The U-Boat War: A Global History 1939–45

Dedicated to Opus

Contents

Acknowledgements

List of Illustrations

Glossary

Introduction

Chapter One: Genesis

Chapter Two: War

Chapter Three: The Myth of the ‘Happy Time’

Chapter Four: Diverging Objectives

Chapter Five: The Descent

Chapter Six: Sun and Snow

Chapter Seven: Drumbeat in the New World

Chapter Eight: Losing the Race

Chapter Nine: Tipping Point

Chapter Ten: Reinforcing Failure

Chapter Eleven: Total Commitment

Select Bibliography

Notes

Plates

Acknowledgements

There are many that should be mentioned, but my word-count is unforgiving. I would like to express my thanks to the U-boat veterans that it has been my pleasure to know over the years. Among them, I would especially like to mention Horst Bredow, Georg Seitz, Volkmaar König, Gerhard Buske, Herbert Waldschmidt, Hans-Rudolf Rösing, Jürgen Oesten, Hans-Joachim Krug, Georg Högel, Ludwig Stoll, Wolfgang Pohl, Wolfgang Hirschfeld, Bernard Geissman and Ernst Göthling. Thanks to my wife Anna, Megan, James and ‘Mumbles and Mr Mumbles’ Paterson, Maggie Bidmead and Paul Milner, for access to his incredible photo collection.

List of Illustrations

The Type IIB U23 before the war. Numbers on the conning tower and small bow plaques were removed before the outbreak of hostilities.

Type VII U33, with the external stern torpedo tube clearly visible; the tube was integrated internally from the VIIB onwards.

U33 in Wilhelmshaven on 28 September 1939 at the end of its first patrol, with souvenirs from the British steamer Olivegrove, the first of three ships sunk during this baptismal war patrol.

Georg Högel, radio operator aboard Lemp’s U30 and U110, was a gifted artist. So too was Guy Griffiths, who was taken prisoner aboard U30 at the outbreak of hostilities. He formed a lifelong friendship with Högel. This illustration taken from a letter from Griffiths to Högel dated 6 January 1980 shows Griffith’s Skua attack on U30.

Günther Prien (with binoculars), his ears wadded with cotton wool as crewmen prepare to test fire an MG34 from U47’s conning tower.

Fritz-Julius Lemp, who had recently been awarded the Knight’s Cross, with Karl Dönitz aboard U30 in Kiel at the end of its final war patrol, 30 August 1940.

Otto Kretschmer (in leather jacket), the highest-scoring U-boat ‘Ace’ of the Second World War.

Schnurzl photographed aboard U30 by Georg Högel.

Lemp’s U110 with ‘Schnurzl’ emblem and Schepke’s U100 with splinter camouflage and his crouching panther emblem, Kiel, 9 March 1941. Both captains wear the traditional white caps. This was to be U100’s last patrol.

The Type IID U139 being commissioned by Oberleutnant zur See Robert Bartels in Kiel on 24 July 1940. The small size of the coastal Type II is immediately apparent. Bartels was killed in action near Madagascar on 20 August 1943 as captain of the Type IXD-2 U197.

The forward torpedo compartment of U43. (Paul Milner)

Victor Oehrn was tasked with restarting the Atlantic U-boat offensive after disastrous torpedo performance caused morale to crash. Here he celebrates the award of his Knight’s Cross following U37’s return to Lorient in October 1940. At left is Günther Prien; second from right Joachim Schepke.

The charismatic Joachim Schepke speaking at the Berlin Sportspalast to thousands of young schoolchildren in a state-sponsored recruitment drive.

Horst Degen, Engelbert Endrass and Erich Topp. Degen was IWO on Topp’s U552 when this photograph was taken.

Victor Oehrn (left) and Eberhard Godt (centre); the latter headed BdU Ops.

Karl Dönitz (seated in car) with Hans-Rudolf Rösing (centre) at the latter’s Angers FdU West headquarters.

Size comparison of a Type VIIC and a Type XB minelaying U-boat, moored in Kiel.

