A submarine pen (U-Boot-Bunker in German) is a type of submarine base that acts as a bunker to protect submarines from air attack. The term is generally applied to submarine bases constructed during World War II, particularly in Germany and its occupied countries, which were also known as U-boat pens after the phrase U-boat to refer to German submarines.
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Among the first forms of protection for submarines were some open-sided shelters with partial wooden foundations that were constructed during World War I. These structures were built at the time when bombs were light enough to be dropped by hand from the cockpit. By the 1940s, the quality of aerial weapons and the means to deliver them had improved markedly.
The mid-1930s saw the Naval Construction Office in Berlin give the problem serious thought. Various factions in the navy were convinced protection for the expanding U-boat arm was required. A Royal Air Force (RAF) raid on the capital in 1940 plus the occupation of France and Great Britain’s refusal to surrender was enough to trigger a massive building program of submarine pens and air-raid shelters.
By the autumn of 1940, construction of the Elbe II bunker in Hamburg and Nordsee III on the island of Heligoland was underway. Others swiftly followed.
It was soon realized that such a massive project was beyond the Kriegsmarine, and the Todt Organisation (OT) was brought in to oversee the administration of labor. The local supply of such items as sand, aggregate, cement, and timber was often a cause for concern. The steel required was mostly imported from Germany. The attitudes of the people in France and Norway were significantly different. In France, there was generally no problem with the recruitment of men and the procurement of machinery and raw materials. It was a different story in Norway. There, the local population were far more reluctant to help the Germans. Indeed, most labour had to be brought in. The ground selected for bunker construction was no help either: usually being at the head of a fjord, the foundations and footings had to be hewn out of granite. Several meters of silt also had to be overcome. Many of the workers needed were forced labor, most especially the concentration camp inmates supplied by the Schutzstaffel from camps near the pens.
The incessant air raids caused serious disruption to the project, hampering the supply of material, destroying machinery, and harassing the workers. Machinery such as excavators, piledrivers, cranes, floodlighting, and concrete pumps which were still a relatively new technology in the 1940s was temperamental, and in the case of steam-driven equipment, very noisy.
Bunkers had to be able to accommodate more than just U-boats. Space had to be found for offices, medical facilities, communications, lavatories, generators, ventilators, anti-aircraft guns, accommodation for key personnel such as crewmen, workshops, water purification plants, electrical equipment, and radio testing facilities. Storage space for spares, explosives, ammunition, and oil was also required.
Types of Bunkers
Four types of bunker were constructed:
- Covered Lock – These were bunkers built over an existing lock to give a U-boat some protection while it was at its most vulnerable – i.e. when the lock was emptying or filling. They were usually constructed with new locks alongside an existing structure.
- Construction Bunker – Used for building new boats,
- Fitting-out Bunker – After launch, many U-boats were fitted-out under their protection.
- Shelter for Operational Boats and Repair Bunker – This was the most numerous type. There were two types that were built either on dry land or over the water. The former meant that U-boats had to be moved on-ramps; the latter enabled the boats to come and go at will. Pumping the water out enabled dry dock repairs to be carried out. Some bunkers were large enough to allow the removal of periscopes and aerials.
Pens were constructed in the northern coastal ports of the Reich and in many occupied countries. Pens protecting the construction of the Type XXI submarine were located in Hamburg with Blohm & Voss doing the work, Bremen – AG Weser, and Danzig -F. Schichau. The German occupying forces built many U-boat pens in the Atlantic ports of France in Bordeaux, Brest, La Rochelle/La Pallice, Lorient, and St. Nazaire. Almost 4.4 million cubic meters of concrete was used.
Norway is to some extent ruled by its weather. Building submarine pens was often hampered by snow and ice. The ground might have been chosen, but the occupation of France only a few months after Norway’s surrender rather overshadowed the Scandinavian country as far as bunkers for U-boats was concerned. Nonetheless, a requirement for protection was identified. With the liberation of France in 1944, Norway regained its importance, but for barely a year.
