At the outbreak of World War Two the Wehrmacht possessed one of the most sophisticated military communication systems in the world. At the heart of this was the secure coded messages transmitted on Germany's supposedly unbreakable coding machine, the Kriegsmarine version of which was known as Sclüsselmaschine M (Code writer M). These electronic coding devices, resembling a small portable typewriter, relied on a variable internal wiring and rotor system to provide an incredible 6,000,000,000 variable code settings, changed daily. While British code-breakers at Bletchley Park's top secret "Government Codes and Cypher School" had managed to begin penetration into Luftwaffe and Heer Enigma ciphers (with enormous and often overlooked help by Polish cryptoanalysts that had been working on the Enigma for years before both before the war and later in England), the more complex Kriegsmarine system defied all attempts at penetration. The first break for British crypto-analysts came with the capture of an intact Enigma code book and rotors from auxiliary supply/weather reporting ship München by a boarding party from destroyer HMS Somali near Jan Mayen island on 7 May. While this enabled the brilliant team of mathematicians at Bletchley Park to crack the low-grade weather code, it still did not enable access to the far more complicated naval code system used by combat units such as destroyers and U-boats. The final breakthrough was not long in coming. On the night of 8 May Kptlt. Lemp's U110, from Lorient's 2nd U-Flotilla, had been successfully shadowing convoy OB 318 while transmitting homing signals that attracted Kaptlt. 'Adi' Schnee's U201. The following morning the two U-boats attacked independently, Schnee achieving success before departing for Lorient. Lemp on the other hand suffered mixed fortune. Firing three torpedoes into the dense convoy U110 sank two British ships, the third "eel" passing wide of its mark. Inside the U-boat, torpedo men frantically attempted to free a fourth "eel" that had failed to leave the tube. As his sweating crew laboured in the forward torpedo room Lemp lined up another attack, this time on a 15,000-ton whaling ship, when he saw to his horror that escort corvette HMS Aubretia was cleaving swiftly through the sea toward him. Crash diving, the U-boat was shaken by accurate depth charges causing considerable damage to the still shallow boat. As Schnee disengaged from the action on the convoy's opposite beam all Allied attention focused on Lemp, destroyers HMS Bulldog and Broadway dropping further charges disabling the U-boat's hydroplanes, rudder and electric engines. Choking gas from flooded batteries filled the boat and she seemed unable to stop her downwards plummet to oblivion. Inexplicably the U-boat slowed her rapid fall and gradually began to rise, her frantic crew rushing to abandon ship as U110 came under concentrated fire after bursting onto the surface. HMS Broadway rapidly bore down on the stricken boat as if to ram her. Aboard Bulldog Commander Baker-Cresswell saw an alternative and ordered Broadway to veer away at the last moment, though the destroyer still struck U110 a glancing blow. Lemp and his crew had meanwhile abandoned ship, the bearded commander having set scuttling charges himself. Aboard stationary Bulldog boats were lowered and struggling Germans rapidly pulled from the water and into captivity, while other boats raced for U110 intent on boarding her. Lemp - now alone in the gentle swell - saw with despair that his scuttling charges had failed to detonate and realised the obvious implications. He or his panicked men had not thrown their Enigma and vital code books overboard in weighted sacks during their hasty escape; they remained on the radioman's small desk. Although exact details of what followed are obscure Lemp did not survive, some accounts claiming he was shot others that he allowed himself to drown aware of the ramifications of his boat's boarding. He was the fourteenth crewman to die during the attack. The surviving crew had been secured below decks and were unaware that their boat had been captured. Within days the intact Enigma, its rotor wheels and code books were in the hands of the British at Bletchley Park. Although the Allies suffered periodic blackouts where the Kriegsmarine changed their code patterns, the intact capture of the code machine was perhaps the most crucial intelligence coup of the war, and one that the German High Command refused to believe possible. Many front line commanders suspected that the codes were compromised but were met with either brusque denials by their commanders or vague reassurances that the breaking of the Enigma code was not possible.
LINK: Andy Carlson's excellent Enigma simulator (in German on the comprehensive Grey Wolf website).
The Polish part in cracking the Enigma.
Turing's treatise on Enigma (written by Dr Alan Turing)
The Bold launcher aboard U2540, the Type XXI now moored outside Bremerhaven's Maritime Museum.
On 28th September 1942 Adolf Hitler and senior Kriegsmarine officers engaged in conference in Berlin, one of the prime reasons being Hitler's desire for a "decoy torpedo" to simulate the destruction of U-boat. His original opinion was that special torpedoes carrying oil and various items of wreckage could be carried and launched in the event of a prolonged hunt by enemy destroyers. However the loss of an operational torpedo tube for the carrying of such a device was unacceptable to U-boat officers.