The cook (‘Smutje’) aboard U43 in his cramped galley. U-boat provisions were among the best issued by the Wehrmacht, though fresh food was soon either eaten or disposed of as rotten, replaced by tinned and preserved items. (Paul Milner)

Hans-Heinz Linder’s U202 docks in the Penfeld River, Brest, used for mooring before the construction of U-boat pens. (Paul Milner)

The forward gun of U111 during severe winter conditions on the boat’s patrol near Newfoundland. (Paul Milner)

Jost Metzler, commander of U69, the first Type VIIC to be launched. He sank the first American ship of the war when he torpedoed freighter Robin Moor on 21 May 1941, after stopping the ship and discovering cargo considered contraband under prize rules.

Adalbert ‘Adi’ Schnee and men aboard U201 taking advantage of the mid-Atlantic ‘air gap’ for coffee, cake and music.

Trainee captains of the 21st U-Training Flotilla aboard U141. Standing left to right: Horst Geider (U61, U761); Hans Döhler (U21, U606, killed 22 February 1943); Jürgen Krüger (U141, U631, killed 17 October 1943); Kurt Eichmann (U151, U98, killed 15 November 1942); Wolfgang Leimkühler (U4, U225, killed 22 February 1943); Siegfried Koitschka (U7, U616). Seated: Hardo Rodler von Roithberg (U24, U71, U989, killed 14 February 1945).

Dönitz – known to his men as Der Löwe (The Lion) – maintained as close personal contact with the men under his command as possible. Here he inspects a crew of the 7th U-Flotilla at La Baule, 30 November 1942. The Obermaat nearest the camera wears the Spanish Cross, signifying his involvement in patrols during the Spanish Civil War.

The time-consuming task of moving a torpedo from its under-deck storage to inside the forward torpedo room – only practical in areas safe from air attack. The external storage canisters were discontinued once Allied air power became preeminent; the containers were easily damaged by depth charge.

Certificate for the award of the U-boat Badge to Edmund Prochnow. Somewhat curiously, it is undated, though Prochnow was a member of the ‘Weddigen’ Flotilla before transfer as part of Otto Kretschmer’s U23 crew to U99 (7th U-Flotilla). He had been hospitalised with severe rheumatism in 1940, reassigned to the 3rd U-Flotilla for a recovery period until 1941, and had returned to the 7th U-Flotilla aboard U93. He survived the boat’s sinking by HMS Hesperus on the early morning of 15 January 1942.

U203 enters the first completed U-boat bunker pens at Saint Nazaire, 30 June 1941.

Helmut Rosenbaum in La Spezia, 5 September 1942, after being awarded t he Knight’s Cross for sinking HMS Eagle. He later headed the 30th U-Flotilla before being killed in a flying accident on 10 May 1944.

Hitler awards the Oak Leaves to Rolf Mützelburg (U203) and Adalbert Schnee (U201), 15 July 1942.

Wolfgang Lüth, one of the two most highly decorated men of the U-boat service, was awarded the Knight’s Cross with Oak Leaves, Crossed Swords and Diamonds on 9 August 1943. He survived the war and was accidentally shot by a German sentry at the naval school, Flensburg-Mürwik, on 13 May 1945.

The stern of the Type VIIC U737 in the North Atlantic. Possessing excellent seakeeping qualities, they could be uncomfortable in heavy swell. (Paul Milner)

Helmut Möhlmann, here as commander of U571 returning to La Pallice (whose activities bear no resemblance to Hollywood’s ‘vision’ of events). Möhlmann later served at BdU before taking command of the 14th U-Flotilla in Narvik.

The bridge watch aboard U737, which mounted nine war patrols as part of the Norwegian 13th U-Flotilla before being sunk on 19 December 1944 in Vestfjord following collision with a Kriegsmarine minesweeper. Thirty-one men were killed and 20 rescued from the freezing water. U737 sank no ships. (Paul Milner)

Reinhard Reche’s U255 returns to Narvik victorious after the attack on PQ17. Reche had sunk four ships, boarding abandoned Liberty ship Paulus Potter and retrieving much material as well as the ship’s flag, seen here streaming from the periscope.

