The Maunsel Sea Forts aka “Fort Madness.”

Sea fort in the Thames Estuary during WWII

Sea fort in the Thames Estuary during WWII

World War II was a desperate time for the British Empire. Early in the war, the Nazi war machine had steamrolled through most of Europe, leaving the British Isles to stand alone against the German onslaught. The German Navy prowled the seas, destroying vital shipping, while German aircraft raided English cities.  The Nazis even attempted to assassinate the British Prime Minister, Winston Churchill, using a scheme straight out of a comic book.

A particular problem was the Nazi strategy of using planes to mine British harbors. The practice effectively brought shipping to a standstill, starving the island of crucial resources. During those hard years, the British looked for any means possible to defend their island from German attack. One odd solution concocted by Sir. Guy Maunsel was what have come to be known as the Maunsel Sea Forts, dubbed by soldiers who had to man them “Fort Madness.”


Artificial islands defend Britain against Nazi navy

The first sea forts were built at the mouth of the Thames estuary. The structures were about 108 feet tall and weighed in at a hefty 4500 tons. They consisted of a seven story concrete tower, floating on giant pontoons. Each tower could support 120 men and all their supplies. A platform on top of the structure supported two 3.7 inch anti-aircraft guns and two 40mm Bofors guns. Four of them were designated to defend the Thames estuary. Between February and June of 1942, they were towed out to their positions and sank into place by flooding sea water into the pontoons.

Even while work was being done on the Thames towers, Maunsel was called upon to do similar work for the Mersey estuary near Liverpool. Conditions differed from the mouth of the Thames, so Maunsel settled on designing the toward there on four concrete legs, which would support a two-story steel structure. Each of these would weight about 750 tons. Seven of them, linked by walkways, formed a fort at the mouth of the Mersey. The center tower was the control station, complete with radar. Four towers surrounding it were armed with 3.7 inch guns while another tower with two 40mm Bofors was set slightly away from arrangement. Another tower was mounted with searchlights. Each tower complex housed 265 men and their supplies. By 1943, the Army ordered three structures similar to the complexes at Mersey be built in the Thames estuary.

603px-The_British_Army_in_the_United_Kingdom_1939-45_H34542Life on the floating fortresses was tough, especially for men in the lower ranks. Long days in close quarters and cold nights spent on the sea began to wear on the men’s nerves, to the point that they dubbed the structures “Fort Madness.” Army psychologists advised the soldiers to take up hobbies such as painting, knitting, or making models to pass the time. Crews were rotated on a six week rotation, spending ten days on land before returning to sea. Many required psychiatric treatment after deployment.

Despite the hardships, the tower crews proved effective in fending off German aircraft. The Thames fort alone shot down 22 aircraft and 20 V-1 flying bombs. They also destroyed  an enemy speedboat.


Life after war

After the war, the forts in the Mersey estuary were destroyed. They were located on a shifting sand bar, and they would often sink to the ocean floor where they posed a danger to commercial shipping. A delegation from the War Office visited the structures in July 1948 to determine whether the forts would be useful in the post war era. With the threat of possible war with the Soviet Union, Britain still had to maintain her defensive footing. The group determined that another 11 sea forts could secure shipping all around the British seaboard. H

However, the project would have come with a steep price tag, 2.8 million pounds, a huge sum in the post-war era. And if the cost made the project unpopular, an accident on March 1, 1953 made it downright unfeasible. A Swedish freighter, the Baalbeck, crashed his vessel into one of the Maunsel forts after becoming disoriented in the fog, destroying two of the seven towers. By July, the powers that be decided to suspend the expansion project.

The forts still remain in the Thames estuary. In the 1960s they became homes for British pirate radio stations.  These days, preservation work is underway to save the four forts that remain from the ravages of weather, water, and neglect. They stand unoccupied, rusting in the salty air, mute relics of an era gone by.



“World War II: ‘Fort Madness:’ Britain’s Bizarre Sea Defense Against the Germans.” November 12, 2010. Spiegel Online International. April 26, 2014. <>

“The Maunsell Sea Forts.” Whitstable Scene. April 26, 2014. <>

4 thoughts on “The Maunsel Sea Forts aka “Fort Madness.”

  1. Pingback: The Shortest War in History–The Anglo-Zanzibar War | Oddly Historical

  2. Laurence Daley

    It would be appreciated if someone could check this account from a chapter in book in progress “Love and War in Cuba” (formal citations for book are omitted here).

    Laurence Daley
    Professor Emeritus
    Corvallis, Oregon USA

    “… It is known that the German bombers flew into Britain, not from their home-land to the east, but apparently coming from France, skirting the south of England and flying northward; or coming down from Nazi occupied Norway, into the Irish Sea west of Wales and Liverpool. The German bombers flying over the Irish Sea, found it easy to locate the general area of Liverpool, by “the lights of Dublin [in neutral Ireland] burning across the Irish Sea. ”

    Over the sea and as they approached inland, they fell into the domain of Valley Aerodrome based fighter aircraft and associated anti-aircraft gun sites. There my father, was training Free French and part of contingent of Royal Engineers, McAlpine staff, and the endless laboring Irish navies were building that place which would end the raids on Liverpool. They would also find quite by accident ancient Celt weapons and other artifacts.

    Then after passing over or by Northern Wales, these German bombers would turn east. Then, apparently to avoid anti-aircraft fire from the forts at the mouth of the Mersey River, they commonly went up the wide sand clogged and scarcely populated Dee estuary. The Dee, at Hoylake resort, is where after the war my brother Lionel and I were rescued once again by our parents from the incoming tide. There under cover of night the German bombers approached Liverpool, Merseyside, Garston docks flying over Barton Marsh. ¬¬¬¬,,,?

    1. Andrew Kincaid Post author

      Sorry it took so long to get back to you. From what I understand, the raids of the Blitz were primarily based in northern France and the Low Countries. This was owing in part to the relatively low range of German fighters and bombers. The bombers especially were lighter than their Allied counterparts, and didn’t carry the fuel to cross a huge distance. The bombing seems to have been concentrated in southeast England, directly across the Channel from air bases in northern France, rather than skirting around and going through Wales to approach from the west/northwest. I believe that they often passed directly over the Maunsel forts. I don’t doubt that bombers flew in from Nazi controlled Normandy, but I was not able to find any articles that talked about that. I add the caveat that I’m no expert on WWII history (or any part of history in general.)

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