Posford Duvivier

The building of off-shore forts on the Thames Estuary.

The film begins with a map of the Thames Estuary whilst the commentary emphasises its vulnerability to aircraft attack. The idea of Guy Maundsell and John Posford, of the engineering company late known as Posford Duvivier was to construct offshore forts that would rest on the sea floor. There are stills of these. Designed by Guy Maundsell, the idea originated c. 1941. The film shows stills of the plans. Also involved in the design were George Smith and David Weller. The idea was for four forts to be built, each staffed by 100 officers and men. They would be built in dry dock and towed in situ at night by tugs and grounded on the sea bed. Initially the Admiralty was sceptical, but gave the go-ahead in mid 1941.The film shows stills of the first fort, 'Rough's Fort,' under construction in dry dock at Gravesend. There was extensive use of pre-fabrication. The base measured 170'x90'x14'. The towers were 60' high and after completion the fort weighted in excess of 4000 tons. There is 'live' film of Rough's Fort being towed to its situation, 8 miles east of Harwich on February 11th, 1942. Film shows the grounding. This had to happen in a hurry because of aircraft attack and the fort was nearly lost. The Admiralty decided that in future the engineer in charge of construction should also be in charge of grounding and this, explains John Posford in the commentary, is what happened. The commentary recalls the next two forts built; Sunk Head was sited 15 miles south east of Harwich on June 1st, 1942, and Tongue Sand 7 miles north of Margate on June 27th, 1942. The film also shows plans and an aerial view of seven linked forts built in the Thames Estuary north of Herne Bay. The film shows the grounding of the fourth fort Knock John on August 1st, 1942. This also slips sideways. The Destroyer HMS Campbell Town, is seen in the picture. This carried naval dignitaries, including Earl Mountbatten, to witness the grounding. The last part of the film looks at the concrete breakwaters and docks that were constructed. There are stills of this and a static shot of the plans.

Featured Buildings

Knock John Fort; Rough's Fort; Sunk Head Fort; Tongue Sand Fort

Keywords

Civil engineering, Coastal defences; Military defence

Intertitles

Grounding the fort. After 4 minutes. After 6 minutes.After 8 minutes.Thames Estuary Defences No. 4 Unit. 1-8-42.

Other Places

Gravesend

Background Information

In October 1940 Guy Maunsell, a civil engineer, was commissioned by the Admiralty to design a series of heavily armed sea forts. These would stand in the north mouth of the Thames and harass German planes on their way to London, deter mine laying by E-boats, and extend radar coverage across the Continental coast. In February 1942, Roughs Fort, the first of these sea forts, was towed from a shipyard at Gravesend to its site off the coast of Harwich. During World War II these sea forts claimed 22 aircraft, 30 V1 flying bombs and helped to sink 1 E-Boat. No servicemen died from enemy action. However by the end of the war they were already becoming obsolete, only able to observe faint trails of the V2 rockets. So great were the mental pressures of serving in the sea forts, they are often accredited with having done more damage to home forces than to the enemy.