Werner Hartenstein, commander of U156, which opened the Neuland attack on Caribbean oil traffic and installations.

Christmas 1942 aboard U604.

An unexploded torpedo from U67 ashore in Curaçao, with the badly damaged Dutch tanker Rafaela being towed into St Anna Bay where it later broke in two.

The Metox ‘Biscay Cross’ is mounted at the front of this Type IX conning tower.

U20 (left) and U19 of the 30th U-Flotilla in Constanta harbour.

Reichsminister Josef Goebbels visits 6th U-Training Flotilla in Danzig during 1942, signing the flotilla guest book. On the right is flotilla commander Kapitänleutnant Georg-Wilhelm Schulz, formerly of U124. He later joined the staff of FdU Ausbildungsflottillen (Commander Training Flotillas) in Gotenhafen, becoming the last commander of 25th U-Training Flotilla in April 1945.

U43 refuelling from the Type XIV U461, July 1942.

The Type3 XIV U462 photographed from U604 during resupply near the Azores on 27 February 1943. The tanker’s octopus Wappen is just visible on the conning tower.

One of nine Type VIIC U-boats handed over for commissioning into the Italian Navy during 1943. None would leave the Baltic training grounds before the Italian armistice in September. They were reclaimed by the Kriegsmarine and use solely for training purposes.

U43’s Oberleutnant zur See Hans-Joachim Schwantke bringing a bottle aboard U109 during a mid-Atlantic meeting. The man in the background is wearing a Tauchretter. Schwantke was killed on 30 July 1943 with his entire crew when U43 was sunk by an Avenger launched from USS Santee. (Paul Milner)

U511 – codenamed ‘Marco Polo I’ – arrives off the Japanese-occupied harbour of Penang, July 1943.

U271 was sunk with all hands by depth charges during this attack by a USAAF Liberator bomber on 28 January 1944.

The lower extended Wintergarten of a Type VIIC with 37mm flak weapon. Steel helmets became common among bridge and flak crews as the danger from aircraft became paramount.

Looking from the conning tower hatch at crewman manning the FuMB-26 Tunis radar detector aboard U861 in the Indian Ocean. Tuned to the 3cm wavelengths used by American radars, Tunis was introduced into service in June 1944.

Oberleutnant zur See Helmut Herglotz bringing U290 into Bergen, 16 June 1944.

U441 showing the fearsome array of weaponry for this original ‘flak boat’.

U290 showing the round dipole antennae of the FuG 350 Naxos radar detector. The large rectangular opening housed the FuMO 61 Hohentwiel radar when not in use.

U43 making a surfaced torpedo attack. While Oberleutnant zur See Hans-Joachim Schwantke stands at right and maintains overall control, the UZO targeting device is manned (third from left) by another for surface firing. (Paul Milner)

The Brest U-boat pens after the fall of the city to Allied troops, September 1944. On the hill behind is the French naval academy that was the headquarters of the 1st U-Flotilla. From the left the first five ‘pens’ were ‘wet pens’ that could hold two boats; the remainder were dry docks.

U802 after its surrender showing (from left) the snorkel, radio DF loop, observation periscope, and attack periscope. The helical wires around the periscope heads reduced vibration.

The Type XXIII U4709 leaving Kiel. It was commissioned on 3 March 1945 and scuttled on 4 May.

The Type XXI U2519, captained by Peter ‘Ali’ Cremer, formerly of U333. This boat was commissioned on 15 November 1944 but never saw action. Cremer left the boat in February 1945 to become commander of a naval tank destroyer unit that was in combat near Hamburg. (Paul Milner)

Kapitänleutnant Heinrich-Andreas Schroeteler of U1023 (though pictured here as captain of U667). He was one of the last U-boat skippers to sink an enemy ship on 7 May 1945. (Paul Milner)

Glossary

ASDIC Term applied to the sonar equipment used for locating submerged submarines. A powerful and effective weapon, it emitted a distinct ‘ping’ when locating the target. ASDIC is an acronym for ‘Anti-Submarine Detection Committee’, the organization that began research into this device in 1917.

