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Surrendered German U-boats moored outside the Dora I bunker in Trondheim, Norway, May 1945

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).

Background

[edit]

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.[1]

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, the occupation of France and Britain's refusal to surrender triggered a massive building programme 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 under way. Others swiftly followed.

General

[edit]

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 labour. 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. Local Norwegian populations were far more reluctant to help the Germans. Indeed, most labour had to be brought in.[2] 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 metres of silt also had to be overcome.[3] Many of the workers needed were forced labour, 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, pile drivers, 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.[4]

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 bunker

[edit]

Four types of bunker were constructed:[citation needed]

  • 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 bunkers
After launch, many U-boats were fitted-out under their protection
  • Shelter for operational boats and repair bunkers
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.

There is no truth in the rumour 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 crews were interrogated by the British.[5]

Locations

[edit]
Map all coordinates using OpenStreetMap Download coordinates as KML

Pens were constructed in the northern coastal ports of the Reich and in many occupied countries.

Germany

[edit]

Pens protecting construction of the Type XXI submarine were located in Hamburg (Blohm & Voss), Bremen (AG Weser), and Danzig (F. Schichau).[6][7][8]

Bremen

[edit]
Forced workers at the construction site of the Valentin submarine pens in Bremen, 1944

The "Hornisse" bunker[a] was not started until 1944 in Bremen; it was never completed.[9]

"Valentin"[b] 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, too, was never completed. Post-war, it was briefly used as a test site for British and American bombs (most of the damage done to the bunker was inflicted at this time[10]) before becoming a storage facility for the German Navy. The labour to construct it was supplied by local concentration camps such as Neuengamme in Hamburg.

Hamburg

[edit]

The city was the site of two structures, "Elbe II"[c] and "Fink II"[d]. The Finkenwerder bunker was constructed by 1,700 slave labourers over four years. After capture, it was demolished with 32 tonnes of bombs.[11]

Helgoland

[edit]

The "Nordsee III" bunker[e] in Helgoland was one of the oldest submarine pens, being started in 1940. It escaped Allied bombing until near the end of the war when it was attacked by the RAF and completely destroyed. It was also used after the end of the war for testing new weapons. No trace of the pen remains.[12]

Kiel

[edit]

This town was constantly bombed in World War II, the targets often being the "Kilian" and "Konrad" bunkers. They were started in 1941 and 1942 respectively. The latter was used for the construction of Seehund midget submarines.[13]

It was in "Kilian" that U-4708 likely became the only submarine to be lost in a bunker. Off target bombs dropped in an air raid on the town caused a wave that crossed the Förde and enter the bunker. The wash swamped U-4708 but U-170 alongside was saved because its captain, Oberleutenant zur See Hans-Gerold Hauber, had ordered all hatches on his boat to be closed, even though it was in the bunker. "This simple precaution saved from sinking while lying next to U-4708".[14]

Wilhelmshaven

[edit]

A U-boat bunker in Wilhelmshaven was planned, but never advanced beyond the preliminary stage.[15]

France

[edit]

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 metres of concrete were used.[16] These Atlantic bases expanded the u-boat striking range–allowing for voyages to the Mediterranean Sea, the west African coast, the Gulf of Mexico, and the United States' eastern seaboard.[17]

Bordeaux

[edit]
Bordeaux U-boat pens

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 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 – the so-called Cockleshell Heroes.[18] It has now been transformed into the largest digital art centre in the world.[19]

44°52′11″N 0°33′31″W / 44.86972°N 0.55861°W / 44.86972; -0.55861 (Bordeaux Submarine Pen)

Brest

[edit]
Brest U-boat pens

The Brittany port only had one bunker, but it was the largest; it was also unnamed.[20] 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.[21][clarification needed]

48°22′00″N 04°31′20″W / 48.36667°N 4.52222°W / 48.36667; -4.52222 (Brest Submarine Pen)

Brest is still a submarine base, now serving the French Navy, and the bunker is still in use today. [22][23]

La Rochelle/La Pallice

[edit]
La Rochelle U-Boat pens
Construction of the U-boat base at La Pallice, 1942
Main article: Allied siege of La Rochelle

Only 6 kilometres (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 (Base sous-marine de La Rochelle [fr]); it was started in April 1941.[24] 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[24] and Raiders of the Lost Ark were shot in La Pallice.[25]

46°09′31″N 01°12′34″W / 46.15861°N 1.20944°W / 46.15861; -1.20944 (La Pallice Submarine Pen)

Lorient

[edit]
Keroman I and Keroman III, Lorient

The largest U-boat base was the Lorient Submarine Base in Brittany. 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 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 I: 47°43′45″N 03°22′12″W / 47.72917°N 3.37000°W / 47.72917; -3.37000 (Lorient - Keroman I Submarine Pen)
Keroman II: 47°43′52″N 03°22′18″W / 47.73111°N 3.37167°W / 47.73111; -3.37167 (Lorient - Keroman II Submarine Pen)

"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.

Keroman III: 47°43′38″N 03°22′02″W / 47.72722°N 3.36722°W / 47.72722; -3.36722 (Lorient - Keroman III Submarine Pen)
Scorff: 47°45′02″N 03°20′53″W / 47.75056°N 3.34806°W / 47.75056; -3.34806 (Lorient - Scroff Submarine Pen)
Dom (East): 47°43′56″N 03°22′02″W / 47.73222°N 3.36722°W / 47.73222; -3.36722 (Lorient - Dom (East) Submarine Pen)
Dom (West): 47°43′55″N 03°22′07″W / 47.73194°N 3.36861°W / 47.73194; -3.36861 (Lorient - Dom (West) Submarine Pen)

Karl Dönitz, head of the U-boat arm and later the chief of the Kriegsmarine, had his headquarters at nearby Kernevel villa.

St-Nazaire

[edit]
Roof of the U-boat base in Saint Nazaire.

The construction of the Saint-Nazaire submarine base was commenced in 1941, including a bunkered lock.[26] (Elsewhere in the reference, it states that "the excavations" for the bunkered lock were begun in October 1942).[27]

47°16′33″N 02°12′09″W / 47.27583°N 2.20250°W / 47.27583; -2.20250 (St.-Nazaire Submarine Pen)

The pens were not affected by the British commando raid in March 1942, whose main objective were the Normandie dock gates.

Norway

[edit]
Main article: German U-boat bases in occupied Norway

Submarine pen construction was often hampered by snow and ice, and the prioritization of French submarine pen construction. With the liberation of France in 1944, Norway regained its importance, but for late in the war and past the prime of the U-boat's capabilities.

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.[28]

Bergen

[edit]

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 labour, along with the acquisition of raw materials in sufficient quantities and poor weather, caused persistent problems. Specialized machinery and materials that could withstand harsh Norwegian winters had to be imported.

Granite blocks were added to reinforce the bunker's strength. However, a cement shortage led to these one cubic meter-sized blocks being placed insecurely, minimizing their protective effect. [29]

Trondheim

[edit]

"Dora I" was started in 1941, shortly after Operation Barbarossa, the German 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), but it is believed to have little effect on submarine operations. Work on "Dora II" started in 1942, but was not completed by the end of the war.[30]

The Allied bombing offensive

[edit]
Main article: Combined Bomber Offensive
Destruction to a U-boat pen caused by the Grand Slam bomb, a larger version of the Tallboy (1944)

U-boat facilities first became a bombing priority in March 1941,[31] and again during the Combined Bomber Offensive. The bunkers did not suffer as much as their surroundings until August 1944 when a new type of bomb was used against them, the "Tallboy" bomb.[32]

U-boat yards and pens were the primary objectives for the US Eighth Air Force from late 1942 to early 1943.[33][34] In the course of the war, the Allies used various tactics and weaponry against German U-boat pens. For example, The United States Army Air Forces, as part of Operation Aphrodite, used US designed and operated radio-controlled aircraft, "Bat" guided bombs. Whereas the RAF Bomber Command, used the Royal Navy designed "Disney" rocket-assisted bombs, and the Barnes Wallis-devised Tallboy and Grand Slam deep penetration bombs.