The Norwegian bunkers in Bergen and Trondheim were originally designed to have two floors, the lower one for U-boats, the upper one for accommodation, workshops, and offices. However, with the project running six months late, plans for the second story were abandoned.
|COUNTRY||LOCATION||NAME OF BUNKER||DESCRIPTION|
|Germany||Bremen||Hornisse||The bunker was not started until 1944 in Bremen. It was never completed.|
|Bremen||Valentin||Valentin was the largest bunker in Germany. Begun in 1943, it was built to be a manufacturing facility, where Type XXI submarines were to be constructed. It was never completed. Post-war, it was briefly used as a test site for British and American bombs in which most of the damage done to the bunker was inflicted at this time before becoming a storage facility for the German Navy.|
|Danzig||NOT NAMED||Since it was out of range of Allied aircraft, no pens were built in Danzig (now Gdańsk in Poland).|
|Hamburg||Fink II||The Finkenwerder bunker was constructed over four years. After capture, it was demolished with 32 tonnes of bombs.|
|Helgoland||Nordsee III||The Nordsee III bunker in Helgoland was one of the oldest submarine pens, started in 1940. It was left alone until near the end of the war when it was attacked by the RAF and like most of the facilities on the island, it was completely destroyed. It was also used after the end of the war for testing new weapons. No trace of the pen has survived.|
|Kiel||Kilian||This town was constantly bombed in World War II, the targets often being the Kilian and Konrad bunkers. Kilian was started in 1941.|
It was in Kilian that U-4708 was probably the only submarine to be lost in a bunker. Misguided bombs from an air raid on the town caused what might today be called a tsunami to cross the Förde and enter the bunker. Oberleutenant zur See Hans-Gerold Hauber, the captain of U-170, had courted ridicule by ordering all hatches on his boat to be closed, despite being in the bunker. This simple precaution saved U-170 from sinking while lying next to U-4708.
|Kiel||Konrad||This town was constantly bombed in World War II, the targets often being the Kilian and Konrad bunkers. Konrad was started in 1942. The bunker was used for the construction of Seehund midget submarines.|
|Wilhelmshaven||NOT NAMED||A U-boat bunker in Wilhelmshaven was planned, but it never got beyond the preliminary stage.|
|France||Bordeaux||NOT NAMED||An unnamed bunker and bunkered lock were constructed in Bordeaux, the fourth-largest French city at the start of the war. Both structures were started in 1941. The bunkered lock was not finished by the war's end. The main building was larger than those in other locations, this was to allow supply boats and minelayers to use it. The Royal Italian Navy established the Betasom base at Bordeaux. The port was also the target of a British commando raid being the so-called Cockleshell Heroes.|
|Brest||NOT NAMED||The Brittany port only had one bunker, but it was the largest; it was also unnamed. Started in 1941, the plans were modified many times before completion a year later.|
By February 1942 the RAF had lost interest in the area; most of the town had already been destroyed and they did not possess large enough bombs to seriously threaten the bunker. Between February 1942 and early 1943, apart from a few American aircraft, the place was left alone. The German garrison surrendered to US forces in September 1944. They had had sufficient explosives to cripple the bunker but did not use them due to the proximity of a hospital.
|La Rochelle/La Pallice||NOT NAMED||Only 6 kilometers (3.7 mi) separate La Rochelle and La Pallice so they are usually considered as one port. An unnamed bunker was built at La Pallice. It was started in April 1941. Similar building techniques to those used in St. Nazaire were employed. Due to the relative ease of construction, the main structure was ready for its first U-boats six months later. A bunkered lock was begun in June 1942. It was completed in March 1944. Scenes for the 1981 films Das Boot and Raiders of the Lost Ark were shot in La Pallice.|
|Lorient||Keroman Submarine Base||The largest U-boat base was the Keroman Submarine Base in Lorient. Three bunkers, Keroman I, II and III, the Scorff bunker, and two Dom bunkers, east, and west were all begun in 1941. Two more were in the planning stage.|
Keroman I was unique in that it required its U-boats to be hauled out of the water, placed on a many-wheeled buggy and then transported into the bunker on a sliding bridge system. This arrangement might have been more vulnerable to air raids, but the damage was minimal and it had the advantage of the U-boat not needing a dry dock. Keroman II, being landlocked, was served by the same system. Keroman III was more conventional, as was the Scorff bunker. The two Dom bunkers, so-called because of their resemblance to the religious building, Dom means cathedral in German, were located around a massive turntable which fed U-boats into the covered repair bays. Karl Dönitz, head of the U-boat arm and later the chief of the Kriegsmarine, had his headquarters at nearby Kernevel.