ASW Anti-Submarine Warfare.

AZ (German) Aufschlagzündung; impact trigger for the Pi1 pistol fitted to both G7a and the G7e torpedoes.

BdU (German) Befehlshaber der Unterseeboote; Commander U-boats.

Eel (German) aal; slang expression for torpedo.

EK (German) Eisernes Kreuz; the Iron Cross, awarded in either First or Second Class.

Enigma Coding machine used by German armed forces throughout the Second World War.

FdU (German) Führer der Unterseeboote; Flag Officer for submarines, responsible for a particular geographical region.

G7a German torpedo propelled by compressed air.

G7e German torpedo propelled by electric motor.

HF/DF High Frequency Direction Finding; radio direction finder for locating high-frequency U-boat transmissions (colloquially known as being ‘DFed’).

IWO See Wachoffizier below.

KG (German) Kampfgeschwader; Luftwaffe bomber group.

Kriegsmarine (German) Navy of the Third Reich.

KTB (German) Kriegstagebuch; War Diary. Kept by the commander during a U-boat’s patrol and later entered into official records.

LI (German) Leitendre Ingenieur, Chief Engineer.

Luftwaffe (German) Air Force.

MGK (German) Marinegruppenkommando; geographic command posts (e.g. MGK West, MGK Nord etc) for naval surface and land-based forces.

MZ (German) Magnetzündung; magnetic trigger for the Pi1 pistol fitted to both the G7a and the G7e torpedoes.

OKM (German) Oberkommando der Marine; Supreme Navy Command.

OKW (German) Oberkommando der Wehrmacht; Supreme Armed Forces Command.

RK (German) Ritterkreuz; Knight’s Cross. Highest level of the Iron Cross award for valour, augmented with Oak Leaves, Swords and Diamonds for further meritorious award.

SKL Seekriegsleitung; Naval High Command.

UZO U-Boot-Ziel-Optik; surface targeting device introduced in 1939 that electrically transmitted targeting information from a pair of mounted binoculars to the torpedo calculator.

Wachoffizier (German) Watch Officer. There were three separate U-boat watch crews, each consisting of an officer or senior NCO, Petty Officer and two ratings. The ship’s First Watch Officer (IWO) would be the Executive Officer (second in command); the Second Watch Officer (IIWO), the ship’s designated Second Officer; and the Third Watch Officer (IIIWO), often the Obersteuermann (Navigation Officer). The duties of the IWO included torpedo and firing system care and maintenance as well as control of surface attacks; the IIWO handled administration regarding food and supplies as well as the operation of deck and flak weapons.

Wehrmacht (German) Armed Forces.

Wintergarten (German) Nickname given to the open railed extension astern of the conning tower, Eisbbuilt, to accommodate increased flak weaponry. Known to the Allies as a ‘bandstand’.

Introduction

The crews of five merchant ships and three escorting armed trawlers that comprised convoy EN491 could have been forgiven for some level of complacency. The date was 7 May 1945, and the small cluster of ships had departed the Scottish port of Methil shortly after 2000hrs that evening, an interim stop in the voyage from Hull to Belfast, expected to take three days. The month had begun with unsettled weather; frequent rain showers and thunderstorms had swept across the United Kingdom until the evening of departure from Methil, when a fresh high-pressure system brought a warmer breeze to the British eastern seaboard. In Europe, the war that had raged since 1939 was but hours from ending. Hitler was dead, and partial surrender documents had already officiated over the preceding days for various geographical combat areas, before the final instrument of complete unconditional German surrender was signed earlier that day at SHAEF headquarters within the red brick building housing the Collège Moderne et Technique de Reims. The final capitulation would come into effect at 2301hrs (Central European Time) the following day, 8 May. U-boats had already been ordered to cease offensive operations four days previously on 4 May.

Captain Johannes Lægland’s 23-year-old Norwegian merchant ship Sneland I carried 2,800 tons of coal bound for Northern Ireland, sailing as Number 2 ship in the small starboard column and also serving as the convoy commodore’s ship for what would be one of the last wartime convoys. Immediately ahead rode the newer Canadian merchant ship Avondale Park, skippered by Captain James Cushnie and also carrying coal to Belfast.

