information can be found in a series of books by Frank R. Turner  - The Maunsell Sea Forts: The World War Two Naval Sea Forts of the Thames Estuary – and from his documentary that can be seen at 

Many of the photographs used to illustrate this article are taken as stills from the documentary produced by Posford Duvivier 

http://www.offshoreradiomuseum.co.uk is great source particularly related to Pirate Radio station activities

http://www.offshoreradiomuseum.co.uk/page461.html  - Fort Roughs

http://www.offshoreradiomuseum.co.uk/page463.html - Sunk Head

http://www.offshoreradiomuseum.co.uk/page464.html - Tongue Sands

http://www.offshoreradiomuseum.co.uk/page459.html - Knock John

https://youtu.be/XMl9PKnJr3E?feature=shared

https://youtu.be/FSX0IREtJ5o?si=1ZzwGriTCqkPRjZv

https://sealandgov.org/en-eu/blogs/news/sunk-head-fort-explosion-1967

https://youtu.be/gbaER7G3C4I?feature=shared

https://sealandgov.org/en-eu/collections/shop

https://youtu.be/bYQphX0zPSc?si=2L7HNnIEtb5yY99D

https://www.facebook.com/FLOJIVECAT/videos/559247029504027?idorvanity=1839860506119353

https://www.facebook.com/groups/1839860506119353/permalink/2235497646555635/?mibextid=oMANbw

https://youtu.be/UeTgAwkbP5E?si=eJH4mhCcf1cUHUW5

https://sealandgov.org/en-eu/pages/the-story