Allied bombardments of naval bases in France by base (1940-44)[35]
Base Bombardment (tonnes) Bombardment (percentage)
Bordeaux 3,718 tonnes 13.77%
Brest 9,462 tonnes 35%
La Rochelle 1,926 tonnes 7.1%
Lorient 6,838 tonnes 25.3%
Saint-Nazaire 5,058 18.7%
Bombing of U-boat pens and yards during World War II
Target Date Details
Saint-Nazaire 15/16 February 1942 10 Armstrong Whitworth Whitleys and six Handley Page Halifaxes; only nine aircraft bombed St Nazaire, in cloudy conditions. No aircraft were lost but three crashed in England[36]
St-Nazaire 7/8 March 1942 17 aircraft bombed St Nazaire[37]
St-Nazaire 25/26 March 1942 Minor Operations: 27 aircraft to St Nazaire—one Vickers Wellington lost[37]
St-Nazaire 27/28 March 1942 35 Whitleys and 27 Wellingtons bombed German positions around St Nazaire in support of the naval and Commando raid to destroy the Normandie dock gates in the port. The submarine pens were incidental to the raid which was aimed at preventing use of the dry-dock by capital ships. The aircraft were ordered to bomb only if the target had clear visibility. Conditions were bad, however, with 10/10ths cloud and icing, only four aircraft bombed at St Nazaire. Six aircraft bombed elsewhere. One Whitley was lost at sea[37]
St-Nazaire 3 January 1943 The first use of Lieutenant Colonel Curtis LeMay's modification of formation bombing to staggered three-plane elements within a squadron and staggered squadrons within a group was the "sixth raid on Saint Nazaire". With LeMay in command of the 305th Bomb Wing, 76 of 101 dispatched aircraft found the target and used a straight and level bomb run. Seven aircraft were shot down and 47 were damaged. The majority of bombs hit the submarine pens
Lorient 15 January 1943 The 317th air raid on Lorient dropped 20,000 incendiary bombs[38]
St-Nazaire 16 January 1943 Two waves of B-17 Flying Fortresses inflicted major damage and killed 27 people[38]
Wilhelmshaven 27 January 1943 The US VIII Bomber Command dispatched ninety-one B-17s and B-24 Liberators to attack the U-boat construction yards at Wilhelmshaven, the very first 8th Air Force heavy bomber attack directed at Germany itself.[39] Three bombers (one B-17 and two B-24s) were shot down, only 53 aircraft actually dropped their bombs on the target due to bad weather conditions
Lorient 23 and 26 January
3, 4, 7, 13 and 16 February
6 March
16 April
17 May 1943 		Lorient was bombed and the city was evacuated[38]
Bremen 3/4 June 1943 170 aircraft attacked in the first large raid on Bremen since October 1941. 11 aircraft – four Wellingtons, two Halifaxes, two Avro Lancasters, two Short Stirlings and one Avro Manchester were lost. Bremen recorded this as a heavy attack, the results of which exceeded all previous raids. Housing areas were badly hit with six streets affected by serious fires. Damage to the U-boat construction yards and the Focke-Wulf factory was described as "of no importance" but there were hits in the harbour area which damaged a pier, some warehouses and the destroyer Z25.[clarification needed] 83 people were killed, 29 were seriously injured and 229 slightly injured (Bremen's third heaviest casualty toll in the war)[40]
Wilhelmshaven 11 June 1943 VIII Bomber Command, Mission Number 62: 252 B-17s were dispatched against the "U-boat yard at Wilhelmshaven" and the port area at Cuxhaven; 218 hit the targets; VIII Bomber Command claimed 85-20-24 Luftwaffe aircraft, with the loss of eight aircraft and 62 damaged. American casualties were 3 KIA, 20 wounded, and 80 MIA. The raid on Wilhelmshaven demonstrated the difficulty of operating beyond the range of escort fighters as enemy fighter attacks prevented accurate bombing of the target[41]
Bremen and Kiel 13 June 1943 VIII Bomber Command, Mission Number 63: 151 B-17s were dispatched against the Bremen U-boat yards; 122 hit the target, claiming 2-2-1 Luftwaffe aircraft, with four lost and 31 damaged; casualties were eight WIA and 32 MIA. A smaller force of 76 B-17s was dispatched to the Kiel U-boat yards; 60 hit the target and claimed 39-5-14 Luftwaffe aircraft; Bomber Command lost 22 aircraft, one was damaged beyond repair and 23 were damaged. The heaviest fighter attacks to date against the Eighth Air Force accounted for 26 B-17s, mostly of the force attacking Kiel[41]
St-Nazaire 28 June 1943 VIII Bomber Command, Mission Number 69: 191 B-17s were dispatched against the "locks and submarine pens at Saint-Nazaire"; 158 hit the target. Bomber Command claimed 28-6-8 Luftwaffe aircraft, for the loss of eight B-17s and 57 damaged[41]
Bergen and Trondheim 24 July 1943 95th Bomb Group, Mission Number 75: First USAAF bombing raid on Norway.[42] 84 B-17s are dispatched against the port area of Bergen, they find 10/10 cloud cover and return to base with their bombs. 45 B-17s are dispatched against the port area of Trondheim, which includes the Dora I submarine pens which have just been placed in service; 41 hit the target, they claim 4-2-3 Luftwaffe aircraft; one B-17 is damaged beyond repair and nine others damaged; casualties were three wounded.[43] Workshops in the area are destroyed, there was large material damage including on civilian targets; German reports indicate three months delay in construction plans which includes a second set of partially built submarine pens Dora II. Damage to existing submarine pens (Dora I) is light. 31 Germans and 8 Norwegian civilians are killed.[44] The U-622 was badly damaged[45] and sunk near Trondheim. It was one of the only U-boats sunk by high-level bombing during World War II.[46]
Deutsche Werke, Kiel December 1943 B-17 and B-24 bombing destroyed one workshop (100%), another workshop and storage building (80%), a factory workship and boat building (67%); a number of other buildings were damaged; a submarine under construction and workshops for engines and engineering were hit[33]
Deutsche Werke, Kiel 23/24 July 1944 In the first major raid on a German city for two months, 629 aircraft – including 10 de Havilland Mosquitos – were dispatched in this first RAF (since April 1943) and heaviest RAF raid of the war on the target. In less than half an hour, all parts of Kiel were hit but the bombing was particularly heavy in the port areas and all of the important "U-boat yards" and naval facilities were hit. The presence of around 500 delayed-action or unexploded bombs caused severe problems for the rescue and repair services. There was no water for three days; trains and buses did not run for eight days; and there was no gas for cooking for three weeks[47]
Brest 5 August 1944 15 Lancasters of No. 617 Squadron RAF, with two supporting Mosquitos, attacked the U-boat pens and scored six direct hits with Tallboy bombs penetrating the concrete roofs. One Lancaster was shot down by flak. Subsequent attempts to reinforce other sites with even thicker concrete diverted resources from other projects.[48]
Lorient 6 August 1944 617 Squadron attacked Lorient again, with two hits.[48][49][50]
Lorient 7 August 1944 The Tallboy bombing mission to Lorient was scrubbed[49]
La Pallice 8 August 1944 Iveson dropped one Tallboy bomb[49]
La Pallice and Bordeaux 11 August 1944 53 Lancasters and three Mosquitos of No 5 Group RAF attacked U-boat pens at "Bordeaux and La Pallice" with 2,000 lb armour-piercing bombs, but the bombs did not penetrate the roofs. No aircraft were lost[48]
Brest, La Pallice, and Bordeaux 12 August 1944 68 Lancasters of No 1 Group and two Mosquitos of No 5 Group attacked "pens at Brest, La Pallice, and Bordeaux" without loss. A U-boat was believed to have been hit at La Pallice[48]
Brest 13 August 1944 28 Lancasters and one Mosquito of No 5 Group attacked the "U-boat pens and shipping at Brest". Hits were claimed on the pens, on the hulk of an old French battleship, the Clemenceau and on a medium-sized tanker. The object of the attacks on ships was to prevent the Germans using any of the vessels in Brest to block the harbour just before its capture by American troops[48]
La Pallice and Bordeaux 16 August 1944 25 Lancasters and one Mosquito of No 5 Group to attack the U-boat pens at La Pallice found the target was cloud-covered and only three aircraft bombed. No aircraft were lost[48]
La Pallice 17 August 1944 Mission 559: A B-17 dropped "Bat" guided bombs on La Pallice.[41] One impacted 1 mile (1.6 km) short and the second about 1 mile to the right of the target[51][52]
IJmuiden 28 August 1944 Iveson dropped one Tallboy[49]
Heligoland 3 September 1944 The US Navy controller flew the Operation Aphrodite SAU-1 drone (B-24D 42-63954)[53][54] into Duene Island by mistake
Heligoland 11 September 1944 During the first Castor mission of Operation Aphrodite, the pilot of B-17 42-30180 (Guzzlers) was killed when his parachute failed to open on bailout[55][56]