As the ships sailed into the Firth of Forth, it is thought likely that the defensive Indicator Loop System comprising two sets of electric cables laid a mile apart on the estuary may have registered the passing of an unexpected large metal object that evening, but the trace had been disregarded as no credible threat at this extremely late stage of the war. If so, it was a grievous mistake. Indeed, at 2145hrs that evening, the war at sea had claimed another victim in the English Channel when a Norwegian minesweeper was torpedoed and sunk by the ageing Type VII U-boat U1023 with only ten of its 32-man crew rescued in Lyme Bay.

The unidentified craft that had activated the magnetic loop and was steadily approaching EN491 was the small electro-U-boat U2336, captained by Kapitänleutnant Emil Klusmeier. This 32-year-old officer had started his naval service in 1930, three years before the accession of the National Socialist Party to power in Germany and five years before the Reichsmarine was renamed Kriegsmarine. Serving as a senior non-commissioned officer within U-boats from 1937, he received promotion to officer one year later. By October 1940, he was transferred to Dönitz’s operational staff where he remained until 1944 and his return to sea as commander-in-training aboard Karl Boddenberg’s U963; their single war patrol together aborted due to the D-Day landings. Klusmeier had then returned to shore and wrote the operating manual for the newly developed and technologically advanced Type XXIII U-boat expected to shortly enter service, Klusmeier volunteering to take one to sea as commander and prove his theoretical instructions correct. Circumstances prevented active operations until, on 1 May, U2336 finally slipped from Larvik bound for the British east coast.

A little before 11 o’clock, the first of two fired torpedoes slammed into the Avondale Park’s laden hull. The ship’s 17-year-old cabin deck boy Sydney Rapley later recalled the unexpected attack: ‘We had come down through the Pentland Firth and on the evening of May 7 I took the helmsman a cup of cocoa and biscuits. I went back to my cabin to sleep. I was reading the Readers’ Digest when the torpedo hit. I got blown up out of my bunk. I was on the top so hit the roof. We got to the lifeboat and were picked up by a Royal Navy vessel minutes later.’¹ The stricken ship began to go down; Chief Engineer Anderson and Donkeyman William Harvey were both killed after being trapped below decks while on engine room watch. Captain Cushnie and 35 other survivors abandoned ship and were rescued by HMT Valse (T-151) and Leicester City (FY-223). Fearing a mine strike and to avoid collision with the sinking Canadian vessel, Captain Lægland swung Sneland I to port when the second torpedo struck the starboard side near No. 2 hatch. Fatally holed, the ship was gone within two minutes, taking Lægland and six others to the bottom and leaving 22 survivors to be rescued. Avondale Park took longer to sink, finally going down on an even keel in 51 metres of water, thereby becoming the last vessel to be destroyed by aggressive U-boat action of the Second World War.

Klusmeier – his boat’s small weapon load exhausted – departed stealthily as desultory depth charges were dropped far behind. U2336 was detected twice more as it passed over the Indicator Loops while circling the Isle of May following the attack. No doubt he was testing the exceptional submerged capabilities of his new vessel, returning unscathed to Kiel on the night of 14 May to find the port already occupied by British troops.

Accused of breaking international law following Dönitz’s order to cease fire, Klusmeier was interrogated by British authorities before they accepted his denial of having received such instructions due to proceeding largely submerged, his U-boat capable of three consecutive days underwater. After two months, he was released from British captivity and returned to his hometown of Bochum.

Emil Klusmeier belonged to a minority group in that he successfully sank at least one single ship. Despite common assertions to the contrary, the U-boats of Hitler’s Kriegsmarine were largely ineffectual in their allotted role as predators of Britain’s commercial trade routes. Despite localized and brief successes, they were constantly hamstrung by rigid tactical doctrine that became increasingly reliant on outmoded and flawed technology. Combined with a fatal misplaced belief in the impregnability of German codes, the U-boats were doomed to fail at their second attempt to force the United Kingdom to its knees through an effective blockade. However, Allied fear of U-boat predation always loomed large through the early years of the war, and subsequently in the various retellings of the tumult that has come to be known as the ‘Battle of the Atlantic’. Customary essays on this struggle tell us that Karl Dönitz admitted defeat within the Atlantic in May 1943. This is not true, as he refused to concede the struggle and always intended to return to the Schwerpunkt – the central focus – of his operations, diverting his strength to peripheral areas of combat in which his boats already fought.

