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Royal Marines Man Island Sea Forts

The Maunsell Forts are armed towers built in the Thames and Mersey estuaries during the Second World War to help defend the United Kingdom. They were operated as army and navy forts, and named after their designer, Guy Maunsell.

The forts were decommissioned in the late 1950s and later used for other activities including pirate radio broadcasting.

The Maunsell naval forts were built in the Thames estuary and operated by the Royal Navy, to deter and report German air raids following the Thames as a landmark, and prevent attempts to lay mines by aircraft in this important shipping channel. There were four naval forts:

  • Rough Sands (HM Fort Roughs) (U1) 51.895194°N 1.480569°E

  • Sunk Head (U2) 51.7773°N 1.50841°E

  • Tongue Sands (U3) 51.492915°N 1.36662°E

  • Knock John (U4) 51.562277°N 1.162059°E

This artificial naval installation is similar in some respects to early "fixed" offshore oil platforms. It consisted of a rectangular 168-by-88-foot (51 by 27 m) reinforced concrete pontoon base with a support superstructure of two 60-foot (18 m) tall, 24-foot (7.3 m) diameter hollow reinforced concrete towers, walls roughly 3.5 inches (9 cm) thick; overall weight is estimated to have been approximately 4,500 tons. The twin concrete supporting towers were divided into seven floors, four for crew quarters; the remainder provided dining, operational, and storage areas for several generators, and for fresh water tanks and antiaircraft munitions. There was a steel framework at one end supporting a landing jetty and crane which was used to hoist supplies aboard; the wooden landing stage itself became known as a "dolphin".

The stages involved in anchoring a naval fort

The towers were joined above the eventual waterline by a steel platform deck upon which other structures could be added; this became a gun deck, on which an upper deck and a central tower unit were constructed.

QF 3.7 inch anti-aircraft guns were positioned at each end of this main deck, with a further two Bofors 40 mm anti-aircraft guns and the central tower radar installations atop a central living area that contained a galley, medical, and officers quarters.

The completed fort with its steel superstructure, guns and personnel is towed to sea. The four feet high wooden bulwarks around the pontoon were removed before sinking the fort. The reinforced concrete cylinder on the pontoon bow was later rolled off and suspended by wires under the bow, when the fort was grounded this cylinder was crushed and thus broke the impact on the sea bed. © IWM A 26877

The design of these concrete structures is equal to a military grade bunker, due to the ends of the stilts, (under water) that are solidly locked into the ground. Many species of fish live near the forts because the forts create cover. They have provided landmark references for shipping. 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 familiarization—before being towed out and sunk onto their sand bank positions in 1942.

The Royal Navy during the Second World War Sea forts in the Thames Estuary: The fort in position and ready for action. Air is still seen escaping from the sea cocks as further consolidating water ballast enters the pontoon base. One of the forts was actually in action half an hour after being put into position. These secret forts of the Thames Estuary were an effective defence against attacks by sea and air on the estuary. Visible on the very top of the fort is a Searchlight Control radar, a medium-accuracy system used to help aim a searchlight which would then be used to illuminate the target for conventional optical aiming. The large guns on either side of the fort are 3.7-inch AA. Just left of center, at the "back" of the fort from this position, is a Bofors 40mm gun. The barrel of a second Bofors can be made out just right of center. © IWM A 26878

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. [1]

Royal Marines Crews

For a time the Naval Forts armaments' were manned by Royal Marine's who made up the majority of the crew under RNVR officers and naval ratings in various support roles.

A close up of a manned Naval fort, the crew mostly Royal Marines are at the alert around a 40 mm Bofors gun.

Marines man "island forts" to protect Britain’s east coast shipping.

2 September 1943, in the North Sea. Island forts protect east coast shipping from low flying enemy aircraft.

These forts are commissioned as HM ships and are called after the sands on which they stand. Each consists of two concrete towers, 50 feet high from the base, connected by a steel superstructure on which the anti-aircraft guns and equipment are mounted.

The Commanding Officer of SUNK HEAD Fort addressing some of the ship's company including Royal Marines before they go on leave. © IWM A 19007

All the Armament is manned by Royal Marines under RNVR officers, with naval ratings for technical duties.

Mail and supplies are regularly delivered to the forts. [2]

Here four enemy aircraft destroyed by SUNK HEAD Fort's guns are recorded by swastikas on the side of the "Bridge". The fourth one is being painted on by a member of the fort's crew. Image also at © IWM A 19008

It was reckoned that, during World War Two, the Thames estuary forts shot down 22 German aircraft and around 30 V-1 flying bombs, the Thames estuary Navy forts also destroyed one German E-Boat.


[1] Wikipedia - Maunsell Forts

[2] Imperial War Museum Search - Marines man Sea Forts

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