41-24340 to 41-30847, 42-001 to 42-30031, 42-30032 to 42-39757, 42-39758 to 42-50026, 42-57213 to 42-70685

Bergen 4 October 1944 The first of the three attacks against the U-boat pen "Bruno" came in the morning. The attack was carried out by 140 British bombers and 12 Mosquitoes, and most of the 1,432 bombs dropped weighed 1,000 lb (450 kg), the rest 500 lb (230 kg). The construction of Bruno was then running behind schedule and was never more than 80 percent finished. Still, after D-day it became increasingly important, and at times 200 U-boats lay hidden in fjords around Bergen. Bruno received seven hits, but in spite of its unfinished roof the damage was insignificant, whereas the adjoining Danziger Werft was seriously ravaged with sunken U-boats and the destruction of valuable equipment. In the harbour some ships were sunk. At first visibility was excellent, but deteriorated rapidly due to artificial fog and the smoke from numerous fires.

As it turned out the attack was not the precision bombing as it was intended to be, and 193 civilians were killed, and a considerable number wounded. The worst tragedy was that Holen school was hit, situated about a hundred meters from Bruno. 61 children and 19 adults were killed while 240 pupils and 20 adults survived, but many of them have had serious psychological problems owing to the traumatic experience.

Heligoland 15 October 1944 Mission 678A:[57] Two B-17s[58] of Operation Aphrodite attacked the Heligoland U-boat pens[55]
Bergen 28/29 October 1944 237 Lancasters and seven Mosquitos of No 5 Group attacked the U-boat pens at Bergen. The area was cloud-covered, therefore the Master Bomber tried to bring the force down below 5,000 ft but cloud was still encountered and he ordered the raid to be abandoned after only 47 Lancasters had bombed. Three Lancasters were lost[59]
Heligoland 30 October 1944 Mission 693A:[41] One Castor Operation Aphrodite drone lost contact, went out of control and crashed near Trollhättan, Sweden. The other drone was B-17 42-3438[clarification needed][55]
Trondheim 22 November 1944 Lancaster bombing raid on Trondheim.[60] 171 bomber raid on Trondheim which includes the in service Dora I submarine pens and Dora II which is still under construction. Bombers turned back and did not drop their bombs because of the low cloud cover/fog and smoke laying by the Germans.[44]
IJmuiden[clarification needed] 15 December 1944 17 Lancasters attacked with Tallboy bombs but the target was obscured by a smokescreen[49][61]
IJmuiden 30 December 1944 13 Lancasters of No. 617 Squadron set out to bomb the "U-boat pens at IJmuiden" but the raid was abandoned because of bad weather[61]
IJmuiden 12 January 1945 No. 617 Squadron attacked the U-boat pens with Tallboys,[49] but smoke obscured the results[62]
Bergen[49] 12 January 1945 32 Lancasters and one Mosquito of No 9 and No. 617 Squadrons attacked "U-boat pens and shipping in Bergen harbour". Three Lancasters of No 617 Squadron and one from No. 9 Squadron were lost; the Germans told the local people that 11 bombers had been shot down. A local report says that three Tallboys penetrated the 3.5-metre-thick roof of the pens and caused severe damage to workshops, offices, and stores[62]
IJmuiden and Poortershaven 3 February 1945 36 Lancasters attacked "U-boat pens at IJmuiden" (No. 9 Squadron) and "Poortershaven" (No. 617 Squadron) with Tallboy bombs without loss. Hits were claimed on both targets[63]
IJmuiden 8 February 1945 15 Lancasters of 617 Squadron dropped Tallboys on the "U-boat pens at IJmuiden" without loss[63]
IJmuiden 10 February 1945 Mission 825: nine of 164 B-17s on a 92nd Bombardment Group mission against the U-boat pens at IJmuiden, the Netherlands, first used the Royal Navy Disney rocket-boosted concrete piercing bomb[41]
Oslo Fjord 23/24 February 1945 73 Lancasters and 10 Mosquitos carried out an accurate attack on a "possible U-boat base at Horten on the Oslo Fjord". One Lancaster was lost[63]
Bremen (Farge)[64] 27 March 1945 20 Lancasters of 617 Squadron attacked the Valentin submarine pens,[65] two Grand Slam bombs penetrated two metres and detonated[66] which rendered the shelter unusable. No aircraft were lost.[63]
Bremen 30 March 1945 303rd BG (H) Combat Mission No. 348: 38 aircraft were dispatched to bomb Bremen. The "submarine building yards" were the first priority target (PDF)
Hamburg/Finkenwerder 4 April 1945
Hamburg 9 April 1945 17 aircraft of 617 Squadron, with Grand Slam and Tallboy bombs, successfully attacked the "U-boat shelters". No aircraft were lost[67]
Kiel 9/10 April 1945 591 Lancasters and eight Mosquitos of Nos 1, 3, and 8 Groups attacked Kiel. Three Lancasters were lost. This was an accurate raid, made in good visibility on two aiming points in the harbour area. Photographic reconnaissance showed that the Deutsche Werke U-boat yard was severely damaged, the German pocket battleship Admiral Scheer was hit and capsized, the cruisers Admiral Hipper and the Emden were badly damaged. The local diary says that "all three shipyards" in the port were hit and that nearby residential areas were severely damaged[67]
Kiel 13/14 April 1945 377 Lancasters and 105 Halifaxes of Nos 3, 6, and 8 Groups attacked Kiel for two Lancasters lost. This raid was directed against the port area, with the "U-boat yards" as the main objective. RAF Bomber Command rated this as "a poor attack" with scattered bombing[67]
Heligoland 18 April 1945 969 aircraft – 617 Lancasters, 332 Halifaxes, and 20 Mosquitos of all groups – successfully attacked the "Naval base, airfield, & town" "almost [creating a] crater-pitted moonscape".[67] Three Halifaxes were lost,[67] the islands were evacuated the following night
Heligoland 19 April 1945 No. 9 and 617 Squadrons used Tallboys against "coastal battery positions"[67][clarification needed]

Post war

[edit]

Yugoslavia

[edit]

The Yugoslav People's Army used submarine pens as well, including ones on the islands of Vis and Brač or in Kotor Bay, carved inside natural hills. The ones in Montenegro fulfilled their purpose, housing and protecting the submarines and missile boats from NATO aerial attacks during Operation Allied Force in 1999. They are now abandoned and freely accessible from sea or by foot.