For, though Dönitz’s vaunted tonnage war was considered fought and lost within the Atlantic battleground, U-boats sailed in every theatre of action in which German forces were present, ra nging from the Arctic seas of northern Russia to waters touched by the icy Antarctic currents swirling towards New Zealand. All these theatres of action were deeply interconnected, even if only by the allocation of resources to one starving others of valuable tools for the job at hand. The U-boat war fought between 1939 and 1945 may have been of most crucial importance within the Atlantic Ocean but was an imperative of the Kriegsmarine in each of its geographic settings once its major surface forces had been decimated and effectively neutralized after only two years of fighting.

Chapter One

Genesis

‘I cannot say that I was altogether pleased. The idea of a cruise to the Far East had been very alluring, while in the formation of the new balanced fleet which we were planning, the U-boat would represent only a small and comparatively unimportant part. I saw myself being pushed into a backwater.’

Karl Dönitz¹

Fregattenkapitän Karl Dönitz arrived in Kiel to take command of fledgling U-boat combat flotilla ‘Weddigen’ during July 1935.* He had captained Emden during a cruise around Africa and the Indian Ocean and was expecting another long voyage to Japan and China before his surprise posting. The light cruiser, the first to be built in Germany after the First World War, was used to train cadets and midshipmen for the Reichsmarine – a shadow of the former Imperial German Navy due to armament restrictions imposed by the Versailles Treaty after the war’s end. Dönitz had served during that conflict on surface ships until 1916, when he requested transfer to the U-boat service, going on to command minelayer UC25 and attack boat UB68 from which he was captured on 4 October 1918 after technical difficulties forced its scuttling.

Kiel also harboured a new U-boat school headed by veteran submariner Kurt Slevogt, which boasted six Type IIA U-boats (U1–U6) and functioned under the umbrella of the Naval Torpedo Inspectorate. Promoted to Kapitän zur See on 1 September, Dönitz was charged with raising a new U-boat service, beginning with ‘Weddigen’s’ three improved Type IIB U-boats: U7, U8 and U9.

The Type IIA was a single hull design, meaning that the bulkhead was the external pressure hull and all diesel was stored internally. A single 20mm anti-aircraft gun could be mounted on the forward deck. The conning tower was small, with two periscopes protruding from it: an aerial (navigation) periscope towards the front and a smaller-headed attack periscope in the middle of the tower. Possessing no watertight compartments, it carried three bow-mounted torpedo tubes, capable of carrying three loaded torpedoes and two reloads under the interior decking, or an alternative load of torpedo mines. Interior space was extremely limited, the small control room (Zentrale) that was the nerve centre of every U-boat positioned below the tower. The majority of the 24-man crew lived in the forward area, sharing 12 bunks between them, while a further four for the engineering crew were provided in the absolute stern of the boat, past the engine room and single WC. Cooking and sanitary facilities were basic. Two six-cylinder diesel engines were capable of 13 knots surfaced; two electric motors were able to reach 6.5 knots submerged. While the small boat possessed certain advantages – such as ability to operate in shallow coastal water, quick diving time and low-surfaced silhouette – its disadvantages quickly became apparent. Not only was its maximum pressure rating limited to 150 metres deep and its weapon load relatively light, but the IIA possessed an operational radius of only 1,050 nautical miles if running at 12 knots – barely capable of reaching northern Scotland from Kiel.

An improved Type IIB was soon produced, 20 models rolling out of three different shipyards. This model had three additional frame spacings inserted amidships, allowing an additional oil bunker beneath the control room, increasing operational range to 1,800 nautical miles at 12 knots. The diving time was also slightly improved to 30 seconds.

