The Maunsell Sea Forts

When I was a young lad I would often visit Herne Bay in Kent. If the weather was good I would look out to sea and could often see a group of large metal "monsters" lurking a few miles off the coast. My parents knew that these forts were built to help defend the south coast from enemy attacks during the second world war, but could not tell me any more than that. Over the years I became more interested in the purpose and function of these forts. I was visiting Herne Bay in the Spring of 1997, and came across a poster advertising a boat trip to these forts. Naturally I decided to go. The trip was organised by Mr. F. Turner who lives in Gravesend. He has researched the forts, and is now an expert on the structures. I have used his booklet "Maunsell Sea Forts" (Revised Edition ISBN 01-901132-01-3) to write the remainder of this article about these fascinating structures.


Maunsell's Naval Forts

The sea forts were designed by Mr. Guy Maunsell, a notable architect, who sent numerous design proposals to the Admiralty during the run up to the start of hostilities at the start of the second world war. In October 1940 Maunsell was instructed to design a sea fortress which could be towed into the Thames Estuary and sunk to defend the Southern coast of Britain from any enemy attack via air or sea.

His design used a pontoon base in a single unit 180 foot long by 80 foot wide. This had a weight of 2623 tons. On top of this was located another unit which housed the guns, men and ammunition. This design was estimated to cost around 3,250,000 at 1994 prices for three such units. However the construction of these forts never actually took place.

Shortly after this the threat of a German invasion increased as France fell to the Nazi onslaught. As well as this, Allied shipping in the Channel was becoming targetted more frequenty by the enemy, and German bombers were finding little resistance to their progress as they travelled above the Thames on their way to bomb the London Docks. The admiralty thus asked Maunsell to design blueprints for five new fortresses. These were largely based on his previous work, but was capable of housing 100 men inside the legs of the structure, as well as having a large gun emplacement built on the top. These fortresses were self sufficient for over a month.

Navy Fort

A Thames Estuary Naval Fort.
The blue line represents the high tide mark. The decking seen on the left of the picture was of a metal & wooden construction and used for access to the fort super-structure.

Work was started shortly after July 1941. The pontoons allowed the structures to be built in dry-docks, then floated into position before being "sunk". Although calculations had shown these designs were adequate it was a tense moment when the first pontoon was set adrift. This however, was nothing compared to the tension faced when the sea cocks were opened and water was allowed to enter the pontoon. The structures were top heavy, and many feared that they would simply tip over and "turn turtle" during final placement. Fortunately, this was not the case. Fifteen minutes after the opening of the seacocks the pontoon submerged. Twenty seconds later the leading edge of the pontoon hit the sea bed. Attached to this edge was a large reinforced concrete "buffer" which was designed to crush when it hit the seabed, and thus disapate some of the force which could have damaged the pontoon itself. Ninety seconds later the pontoon became completely filled with sea water, and the whole structure became fixed firmly to the seabed.

Please note the top of each naval fort has been removed to aid clarity.

Sea forts floating

The forts were systematically floated to their locations.

Sea Forts 20 degrees

The seacocks are opened and the pontoon begins to fill. After about fifteen minutes the pontoon dips below the surface.

Sea Forts 30 degrees

Twenty seconds later the pontoon reaches the sea bed. The reinforced "buffer" absorbs the huge impact.

Seaforts in position

The fort comes to its final position on the seabed.


The first naval fort, named "Roughs Tower", was towed into position and sunk on the 11th Feb 1942. The second fort, "Sunk Head Tower" was positioned on the 1st of June the same year. "Tongue Sands" was the third naval fort to be compleated. She was finished on the 17th of June 1942, and towed into postion ten days later. Fort number four was built on the 16th of July, and towed into postion on the 1st of August 1942. She was named "Knock John". For undisclosed reasons the Admiralty decided not to have the fifth fort built. Each naval fort cost aproximately 3,000,000 at 1994 prices. This was not including any military equipment or weapons.


Maunsell's Army Forts

After competetion of the Naval Forts, Maunsell was commissioned to design Army forts for position in both the Thames and Mersey estuaries. Their purpose was to attack enemy bombers who would be approaching the dock areas further up river.
Maunsell was faced with the problem of having to redesign the bases of these forts, as the tide was known to cause the seabed to shift rather a lot. This meant that the pontoon idea which he had employed for the Naval forts would not be suitable. He elected to solve the problem by designing a foundation which when sunk into position would bury itself into the seabed, but allow the natural movement of sand and shingle to go on around it.

Each fort consisted of seven towers. One search light tower, a Bofors gun tower, four 3.7 " gun towers and a central control tower. Each of which were interconnected via tubular steel walk-ways. The towers were positioned in a similar fashion to gun batteries found ashore. This was to enable the towers to defend themselves with the greatest degree of success, and had been a proven design.

Army Fort Positioning

A Plan View of the lay-out of the Thames Army Forts.
Blue = Control Tower
Green = 3.7" Gun Towers
Red = Search Light Tower
Yellow = Bofors Gun Tower

Construction of the Thames army forts started in August 1942, and the final tower was completed by December of the following year. The towers were sunk in a specific order starting with the Bofors tower, as this was then able to defend the tugs and other vessels used to construct the remaining six tower units. The total cost of building the twenty-one towers which made up the three Thames Army forts was estimated to be around 21,000,000 at 1994 prices.

Army Fort

A Bofors Tower from a Maunsell Army Fort
The two blue lines represent high and low tide marks.


The Maunsell Forts After The War

After hostilities ceased in 1945 the army forts were put on Care & Maintainance, and were controlled by The Anti-Aircraft Fort Maintainance Detachment. In 1952 new search lights, and updated radar equipment was fitted to them.
In 1956 the army decided the forts were of no purpose to them, and so enlisted the help of a large crane from Chatham Dockyard to remove the heavy guns and equipment from the three Thames forts.

The Naval forts were also put under a care & maintainance contract, being serviced by the Thames Estuary Special Defence unit. Two of the forts were used as post-war light ships, but the difficuties encountered whilst gaining entry to these forts in rough weather meant that Trinity House had to re-think their plans.

In the mid 1960's pirate radio stations used all the forts as bases. All of the forts (except Knock John) were outside the three mile limit for UK jurisdiction, and so provided good locations for such activities. Sunk Head Tower was at one time occupied by an organisation calling itself Radio Essex. It had planned to broadcast TV pictures to the UK after the BBC closed down each evening. However lack of money prevented this from happening.

It it said that Roughs Tower is still occupied to this day by the family of Roy Bates, a Southend entrepreneur. Mr. Bates proclaimed the Tower a prinicipality and called it "Sealand". Indeed he even had his own postage stamps printed, although they were never recognised by th Postal Union.


The Maunsell Forts Today

Below are two photographs taken of the sea defences as they were a few years ago. As you can see they are no longer operational, but are still standing.

Red Sands Army Fort

Red Sands Army Fort. Photograph taken August 1995.

Another shot of Red Sands

Another recent view of Red Sands Army Fort

Knock John Fort 1942

Knock John Navy Fort. Circa 1942

Knock John Navy Fort

Knock John Navy Fort. Photograph taken August 1995.


Much of the information on this page has come from the research conducted by Mr. F. Turner.
Trips to the forts are still conducted in the Summer months from Herne Bay Quay. Ask at Herne Bay Library for more details.

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