See also

[edit]

Notes

[edit]

Bibliography

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Submarine pen

View on Grokipedia
from Grokipedia
A submarine pen is a type of fortified designed as a to shield submarines from aerial attack, with the term most commonly referring to the massive reinforced concrete structures built by during to protect its fleet. These pens were constructed primarily along the Atlantic coast in occupied French ports such as Brest, Lorient, Saint-Nazaire, La Pallice, and Bordeaux, utilizing forced labor to erect enormous enclosures capable of housing multiple submarines for refueling, repairs, and resupply while minimizing vulnerability to Allied bombing raids. Featuring walls and roofs several meters thick—often equivalent to multiple stories of —these structures proved highly resistant to conventional high-explosive bombs, requiring specialized "" munitions like the British Tallboy and Grand Slam to achieve penetrations, though many pens endured thousands of tons of ordnance with only superficial damage. The pens enabled extended patrols by providing secure forward bases closer to the Atlantic convoy routes, contributing to the German submarine campaign's initial successes before superiority and anti-submarine tactics shifted the balance. Notable examples include the Keroman complex at , which sheltered over 20 s at its peak, and the incomplete bunker near , intended as the largest such facility but abandoned after Allied strikes rendered it unusable.

Definition and Purpose

Protective Function and Design Rationale

Submarine pens functioned as fortified shelters to protect docked from aerial , permitting essential operations such as refueling, repairs, and rearming under cover during periods of heightened activity. These structures addressed the acute of s in exposed harbors, where prior to their construction, faced frequent destruction or damage from RAF and USAAF raids targeting Atlantic bases after the 1940 occupation of . The design emphasized massive construction to exploit the material's high and ability to distribute explosive forces, with roofs typically 3.5 to 4.5 meters thick and walls up to 3.5 meters, as seen in facilities like those at . This thickness proved effective against standard high-explosive bombs, which generally failed to penetrate or cause structural collapse despite thousands of tons dropped; for instance, pens withstood impacts that cratered surrounding areas but left interiors intact. Rationale for such robust engineering derived from causal assessments of bombing efficacy: conventional ordnance's limited penetration necessitated defenses prioritizing overhead protection over mobility, enabling to maintain sortie rates into 1943 despite air interdiction efforts. Later adaptations, like increased layering in the Valentin pen reaching 7 meters, responded to evolving threats but highlighted initial designs' success in minimizing losses until specialized penetrator bombs, such as the 5-ton Tallboy introduced in 1944, finally inflicted breaches.

Strategic and Operational Benefits

Submarine pens provided critical protection for German against Allied aerial bombardment, enabling sustained operations in the Atlantic theater. Constructed with walls up to 11 feet thick and roofs reaching 16 feet, facilities like those at withstood conventional high-explosive bombs, rendering them largely impervious during early and mid-war campaigns. The conducted over 2,000 sorties against pens in bases such as and from October 1942 to October 1943, incurring a 5.9% loss rate while inflicting minimal structural damage, which preserved readiness in port. This resilience contrasted with the vulnerability of exposed submarines, allowing the to avoid significant harbor losses despite intensified RAF and USAAF efforts. Strategically, the pens' locations on the French Atlantic coast conferred a positional advantage by shortening transit distances to operational areas by approximately 450 miles compared to ports, facilitating direct access to the mid-Atlantic without navigating around the . This proximity supported the wolfpack tactics, contributing to peak successes such as the sinking of 779 Allied merchant vessels between January and October 1942, with October alone accounting for 585,000 gross tons. By mitigating the risks of aerial interdiction during refits, the pens prolonged the effectiveness of the submarine campaign, which aimed to sever Britain's supply lines and challenge Allied dominance in the . Operationally, the pens enhanced efficiency through bomb-proof bays and integrated wet and dry docks, permitting simultaneous maintenance, rearming, and repairs for multiple submarines under secure conditions. Features like Fangrost screens absorbed bomb impacts, while the structures' design as free-standing drydocks minimized downtime between patrols, supporting over 1,170 U-boat sorties from key Biscay ports. Rapid infrastructure repairs by German forces further maintained operational tempo, even amid sustained bombing, until the introduction of specialized munitions like the 11-ton Grand Slam bomb in 1945 rendered some pens vulnerable—by which point the war's outcome was assured.

Historical Development

Pre-World War II Concepts

The concept of fortified submarine shelters emerged during , driven by the increasing vulnerability of docked submarines to aerial bombing and naval raids, as demonstrated by attacks on German bases. Initial protections consisted of rudimentary open-sided structures featuring partial wooden foundations, which provided minimal shielding against low-yield bombs of the era. By 1917, German forces advanced toward more durable designs amid intensified operations from bases. Construction of a large shelter began in August 1917 at (), , connected via canal to the outpost for secure transit past British blockades. This facility represented a shift from hybrid wood-steel frames to fully enclosures, with walls designed to withstand and early aerial ordnance, accommodating multiple U-boats for repairs and resupply. The shelter, completed in phases through 1918, incorporated compartmentalized bays and canal integration, foreshadowing later enclosed pens by prioritizing blast resistance and operational continuity. Similar concrete-reinforced shelters were erected at to support submarine flotillas raiding Allied shipping, featuring arched roofs and protective quays that mitigated exposure during tidal operations. These WWI innovations, born from empirical lessons in coastal defense, established core principles of overhead cover and material resilience, though limited by contemporary concrete technology and bomb payloads. Post-1918, under the restrictions, German naval development stalled, but operational experience from and informed interwar planning, emphasizing the causal need for impenetrable harbors to sustain undersea campaigns against superior surface fleets.

World War II Construction Phase

Following the German occupation of western France in , construction of reinforced concrete submarine pens commenced rapidly to safeguard during refit and repair from intensifying Allied air attacks in the . These facilities, overseen by the , prioritized ports along the for their proximity to Atlantic convoy routes, enabling quicker turnaround for submarines. Initial efforts targeted existing naval infrastructure, with work starting within days of France's capitulation to establish bomb-resistant shelters featuring thick roofs up to 7 meters in places. In occupied , major pens at Brest began in January 1941 and were largely completed by July 1942, spanning 333 by 192 meters to house 20 U-boats under a 6.2-meter-thick roof. Lorient's Keroman facility followed in February 1941, with the first three pens operational by June 1941 despite setbacks from British bombings that killed around 200 workers; it utilized both paid French labor and forced laborers from across . Saint-Nazaire's bunker initiated in February 1941, while La Pallice started in April 1941, and in September 1941, the latter requiring 60,000 cubic meters of concrete. Extensions occurred in and as the war progressed. Trondheim's in started in 1941, finishing two years later with dimensions of 153 by 105 meters accommodating 16 . In , early pens like those in and began in autumn 1940, but the massive project near Bremen-Farge launched in 1943, employing up to 15,000 forced laborers in grueling 24-hour shifts until its incompletion in 1945 due to Allied raids and material shortages. These endeavors demanded vast resources, underscoring Germany's prioritization of U-boat protection amid mounting losses, though many pens proved resilient only to conventional ordnance.

Engineering and Construction

Materials and Building Techniques

Submarine pens were built predominantly using , a consisting of , aggregates, water, and embedded reinforcement bars to enhance tensile strength against cracking and impacts. The concrete mix emphasized high , typically achieved through low water- ratios and quality aggregates, though exact formulations varied by site and wartime constraints. rebar grids, often densely placed, formed the primary reinforcement, with additional elements like layers in some roofs for enhanced durability. Wall and roof thicknesses were engineered for bomb resistance, commonly ranging from 3 to 5 meters for walls and up to 8 meters for roofs, with specific examples including 8.75 meters at 's dock and 7 meters at the facility near . These dimensions required staggering volumes of material; for instance, utilized nearly 500,000 cubic meters of , while some bunkers incorporated 650,000 cubic yards overall. Construction techniques involved extensive site preparation, including deep excavation for foundations and submarine berths, followed by erection of wooden supported by to shape the massive pours. was transported via tilting lorries and poured in layers to manage curing and structural integrity, with the coordinating rapid assembly—such as completing Saint-Nazaire's main structure in 16 months from February 1941 to June 1942. Ferro-concrete arches and girders distributed loads, enabling large-span interiors for multiple submarine bays while minimizing vulnerability to penetration.