|St-Nazaire||Saint-Nazaire Submarine Base||The construction of the Saint-Nazaire submarine base was commenced in 1941, including a bunkered lock. The pens were not affected by the British commando raid in March 1942, whose main objective were the Normandie dock gates.|
|Norway||Bergen||Bruno||Control of the Bergen project came under the German Naval Dockyard. Construction of Bruno commenced in 1941, with a Munich-based firm taking the lead. A shortage of labor was, along with the acquisition of raw materials in sufficient quantities, and poor weather was always going to cause problems. Specialized machinery had to be imported, as did accommodation that could stand up to the harsh Norwegian winter. In a bid to increase its protection, the bunker had granite blocks, each about a cubic meter in size, positioned on its roof. The shortage of cement ensured that the blocks could not be properly stuck down.|
|Trondheim||Dora I||Dora I was started in 1941, shortly after Operation Barbarossa, the invasion of the Soviet Union. It was constructed by Soviet prisoners of war. Despite any number of precautions being taken when putting in the foundations, Dora I developed a noticeable sag of 15 cm (5.9 in). It did not seem to bother the submariners as much as the builders.|
|Dora II||Work on Dora II started in 1942, but it was not completed by the end of the war.|
There is no truth in the rumor of an underground bunker on Fuerteventura in the Canary Islands. This story was gleaned from a similar situation in Le Havre in France when captured U-boat men were interrogated by the British.
Keroman U-Boat Base
Keroman Submarine Base was a German U-boat base located in Lorient during World War II. Großadmiral Karl Dönitz decided to build the base on 28 June 1940. Between February 1941 and January 1942 three gigantic reinforced concrete structures were built on the Keroman peninsula. They are called K1, K2, and K3. In 1944 work began on a fourth structure. The base was capable of sheltering thirty submarines undercover. Although Lorient was heavily damaged by Allied bombing raids, this naval base survived through to the end of the war. Lorient was held until May 1945 by the Germans’ Wehrmacht Heer regular army forces, though surrounded by the American Army; the Germans refused to surrender.
Since they could not destroy the base and its submarine pens, the Allies had decided to flatten the city and port of Lorient to cut the supply lines to the U-boat bases. Without resupply of fuel, weapons (e.g. torpedoes), and provisions, it became impossible for those U-boats to return to war patrols in the Atlantic Ocean. Between 14 January 1943 and 17 February 1943, Allied aircraft dropped as many as 500 high-explosive bombs and more than 60,000 incendiary bombs on Lorient; nearly 90% of the city was flattened.
After the war, the base was used by the French Navy until 1997. Rechristened by the French as Base Ingénieur Général Stosskopf in July 1946, the new name commemorated Jacques Stosskopf, a German-speaking Alsatian Frenchman who had been the deputy director of naval construction for the Germans at the base while secretly in the French Resistance, and had given valuable information on submarine movements to the Allies during the war until his activities were discovered and he was killed.
At present, the Keroman U-boat base is open to the public. During tours, the submarine pens of block K3 can be seen. Its roof (3.40 to 7.0 meters (11.15 to 22.97 ft) of steel-reinforced concrete) can be visited, as well as a former anti-aircraft tower on top of the U-boat base. The tower affords an excellent view of the harbor and of the former headquarters of Großadmiral Karl Dönitz of the Kriegsmarine across the bay at Larmor-Plage. The FloreS645, a Daphné-class submarine, launched in 1961, can also be visited. Another part of the base has been reconverted for industrial naval activities, with the preparation of racing multihulls.