After the successful invasion of France in 1940, the Commander of the German U-boat arm Karl Dönitz, saw a great opportunity in using the French ports along the Atlantic coast as forward bases for his U-boats. These bases would greatly expand their striking range in the Atlantic, the Mediterranean Sea, the West Coast of Africa, the Gulf of Mexico, and even the United States’ eastern seaboard. They would also increase the threat to Great Britain from the South.
From as early as June 1940 the Kriegsmarine send its engineers to start working on various suitable locations, among which was the former French Naval base of Lorient which became operational for German U-Boats at the end of June. Other U-boat bases were built in Bordeaux, Brest, La Rochelle/La Pallice and St. Nazaire.
Development
The first docks for repairing and resupplying U-Boats were located on the fisherman’s wharf of the Keroman district of the port of Lorient, because this already had a facility to lift fishing trawlers from the water for repairs which could also be used for the Type VII U-boats. These installations were mainly out in the open and provided no protection against Allied air-attacks or bombing-raids. So from November 1940 the construction of a number of bomb proof shelters and enclosed pens were started. At the fishing wharf two large “Dom” bunkers were created to protect U-boats during repairs or refit. At the same time the construction of a U-boat pen started further up the estuary on the banks of the Scorff river, which had two wet-docks that were also capable of accommodating the larger Type IX U-boats. Unfortunately during construction it was found that the ground was too soft to hold the weight of the large concrete structure and the site required constant dredging to keep it accessible.
For this reason the construction of a series of large bomb-proof submarine pens, K1, K2 and K3 started from Februari 1941 on the rocky Keroman peninsula.
Keroman 3
Keroman 3 was built between October 1941 and January 1943 and was designed as a wet bunker with seven pens to accommodate a total of 13 U-boats. The overall length is about 168 m, the width about 142 m, and the height about 18 m. The built-up area is approximately 23,000 m², equivalent to 3.2 football fields.
The bunker housed a total of seven pens, designated as K13 through K24. Pens K23 and K24 were available as dry docks with a length of approximately 95 m. Both these docks could each accommodate one U-boat. Pens K13/14, K15/16, K17/18, and K21/22 could each accommodate two U-boats, while pens K19/19a/20 could hold three U-boats. The usable length of pens K13/14 and K15/16 was about 84 m, while the usable length of the other pens was approximately 98.50 m.
At the back of the pens were the workshops. These workshops had multiple levels of which the first level often gave access to the mobile cranes that could move along the length of the pen and hoist heavy loads during repairs, refitting or resupplying.
Bunker Exterior
Walk around
The entrance to the pens was protected from torpedoes by a network of cables fixed to the hulls of the Strasbourg and the Crapaud; former warships voluntarily sunk in front of the pens and whose wrecks are still visible today. Especially at low-tide.
The roof
The bomb-proof roof, safeguarding five wet docks and two dry docks for maintenance and repairs, was composed of multiple layers of concrete of which the bottom layer was around 3.8 meters thick and a top layer that varied between 1 meter to 2.5 meters thick, depending on the positioning of the so-called “Hoyer-Träger” which handled the roof’s weight distribution. On top of this, the part above the pens had an extra layer of safety consisting of around 2 meters high open galleries at the top level covered by 1.5 meters thick concrete slabs called “Fang-Rost-Rippen” to disperse the bomb-blast and minimise the damage to the bunker’s base roof structure.
Above the Pen bay entrances Keroman 3 also had three concrete towers for Anti-Aircraft (FlaK) batteries.
Bunker Interior
After World War Two, the K3 bunker was used by the French Navy until 1997. During this time many changes were made to the bunker interior and facilities to accommodate more modern submarines. Nowadays most of the bays are used by commercial companies for various causes. Still, little alterations have been made to the base structure of the bunker, so visiting the interior provides a good experience of what it must have been like during the war.
U-boat Pens
Workshops
After World War Two
Despite relentless Allied efforts to destroy the base, K3 remained operational until the German surrender in 1945. After the war, the bunker was used by the French Navy until the 1990s. Today, K3 stands as a monument to the dark history of the U-boat war, attracting visitors and serving as a symbol of Lorient’s strategic importance during the conflict.
Visit
The U-boat base with the Keroman 3 bunker in Lorient is open to the public. You can freely walk around the impressive K1, K2 and K3 bunkers. To visit the Keroman 3 bunker interior you will have to book a guided tour at the tourist office which is situated along the waterfront near the massive K2 bunker.