Structural Innovations and Variations

Submarine pens incorporated reinforced concrete construction with progressively thicker roofs to withstand aerial attacks, evolving from early thicknesses of 1.5 to 3.6 meters in 1941 bunkers like Lorient Dom to 7.5 meters by 1943 in Keroman III, designed to counter heavy ordnance such as the Tallboy bomb. Layered concrete slabs absorbed impact forces, while embedded steel rebar provided tensile reinforcement, building on interwar engineering advances. Internal innovations included compartmentalization into alveoles—individual bays for submarines—and integrated dry docks for , enabling operations without surface exposure; for instance, some designs featured seven such pens with dual wet and dry access. Ventilation shafts and crane rails supported sustained use, though these features varied by site to balance protection and functionality. Design variations adapted to strategic priorities and geography. French Atlantic bases emphasized capacity, with Brest's bunker spanning 333 by 192 meters under a 6.2-meter roof for up to 20 submarines across multiple alveoles, completed between January 1941 and July 1942. featured a 7-meter roof for similar scale. In contrast, Germany's near prioritized production with a 7.3-meter roof over vast assembly halls, though unfinished by 1945. Norwegian pens like Bruno, with 6-meter roofs for nine boats, suited constraints. Early wartime structures held 2-15 submarines, while later expansions reached 24 in incomplete designs like Keroman IVa.

Labor and Logistical Challenges

The construction of submarine pens imposed severe demands on German manpower and logistics during , necessitating the mobilization of tens of thousands of workers for individual large-scale facilities, such as the approximate 15,000 laborers required for a massive . The Organization Todt, responsible for overseeing these projects, addressed chronic labor shortages by relying extensively on forced labor, including prisoners of war, conscripted foreign civilians, and inmates from concentration camps provided by the . This approach was driven by wartime depletion of domestic skilled workers and competition from other construction priorities, such as the broader defenses. At the Valentin submarine pen near , construction from summer 1943 to spring 1945 employed up to 10,000 forced laborers, comprising civilian conscripts, POWs, and concentration camp prisoners, under conditions that resulted in nearly 2,000 deaths due to exhaustion, , and executions. Similar patterns prevailed at French sites like and Brest, where slave laborers endured brutal oversight to pour roofs up to 6 meters thick, often working in shifts amid incomplete infrastructure and risks from unwilling workers. Logistical strains intensified as campaigns targeted rail and port facilities, complicating the delivery of essential materials; U-boat pen projects alone consumed 80,000 to 130,000 cubic meters of monthly, diverting resources from frontline needs. Secrecy measures, including predominant nighttime operations, further hampered efficiency by limiting daylight productivity and increasing accident rates in poorly lit environments. faced additional hurdles from Allied blockades and bombing, which disrupted reinforcements and supplies, while seasonal like winter frosts delayed pouring and curing processes critical to structural integrity. These factors collectively extended timelines and escalated costs, underscoring the unsustainable nature of the pens' rapid expansion amid escalating U-boat losses in the Atlantic.

Major Sites and Operations

Facilities in Germany

The primary submarine pen facility constructed within was the Valentin bunker, located in Bremen-Farge along the River. Initiated by the in mid-1943, it aimed to shield from intensifying Allied air campaigns and facilitate the assembly of advanced Type XXI submarines. The structure measured approximately 426 meters in length, 97 meters in width, and 33 meters in height, featuring multiple berths protected by walls up to 4.5 meters thick. Construction relied heavily on forced labor, including prisoners from the Neuengamme concentration camp's satellite camp at Bremen-Farge, where over 10,000 workers toiled under brutal conditions, resulting in thousands of deaths from exhaustion, disease, and executions. Progress advanced rapidly despite resource shortages, but Allied bombing raids in 1944 and 1945 inflicted significant damage, including roof collapses from high-explosive and Tallboy bombs, rendering the facility inoperable before its completion deadline. No U-boats were ever berthed there, as the project shifted priorities amid Germany's deteriorating war position. Other proposed pens in , such as the Hornisse bunker in and Elbe XVII and Wenzel in , remained in planning or early stages without substantial completion due to material constraints and Allied advances. represented a desperate inland effort, contrasting with the more numerous coastal bases in occupied territories, but its failure underscored the limits of even massive concrete defenses against . Post-war, the ruin survived demolition attempts and now serves as a site highlighting wartime forced labor atrocities.

Bases in Occupied France

![Concrete U-boat pens at Brest][float-right]
Following the fall of in June 1940, the German rapidly developed submarine bases along the to support operations in the Atlantic, benefiting from shorter transit times to operational areas compared to German ports. These facilities, constructed primarily between 1941 and 1943 under the , included reinforced pens designed to withstand aerial bombardment, housing repair workshops, fuel depots, and accommodations for crews. The bases at Brest, , La Pallice, , and collectively supported multiple flotillas, with serving as the largest and primary headquarters for Admiral until 1942. The Brest base, transformed from the existing French naval after German capture in June 1940, saw construction of pens begin in 1941 on the Lanninon seafront. These structures provided for up to a dozen s, enabling rapid refits amid intense Allied bombing campaigns that devastated the surrounding city but left the pens largely intact. Brest hosted the 1st and 9th Flotillas early in the war. At , the Keroman complex comprised multiple s: Keroman I, a dry-dock facility completed by mid-1941 measuring 119.5 meters long, 85 meters wide, and 18.5 meters high with five pens; Keroman II; and Keroman III, a wet built from October 1941 to January 1943 with seven pens accommodating 13 U-boats. The site, initiated in February 1941, could berth around 30 submarines total and withstood over 500 RAF raids, serving as Dönitz's base until his relocation to . More than 200 U-boats passed through during the war. ![Keroman I et Keroman III accès alveoles][center]
The , started in April 1941, formed a 192 by 159 meter structure with a double roof averaging 3.5 meters thick, including three wet and seven dry docks for 13 submarines across 23,000 square meters. This base supported Italian submarines alongside German U-boats and featured explosion chambers to absorb bomb impacts. Saint-Nazaire's facility, with construction commencing in February 1941 and initial pens operational by June, spanned 288 by 139 meters with a roof up to 9.6 meters thick, sheltering and workshops against thousands of Allied sorties. It housed the 2nd and 7th Flotillas, with the first arriving in September 1940. In , an inland base adapted for smaller Type VII U-boats via the , pens were built from autumn 1941 to summer 1943, measuring 245 meters long, 162 meters deep, and 19 meters high with a 5.6-meter-thick roof over the pens. Completed in 22 months by 7,000 workers, it supported the 12th Flotilla from October 1942, primarily for refits rather than frontline deployments.