The compromise that was reached resulted in probably the most effective of the German anti sonar counter-measures. Named Bold (short for Kobold meaning "deceiving spirit" or "goblin") this device comprised a 15cm diameter capsule filled with 370grams of calcium and zinc that was expelled from the stern compartment of a U-boat through a purpose built ejector (named the Pillenwerfer - pill-thrower - by crewmen). The calcium-zinc compound was packed within a wire mesh bag, in turn stored inside an aluminium canister. Sealed within a waterproof outer layer there was a hydrostatic valve that controlled the entry of seawater into the canister. Upon contact with sea water the compound within the capsule produced hydrogen gas resulting in a large mass of bubbles that resembled on ASDIC the echo produced by a submarine contact. The hydrostatic valve regulated the entry of seawater to such an extent that the Bold capsule could continue to emit bubbles for up to 25 minutes.
Two other canisters were developed to be launched by the U-boat's Pillenwerfer. The first was a miniature torpedo that carried no warhead but only an electric motor that would circle at a given depth and simulate the noise created by a U-boat's electric drive before its small battery was exhausted. The second more closely mirrored Hitler's original intention and contained oil which rose to the surface creating a small oil slick for attackers to see.
The bow jumper wires of a Type VIIC - U570 before her capture by the Royal Navy.
The jumper wires that are such a distinctive part of most Second World War U-boats were in fact short-wave radio antennae (3-30MHz). Stretching both fore and aft of the conning tower, the forward portion was used to transmit while the stern twin wires were for reception. The antennae were connected to the radio within the U-boat by a thin insulated wire entering the conning tower facade. Coupled with their primary use was also the value of using the jumper wires as a means of attaching life lines for crew working on deck in bad weather.
The conning tower of U564 showing the insulated connector (below the cat's front paws) by which means the radio antenna entered the U-boat's hull
TMA (Torpedo mine A) Mines were pre-war standard design floating mines, phased out by TMB from mid-September 1939. Stored aboard U-boats in the same manner as torpedoes, TMA mines were shorter than “eels”, measuring only 3.64 metres long. Their detonator was of the magnetic influence type. However because the complete mine comprised the anchor by which to moor it to the seabed plus the connecting cable, the actual warhead was relatively small at 215kg.
TMB was also designed especially for submarine use. Again deployed via the torpedo tubes in the same manner as a torpedo, they were slightly smaller than TMA mines — measuring 2.31 meters — thereby allowing the submarine carrier to load even more into its confined storage space. Up to three could be fired from a single torpedo tube as a single shot. This was an unearthed “ground mine” that utilized a magnetic firing pistol, activated on a timer allowing the U-boat to depart before the mine became “live”. It carried an explosive TNT charge of 567kg — twice that of period torpedoes. The mine was laid on the bottom in shallow seaways and detonated by the magnetic field generated by large ships passing above. The explosive force beneath a ship's keel carried by shock waves through incompressible water was usually more effective than a contact hit against a hull side, often snapping a vessel's spine and dooming her instantly.
These mines were originally ordered laid at a depth of 30 metres. However questions arose as to the weapon's reliability at that depth and further trails were conducted. Following these it was established that 25-metres was a more suitable depth, and more likely to prove fatal to shipping above. It began to be noticed that often the magnetic trigger was itself overly sensitive and smaller ships were sunk than those intended. The blast from such a mine against smaller trawlers was particularly devastating. U-boat crews therefore began sowing their minefields in progressively shallower waters as well as de-magnetizing them so as to reduce their sensitivity.
TMC was designed after Dönitz' worries that the TMB's 576 kg warhead might not be powerful enough to sink capital ships. Measuring 3.39 metres in length the maximum number that could be deployed per tube at one time was reduced to two. However the TMC was packed with an explosive charge of 1 000 kgs believed to be lethal from a depth of 36 meters.
SMA (Schachtmine or Shaft mine) mines were designed to be laid by specially designed mine laying submarines equipped with vertical shafts. An anchored mine, of the same style as the TMA, SMA mines were 2.15 metres in length with a diameter of 1.33 metres. Type VIID and Type X U-boats were to be the principal U-boat carriers of this weapon. However, as the U-boats became operational it was found that an internal fault within SMA weapons would lead to premature detonation once deployed. An embargo was placed on their use until March 1943 when the problem had been rectified. U117 was the first U-boat to deploy the redesigned SMA off the Moroccan coast after transfer to Bordeaux's 12th U-Flotilla.