Strictly forbidden by the Treaty of Versailles, development of this new generation of U-boat designs had been undertaken in secret by a complicated network of fake business fronts, illegal money and design and testing on behalf of ‘client’ nations outside of Germany. However, Commander-in-Chief of the Reichsmarine, Admiral Erich Raeder, had established a clandestine U-boat department as early as 1927 under the cover-name ‘Au’ (Anti-U-boat Defence Questions). Through a series of prototypes built for Spain, Turkey and Finland, three different U-boat types in increasing size gradients were chosen for the Reichsmarine: a 250-ton coastal type, and 500- and 750-ton ocean-going models. These became the Type II, Type VII and Type I respectively. Raeder finally gave the order to begin assembly of the first six Type II U-boats on 8 February 1935, and after years of covert building, the Kriegsmarine replaced the Reichsmarine as one of three Wehrmacht branches; rearmament was officially announced by Adolf Hitler on 16 March 1935, a little under two years after he had taken full dictatorial control of Nazi Germany.

In 1935 Dönitz exerted little control over future U-boat development, which remained in naval command’s hands, the U-boat regarded as merely one component of a balanced fleet. Its potentially war-winning capability demonstrated in 1917 appears to have been forgotten, thought to have been nullified by British anti-submarine sonar technology, ASDIC. Effectiveness of this active sonar had been deliberately inflated by the British Admiralty via carefully worded press stories and ‘intelligence leaks’.

However, ASDIC had limitations, its efficacy deteriorating rapidly in water turbulence created by high speeds (above 20 knots) and rough weather (as frequently encountered in the North Atlantic), proving unreliable in water with steep temperature gradients and strong thermoclines (as in the Mediterranean). It possessed both a maximum range of 1,500 yards and a minimum range, meaning contact w as lost immediately before depth-charge attack. Bearings were inexact, target depth impossible to ascertain, and estimated ranges carried a 25-yard margin of error – crucial given that early British depth charges possessed a lethal range of only 7 yards.

Though many German naval officers’ views of U-boat value were diminished by their perception of ASDIC, Dönitz was not one of them. While exerting little control above his station, he exercised considerable influence over his subordinate command and, assisted by experienced U-boat engineer Otto Thedsen, energetically set about their task. Existing operational doctrine was revised. Previously, trainee commanders had been taught to attack submerged, from 3,000 metres’ distance, with a salvo of torpedoes. Dönitz amended this to 600 metres maximum. The art of ‘shadowing’ was also strenuously taught, relying on a steely nerve to constantly open and close distance between U-boat and target, alternating between submerging, surfacing, then pursuing at speed, all while transmitting location information and avoiding counterattack by any escort vessels alerted by the radio signals.

Furthermore, he urged surfaced attacks under cover of darkness, thereby negating ASDIC and allowing the U-boats’ surface speed, manoeuvrability and low silhouette to be maximized. Dönitz emphasized the importance of aerial reconnaissance reports, the visible horizon from low conning towers providing only a short radius of vision, and advocated group attacks against a single objective, a principle known as Rudeltaktik, now most commonly called the ‘wolfpack’.

The concept of a ‘pack’ attack had originated with Kommodore Hermann Bauer, U-boat commander-in-chief 1914–17. Responding to defensive convoying, Bauer advocated grouping U-boats via radioed instructions from a large ‘headquarters’ U-boat stationed beyond enemy patrol range, which carried wireless and cypher experts, fuel, and weapon stocks for combat boats as its staff plotted enemy convoy traffic and directed the battle. Fortunately for the Allies, Bauer’s proposal was dismissed by the Imperial Navy Staff, who preferred to rely on large numbers of individual U-boat operations.

Dönitz acknowledged tactical questions posed by Bauer’s method. Should the group be coordinated by a tactical commander within one of the U-boats, a surface vessel, or a land-based headquarters equipped with powerful radio equipment and immediately privy to the latest intelligence information? Furthermore, to what degree would individual commanders be granted freedom of action within what should be a tightly coordinated offensive operation? The complex and purportedly impenetrable Enigma machine at least seemed a fine solution to the question of how information could be encoded, decoded, transmitted and received between U-boat and controller.