Installations in Norway and Other Areas

![Captured German U-boats outside their pen at Trondheim in Norway, 19 May 1945][float-right] In occupied Norway, the Germans constructed several submarine pens to support U-boat operations in the Arctic and North Atlantic, leveraging the country's strategic fjords for protection against Allied attacks. The primary facility was Dora I in Trondheim, where construction began in 1941 and was completed by 1943, featuring reinforced concrete roofs up to 3.3 meters thick capable of sheltering multiple Type VII and Type IX U-boats. This bunker accommodated the 8th and 13th U-boat Flotillas, facilitating patrols against convoys to Murmansk and serving as a repair and resupply hub until the war's end. An adjacent Dora II bunker remained unfinished due to resource shortages and Allied bombing pressures. Another significant installation was the Bruno bunker in , initiated in 1941 and operational by 1943, with three dry pens for maintenance, three wet pens for quick turnaround, and one storage section. It housed the 11th , transferred from , and supported over 100 patrols departing from July 1940 to May 1945, including the only Type XXI to conduct a war patrol. On October 4, 1944, RAF bombers targeted Bruno, but the thick structure withstood the assault, though the raid caused civilian casualties in nearby areas. Beyond , German submarine pens in other occupied territories were limited, as the prioritized Atlantic-facing bases. Minor facilities existed in and the for training and support, but these lacked the fortified scale of Norwegian or French pens due to lower aerial threat levels. No equivalent large-scale pens were built in the Mediterranean or Adriatic under direct German control during , with operations there relying on Italian ports or improvised shelters rather than dedicated bunkers.

Allied Countermeasures and Bombing Campaigns

Early Air Attacks and Limitations

The Royal Air Force initiated bombing campaigns against German submarine pens in occupied as early as October 1941, targeting facilities at with small-scale raids involving 1 to 6 aircraft per mission, escalating to 12 attacks that month but inflicting only superficial damage due to the pens' roofs. These early efforts by focused on key Atlantic bases like Brest, , and St. Nazaire, aiming to disrupt operations by striking entrances, surrounding infrastructure, and repair yards, yet photographic reconnaissance consistently showed the pens themselves remaining structurally intact, allowing submarines to shelter securely during port calls. By early 1942, the RAF had largely shifted emphasis away from direct pen attacks after recognizing their futility against the bunkers' design, which prioritized overhead over to conventional high-explosive ordnance. The joined the effort in October 1942, conducting its first major raid on with 90 B-17 and B-24 bombers, followed by at least 27 strikes on submarine pens through October 1943, dropping thousands of tons of bombs but achieving negligible penetration of the primary structures. Official U.S. Army Air Forces assessments concluded that these operations failed to destroy U-boats in their pens or render the facilities inoperable, with most bombs either glancing off the sloped, multi-layered roofs or exploding on impact without breaching the interior spaces. For instance, raids on Brest and in 1942–1943 devastated adjacent civilian areas and dockyards—Lorient alone received over 6,800 tons of bombs by mid-1943—but the pens sustained only minor superficial scarring, enabling continued U-boat maintenance and deployment. Fundamental limitations stemmed from the pens' engineering: roofs typically 3.5 to 7 meters thick, constructed with layered and steel, were calculated by German engineers to withstand direct hits from bombs up to 7,000 pounds—exceeding the payload of most Allied bombers at the time, which relied on 500–4,000-pound general-purpose munitions that detonated prematurely on the hardened surfaces. These lacked the deep-penetration or earthquake-effect kinetics needed to compromise the bunkers' load-bearing arches or collapse internal bays, resulting in zero confirmed losses from bombing inside the pens during this phase. Additionally, intense flak defenses and intercepts raised sortie costs, with the losing seven bombers and damaging 47 in one mission alone, prompting debates over as area bombing of vicinities proved more feasible but strategically indirect.

Advanced Tactics and Special Ordnance

![Damage to a German U-boat pen from a Grand Slam bomb attack][float-right] As conventional high-explosive bombs proved inadequate against the reinforced concrete roofs of submarine pens, which often exceeded 3.5 meters in thickness, the Royal Air Force developed specialized "earthquake" bombs designed to penetrate deeply before detonating, creating underground shockwaves that undermined structural integrity rather than relying on direct surface penetration. The Tallboy, weighing 12,000 pounds (5.4 tonnes), was the first such ordnance, introduced in June 1944 and capable of achieving high terminal velocities from altitude drops by modified Avro Lancaster bombers, allowing it to burrow into the earth or concrete. This was followed by the larger Grand Slam, a 22,000-pound (10-tonne) bomb operational from March 1945, which amplified the destructive effect through its greater mass and aerodynamic casing filled with Torpex explosive. These weapons were deployed by elite units like No. 617 Squadron, employing precision high-altitude bombing tactics to maximize kinetic energy upon impact. Specific applications against submarine pens included Tallboy strikes on Norwegian facilities on 12 January 1945, where three bombs penetrated a 3.5-meter-thick roof, inflicting serious structural damage. In March and April 1945, RAF Lancasters targeted the Valentin bunker near and pens in with Grand Slam bombs, achieving substantial damage including partial roof penetrations up to 4.5 meters deep in the Valentin structure. These attacks often damaged or destroyed U-boats berthed inside, as seen in instances where explosions compromised internal compartments and support infrastructure. While complete destruction of the pens remained elusive due to their massive scale and compartmentalized design, the special ordnance disrupted repair operations, flooded sections via breaches, and forced German forces to disperse U-boat maintenance, contributing to the overall curtailment of submarine campaigns in the war's final months. Effectiveness was enhanced by the bombs' ability to create camouflets—subsurface voids that led to collapses—outperforming thousands of tons of earlier conventional raids that merely scarred exteriors.

Assessment of Effectiveness

Allied bombing efforts against German submarine pens demonstrated limited structural efficacy, as the bunkers' roofs—typically 3 to 7 meters thick—resisted penetration by conventional ordnance throughout much of the war. The concluded that early attacks accomplished little, with bombs failing to breach the 3.7-meter-thick roofs, leaving the pens largely intact and s protected from direct hits. In occupied , five major fortified pens endured sustained raids, remaining standing post-war with only superficial damage to non-essential areas. The deployment of specialized "earthquake" bombs, such as the 5.4-tonne Tallboy and 10-tonne Grand Slam, marked an advancement, designed to burrow deep before detonating and undermine foundations via shockwaves rather than direct penetration. A Grand Slam bomb successfully pierced a 4.6-meter-thick roof at one site, causing localized collapse, while multiple Tallboy strikes at La Pallice destroyed protective grill layers but did not render the bunker unusable. Despite these instances, comprehensive destruction proved elusive; pens at Brest, Lorient, and elsewhere sustained operations, with repairs often completed swiftly using local resources. In terms of U-boat attrition, bombing pens yielded negligible results compared to open-sea engagements. Of the 785 U-boats lost or scuttled by war's end, fewer than a handful were confirmed destroyed within pens by aerial attack, whereas at sea accounted for the overwhelming majority—519 by British and forces, 175 by American units, and others by miscellaneous means. Specific raids, like the strike on Bordeaux's Bruno pen, damaged three U-boats but sank none, alongside collateral hits on adjacent . Broader assessments highlight indirect effects: raids forced German dispersal of repair facilities, consumed concrete and labor for reinforcements, and imposed temporary operational halts, yet the pens' survival enabled sustained sorties into 1945, underscoring the primacy of naval countermeasures over aerial ones in defeating the . Empirical from post-war evaluations affirm that while bombing diverted resources, it did not decisively impair pen functionality or output, challenging narratives of air power's singular role in the .