Among the interesting technological developments that were used to assist hunting Type IXD2 U-boats was the Focke-Achgelis Fa330, commonly known as Bachstelze (water wagtail), a towed gyro copter.
Germany possessed a pioneer in the field of rotorcraft technology in the shape of Heinrich Focke. He had flown his first helicopter in 1937 and by 1942 the Kriegsmarine was engaged in testing another twin-rotor helicopter, the Fl282 for a possible future ASW role. Progressing from this initial development, BdU soon began investigating the use of gyrocopters to extend the long-range surveillance capabilities of U-boats in action. On average U-boats rode so low in the water that the visible horizon lay little more than eight miles distant. A small aircraft offered a novel solution to this problem, but only if the submarine could remain undetected on the surface long enough to launch and recover it.
During the First World War several nations had experimented with submarine-based observation aircraft, but with discouraging results. In the inter-war years although the Japanese navy continued to develop such submarines and aircraft, German attempts at utilising the collapsible Arado Ar 231 seaplane proved a failure, its handling poor at best and assembly and disassembly times overlong and exposing the submarine to possible attack. However, by 1942 Kriegsmarine officials asked Focke-Achgelis GmbH to build a rotor kite that could be towed aloft from a moving U-boat. The aircraft had to be capable of an altitude that would boost the U-boat's scouting range, yet remain compact and mechanically simple to maintain and operate. The Fa330 was the result.
Easy to produce and quick to assemble, launch and recover, it weighed so little that two men could easily lift the machine onto its small launch pad. The Fa330 needed no engine, towed through the air so that it flew by autorotation; the movement of relative wind through the rotor blades causing them to turn with sufficient speed to generate lift. The airframe consisted of two 6.35cm diameter steel tubes joined to form an inverted 'T'. The bottom of the 'T' acted as the gyrocopter's fuselage, mounting the pilot's small seat as well as the rear control surfaces. The vertical tube housed the rotor mast. A control stick hung from the hub of the blade that sat atop the mast, moving the stick allowing accurate pitch and roll control. Foot pedals were provided for the pilot to move the large rudder, controlling the copter's rate of yaw. A simple altimeter, airspeed indicator and tachometer were mounted for the pilot to use.
While the Bachstelze was inoperative it was stored in two watertight metal canisters nearly 4 metres long built onto conning tower's Wintergarten platform. One canister housed the blades and tail, the other the fuselage, a third horizontal container holding the cable and winch used to tether the kite to the U-boat. In calm seas it took four crewmen three minutes to assemble the rotor kite in preparation for takeoff. It was possible to rotate the blades by hand immediately before lift off, but a rope wrapped around a drum on the rotor hub was also provided to spin the rotor for takeoff. The Fa330 took to the air from a small platform attached to the aft railing of the Wintergarten, taking off and landing on two hollow-tubing skids mounted on the craft's underside. The 300 metres of towline that extended from the electric winch included a quick release coupling on the gyrocopter's end. Once aloft the pilot communicated with the U-boat via an interphone system, its wire wrapped around the tow cable. When the pilot indicated that he was ready to land the crew winched him back aboard, the pilot applying a rotor brake upon touchdown before disassembly could begin. However, if the U-boat came under threat and needed to dive, the pilot pulled a large red lever above the seat, activating the quick release coupling which jettisoned the towline and simultaneously separated the spinning rotor blades from the mast. As the rotors detached and spun away they pulled a cable deploying a parachute for the pilot who in turn ditched his seat and harness before falling into the sea.
In theory, the U-boat would later surface and recover the pilot once clear of any potential danger. By the middle of 1942 the first Fa330s were completed by the Weser-Flugzeugbau in Hoyenkamp and had begun testing aboard the Type IXC U523, engaged in Baltic exercises as part of the 4th U-training Flotilla. Results were judged to be excellent. A wind tunnel at Chalais-Meudon, France, served as a flight simulator to train several crewmembers from each submarine that would carry a Fa330 and the gyrocopter was cleared for operational use at the beginning of 1943. Unfortunately the Fa330's large radar signature made it impractical to deploy the rotor kite in the Atlantic Ocean and they were relegated to use with the Type IXD2s headed for the Indian Ocean where escorts were considerably less well equipped with radar gear and merchantmen still sailed independently.
Robert Gysae's U177 was the first to use this device in action, deploying the Fa330 at least thirty times, although only a single sinking resulted from these attempts when the Greek steamer SS Efthalia Mari was sighted and successfully attacked on 5 August east of Madagascar.
Two photographs of the Bachstelze on display in Munich's excellent Deutsches Museum.