In training, prospective commanders and crew undertook 66 surfaced and 66 submerged simulated attacks using compressed air ‘water slugs’, before graduating to practice torpedoes with dummy warheads. Emergency dives, deep dives, surfaced and underwater navigation drills were relentlessly repeated within the Baltic, while academic study in all aspects of submarine warfare provided a firm grasp of theoretical war at sea, as well as weapons maintenance and capabilities.

Despite their strength as torpedo carriers, the mine would unexpectedly become the most effective U-boat weapon of the early war. Three types of mines could be launched by torpedo tube: TMA, TMB and TMC. Torpedomine A was a moored mine that detached from its weighted plate after launching. Two could be carried within each torpedo tube, and they were capable of being moored in water up to 270 metres deep, the 215kg warhead attached by chain to float just below the water’s surface, to be detonated by its magnetic influence trigger. However, the mine proved to have too little buoyancy and too thin a mooring rope; after usage during the early months of war, it was redesigned late in 1939, eventually becoming the TMC mine.

Phasing out the TMA was the TMB, a ground mine and therefore only able to function to a maximum 20 metres’ depth. Measuring only 2.3 metres in length, up to three TMBs could be carried in each torpedo tube. The magnetic fuse was timer activated, allowing the U-boat to move out of range before going live. The TMB warhead comprised 576kg of TNT, twice that of period torpedoes. Like magnetically fused torpedoes, it was triggered by a ship’s metal hull, designed to explode beneath the keel, the resultant shock wave amplified through incompressible water. More effective than any contact hit against a hull side or bottom, it exerted huge stresses on the hull and would snap the ship’s spine. The TMC – derived from the original TMA – was a later development following concerns that the TMB was insufficient to sink capital ships. Measuring 3.4 metres in length, only one TMC could be carried in each tube, but the warhead was packed with 1,000kg of explosive, effective from depths of up to 36 metres.

The two torpedo types with which U-boats were equipped were the G7a (TI) and G7e (TII). The ‘G’ was a hangover from early naval weaponry days before the term ‘torpedo’ was in common usage, standing for ‘Geradelaufapparat’, literally ‘straight running device’. The ‘7’ denotes the 7-metre length of the weapon, and ‘a’ or ‘e’ either ‘air’ or ‘electric’. The former (known as an ‘Ato’) was the standard pre-war German torpedo, powered by a mixture of fuel, hot air and steam burning within a four-cylinder combustion engine giving a maximum speed of 40 knots and theoretical range of 75km. Due to this mechanism, the torpedo left a small trail of bubbles in its wake. The G7e (known as an ‘Eto’), by contrast, was fully electric with lead-acid batteries powering the motor, slower at 30 knots and a range of 50km, but leaving no tell-tale wake.

Dönitz’s tenure as ‘Weddigen’ commander ended at the beginning of 1936, when he relinquished the position to Otto Loycke and was promoted to the post of Führer der Unterseeboote (FdU). Despite his advancement, Dönitz still exercised no influence over any aspect of U-boat development, holding only operational control.

During June 1936, the first medium U-boat, the 500-ton Type VII U27, was launched; three months later, the ‘Saltzwedel’ Flotilla formed in Kiel before moving west to the North Sea base at Wilhelmshaven.† Two prototype heavier boats, Type Is U25 and U26, were also attached to ‘Saltzwedel’, though they ultimately proved an unsatisfactory design.

The first-generation Type VII was a single-hulled, single-rudder U-boat with four bow torpedo tubes and one external stern tube. Despite remaining relatively spartan and cramped internally, the Type VII proved popular with crewmen, it being agile and relatively fast on the surface as well as carrying an 88mm fast-firing deck gun. However, its radius of action was limiting for an ocean-going U-boat – 4,300 nautical miles at 12 knots – and it proved slightly unstable when submerged.

Internally, the pressure hull was divided into three separate compartments by two pressure-proof bulkheads, watertight bulkheads further subdividing the internal space into six