Post-War Fate

Demolition Attempts and Survival

Following the German surrender in May 1945, Allied forces and local authorities across occupied Europe undertook demolition efforts to neutralize submarine pens, aiming to prevent their potential reuse in future conflicts. These structures, engineered with reinforced concrete roofs up to 9 meters thick, proved extraordinarily resilient, often rendering explosive charges ineffective or excessively risky due to the scale of materials required and the proximity to civilian areas. In France, where the largest concentrations of pens existed—at Brest (Dora), Lorient (Keroman), Saint-Nazaire, La Pallice, and Bordeaux—post-war attempts by Allied engineers to breach the bunkers using dynamite and other explosives largely failed. For instance, at Lorient's Keroman complex, transferred to French control on May 10, 1945, the pens sustained only superficial damage from wartime raids and evaded complete destruction afterward, as the immense concrete volume (over 60,000 cubic meters per major bunker) made full demolition cost-prohibitive. Similarly, Brest's Dora pen, scarred by prior Tallboy bomb impacts, withstood further sabotage efforts intact. Norway's Dora I pen in faced multiple post-1945 explosive demolitions, which shattered windows in surrounding buildings but inflicted minimal structural harm, prompting authorities to abandon the operations due to the 's and the of unintended damage. In contrast, Germany's II in was successfully razed by British in late 1945 using captured bombs, collapsing the roof and entombing three scuttled Type XXI U-boats (U-2505, U-3008, and U-3506), which lay forgotten beneath gravel fill until rediscovered in 1985 during port expansion. On , the III pens and associated installations were obliterated in April 1947 blasts, appearing fully destroyed amid the island's broader demolition to remove remnants. The partial successes and widespread failures highlighted the pens' over-engineering for wartime protection, which inadvertently ensured long-term survival for most sites. Un demolished structures transitioned to storage, industrial, or cultural uses, exemplifying how initial Allied priorities shifted from eradication to pragmatic adaptation amid reconstruction constraints.

Repurposing for Civilian and Military Use

Following World War II, several German-constructed submarine pens were adapted for ongoing military purposes by successor navies. The French Navy repurposed the Keroman base at Lorient, renaming it Base Ingénieur Général Stosskopf in 1946 and using it as the Atlantic submarine squadron headquarters until its decommissioning in 1997. At Brest, the bunkers were integrated into French naval operations, supporting submarine maintenance and forming part of the enduring Base Navale de Brest, which remains an active facility as of 2025. In Norway, initial post-war assessments considered demolition of the Dora I pen in Trondheim, but cost prohibitive efforts led to its handover to the Royal Norwegian Navy for temporary submarine basing before broader civilian transition. Civilian repurposing became prevalent after military disuse, transforming the reinforced structures into economic assets. At Lorient's Keroman, the site evolved into a multifaceted nautical and cultural center post-1997, housing the Cité de la Voile sailing museum, the preserved Flore with its onboard exhibit, the Musée des Sous-Mariniers, marinas for civilian vessels, restaurants, bars, and venues, attracting over 500,000 visitors annually by the . In Trondheim, served as a , , municipal and state archives repository, and even a facility after 1945, later privatized in 1961 and adapted into a multi-use harbor for civilian boating and storage. Germany's unfinished bunker near , damaged by RAF bombing on March 27, 1945, withstood post-war demolition attempts and opened as the Denkort Bunker Valentin memorial in 2017, serving educational purposes on forced labor and Nazi while permitting limited public access for historical tours. Other sites, such as La Pallice in , have been partially converted for commercial shipping storage and repair, leveraging the pens' durability for modern maritime logistics without full-scale tourism development. These adaptations highlight the pens' resilience, enabling sustained utility despite their wartime origins, though structural assessments often note ongoing maintenance challenges from .

Legacy and Impact

Engineering and Architectural Lessons

The German submarine pens exemplified advanced use of reinforced concrete in military architecture, with roofs typically 3 to 5 meters thick and walls up to 3.5 meters, designed to distribute impact loads from aerial bombs across massive structural volumes rather than relying on tensile strength alone. This mass-concrete approach, incorporating steel rebar grids, prioritized compressive resistance to fragmentation and blast effects, rendering early pens largely impervious to standard 1,000- to 2,000-pound high-explosive ordnance dropped by Allied bombers in 1942–1943. Architectural features included interconnected bays sharing load-bearing walls to minimize material use while maximizing covered drydock space—up to 200 meters long and accommodating multiple U-boats—often topped with fangrost gratings to trigger premature bomb fuzes on contact, further enhancing external blast absorption. Construction methods emphasized rapid scaling under wartime constraints, employing 24-hour shifts with up to 15,000 laborers per site, including forced workers, to pour concrete in situ using local aggregates and imported cement transported by rail and barge. Later designs, such as the Valentin bunker initiated in 1943 near Bremen, escalated roof thickness to 7 meters to counter escalating threats, incorporating innovative arching and buttressing for seismic-like stability against potential penetration. However, reliance on unskilled, coerced labor under Organization Todt oversight led to inconsistencies; Valentin's incomplete structure suffered from uneven curing and reinforcement lapses, exacerbating vulnerabilities exposed during RAF raids on 27 March 1945, when two 22,000-pound Grand Slam bombs penetrated the roof, causing localized collapse through shock-wave propagation rather than surface cratering. Empirical assessments post-war confirmed the pens' causal effectiveness against conventional bombing: despite over 2,000 sorties by the U.S. against bases like and Brest from 1942 to 1944, structural integrity persisted, with internal berths sustaining minimal operational disruption from direct hits. This underscored a key lesson in causal realism for hardened targets—external blast energy dissipates rapidly in thick, monolithic , but deep-penetration munitions exploiting inertial delay fuzes can couple explosive forces internally, fracturing and inducing progressive failure. Architectural overreach in scale, as at (426 meters long), amplified risks from incomplete integration, where untested spans under dynamic loads failed under combined bombing and material flaws. Demolition efforts after 1945 highlighted enduring durability: conventional explosives at sites like Trondheim's Dora pen in 1947–1953 proved inadequate against the concrete's low and high , requiring specialized shaped charges or abandonment, informing modern on decommissioning megastructures. Overall, the pens demonstrated that first-principles material science—leveraging concrete's superior compression over alternatives like plating—could defy aerial supremacy for protected assets, though at the cost of immobility and vulnerability to purpose-built countermeasures, influencing bunker designs toward deeper, earth-integrated forms.

Role in Prolonging U-Boat Operations

The submarine pens constructed by in occupied French ports, such as , Brest, and , critically extended operational endurance by safeguarding vessels during refitting, repairs, and resupply—phases when submarines were most vulnerable to air attack. Prior to pen completion around 1941–1942, in open docks faced substantial risks; for example, RAF raids in 1940 sank or damaged several at and Brest before construction accelerated under . The pens' multi-layered concrete roofs, often 5–7 meters thick and , resisted penetration by conventional bombs, housing up to 20–30 Type VII per facility while minimizing structural failures from thousands of tons of Allied ordnance dropped between 1942 and 1944. This port security drastically curbed non-combat losses, which had previously eroded fleet strength; historical analyses confirm pens were "unquestionably effective" at protecting berthed , with virtually no confirmed sinkings inside intact bunkers until specialized weapons emerged. By enabling rapid cycles—often reducing downtime from weeks to days—the structures supported sustained rates, facilitating over 1,000 patrols from Biscay bases alone during the campaign's peak from 1941 to mid-1943, when monthly sinkings of Allied shipping exceeded 500,000 gross tons despite intensifying anti-submarine warfare. Without such protection, higher attrition from base strikes would have compounded at-sea losses (which accounted for over 90% of the 785 U-boats sunk by war's end), potentially collapsing the wolfpack system earlier; instead, pens allowed Admiral Dönitz to maintain operational tempo, arguably delaying Allied convoy supremacy by sustaining pressure on transatlantic shipping into 1943's "Black May," when 43 U-boats were lost primarily in open ocean ambushes. Late-war innovations like the 5-ton Tallboy bomb finally inflicted roof breaches, as at Brest in August 1944, but by then U-boat effectiveness had already declined due to radar, escorts, and air cover rather than base denial. Thus, the pens' durability shifted attrition burdens seaward, prolonging the U-boat threat through preserved hulls and crew readiness amid mounting Allied material superiority.

Debates on Strategic Value and Air Power Myths

![U-Boat Pen damaged by Grand Slam][float-right] The construction of submarine pens represented a significant strategic investment by , utilizing millions of cubic yards of to shield from Allied air attacks, thereby enabling safer refitting and deployment closer to the Atlantic convoy routes. These facilities, such as those at and , proved highly effective in this role, with walls up to 11 feet thick and roofs up to 16 feet rendering them resistant to conventional bombing throughout much of the war. Proponents of their value argue that the pens extended the viability of the by minimizing port losses, allowing wolfpack operations to persist into 1943 despite growing Allied air superiority; without such protection, exposed docks would have suffered devastating strikes, potentially accelerating the defeat in the . However, critics contend that the immense resource demands— including forced labor for projects like the Valentin bunker, which spanned 1.7 million cubic meters—diverted materials and manpower from production or defensive enhancements, yielding diminishing returns as Allied antisubmarine technologies like and escort carriers shifted the balance at sea rather than in port. Debates intensify over whether the pens' protection justified their opportunity costs, estimated indirectly through U-boat construction figures where each Type VII submarine cost approximately $2.25 million, while pen projects consumed concrete volumes equivalent to multiple naval vessels or fortifications. Historical analyses suggest that while the pens facilitated rapid turnaround times—critical during peak sinkings of 500,000 tons monthly in —they failed to alter the inflection point, where 73 s were lost against 120 Allied ships, underscoring that sea-based countermeasures, not port bombing, proved decisive. German admiral himself assessed the structures as largely impervious, affirming their tactical success but highlighting how Allied focus on adjacent caused only temporary disruptions, repaired swiftly without halting operations. Myths surrounding air power's impotence against such hardened targets persist, often overstating the pens' invulnerability; while early high-altitude raids with 250-1,000-pound bombs achieved negligible penetration against 20-25 feet of steel-reinforced concrete, the introduction of specialized "earthquake" ordnance like the 12,000-pound Tallboy and 22,000-pound Grand Slam bombs in 1944-1945 demonstrated evolving capabilities, collapsing roofs at sites including Brest's Simone pen in February 1945. Another fallacy posits that strategic bombing of pens was futile and resource-wasting for the Allies, sinking few submarines inside (only 46 by the Eighth Air Force across campaigns), yet this ignores broader air contributions: land-based aircraft destroyed 307 of 368 U-boats overall, primarily through patrols and mining rather than direct port assaults, comprising just 4% of total bombing tonnage. In causal terms, the pens compelled Germany into static, high-cost defenses, indirectly aiding Allied victory by immobilizing industrial efforts, though their endurance fueled postwar narratives exaggerating air power's limitations against fortified naval assets.

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  19. https://www.battlefieldsww2.com/dora_trondheim.html
  20. https://www.bremen.eu/bunker-valentin
  21. https://warhistorybuff.com/2019/03/20/nazi-megaweapons-a-concrete-tale/
  22. https://mechtraveller.com/2023/04/the-u-boat-base-saint-nazaire/
  23. http://www.uboataces.com/articles-uboat-bunker.shtml
  24. https://www.uboat-bases.com/en/category-en-gb/lorient/
  25. https://www.denkort-bunker-valentin.de/sprachen/english
  26. https://warfarehistorynetwork.com/article/building-the-atlantic-wall/
  27. https://www.usni.org/magazines/proceedings/1955/april/german-u-boat-construction
  28. https://www.kz-gedenkstaette-neuengamme.de/en/history/satellite-camps/satellite-camps/bremen-farge-1/
  29. https://www.warhistoryonline.com/world-war-ii/the-valentin-submarine-factory.html
  30. https://www.dark-tourism.com/index.php/210-bunker-valentin
  31. https://ww2db.com/facility/Arsenal_de_Brest/
  32. https://www.researchgate.net/figure/Even-the-most-solid-constructions-in-Brest-fall-victim-to-heavy-targeted-allied-bombings_fig2_326773774
  33. https://uboat.net/flotillas/bases/lorient_bunkers.htm
  34. https://www.landmarkscout.com/u-boat-bunker-keroman-3-lorient-france/
  35. https://www.historynet.com/frances-u-boat-bunkers-survived-the-war-and-thrive-today/
  36. https://www.atlasobscura.com/places/nazi-uboat-pens
  37. https://www.battlefieldsww2.com/la-pallice-u-boat-bunker.html
  38. https://www.facebook.com/immhh/posts/the-u-979-in-the-submarine-bunker-of-la-pallice-la-rochelle-is-one-of-the-two-la/601762798658627/
  39. https://www.saint-nazaire-tourisme.uk/explore/discover-the-port/the-submarine-base-of-saint-nazaire/
  40. https://avgeekery.com/saint-nazaire-uboat-base-that-defied-2000-sorties/
  41. https://uboat.net/flotillas/bases/saint_nazaire.htm
  42. https://www.battlefieldsww2.com/bordeaux-u-boat-bunker.html
  43. https://uboat.net/flotillas/bases/bordeaux.htm
  44. https://www.bassins-lumieres.com/en/discover/place-of-history
  45. https://uboat.net/flotillas/bases/
  46. https://www.battlefieldsww2.com/bergen-u-boat-bunker.html
  47. https://uboat.net/flotillas/bases/bergen.html
  48. https://uboat.net/forums/read.php?3,88693,88742
  49. https://www.uboatarchive.net/Misc/VIIIBC21OCTReport.htm
  50. https://www.academia.edu/38416155/The_Devastation_of_Lorient_Why_Did_the_Allies_Area_Bomb_the_Biscay_Ports_in_1943
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  53. ./assets/U-Boat_Pen_Grand_Slammed.jpg
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  55. https://www.facebook.com/SSAFAOfficial/posts/on-19-march-1945-a-22000-lb-mc-deep-penetration-bomb-bomber-command-executive-co/1091457316356607/
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  57. https://www.facebook.com/groups/754427714964136/posts/1851329451940618/
  58. https://www.warhistoryonline.com/world-war-ii/valentin-u-boat-pen.html
  59. https://www.avi-8.com/blogs/the-aviation-journal/from-the-tallboy-bomb-to-the-grand-slam-bomb-evolution-of-allied-precision-bombing-in-world-war-ii
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  61. https://www.normandybunkers.com/uboatbunkers-1/la-rochelle-u-boat-bunker
  62. https://www.facebook.com/groups/754427714964136/posts/1781938398879724/
  63. https://tchile.wordpress.com/2010/11/14/dora-i-submarine-pen-and-archive-building/
  64. https://www.twz.com/how-entombed-u-boats-were-re-discovered-decades-after-wwii
  65. https://www.theguardian.com/world/2022/apr/19/naval-installations-blown-up-in-heligoland-1947
  66. https://www.la-flore.fr/en/home
  67. https://www.hostelman.com/attractions/france/bretagne/lorient/landmark/keroman-submarine-base/
  68. https://www.tripadvisor.com/Attraction_Review-g187095-d7625067-Reviews-Base_Navale_de_Brest-Brest_Finistere_Brittany.html
  69. https://www.gpsmycity.com/attractions/dora-1-29814.html
  70. https://www.visit-lorient-brittany.com/unmissable/lorient-la-base/
  71. https://www.tripadvisor.com/Attraction_Review-g196530-d10152666-Reviews-Keroman_Submarine_Base-Lorient_Morbihan_Brittany.html
  72. https://www.researchgate.net/publication/325732031_Transforming_the_transformed
  73. https://navyhistory.org/2019/09/building-for-battle-u-boat-pens-of-the-atlantic-battle/
  74. https://digital-commons.usnwc.edu/cgi/viewcontent.cgi?article=1035&context=usnwc-newport-papers
  75. https://historyarticles.com/gray-wolves-den/
  76. https://uboat.net/forums/read.php?3,46440
  77. https://www.airuniversity.af.edu/Portals/10/AUPress/Books/B_0091_MEILINGER_AIRPOWER_MYTHS_FACTS.PDF
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