Stealth U-Boats

U-480 with Olt z S Hans-Joachim Förster saluting.

In order to prevent being detected either by acoustic or visual means, the submarine fleets had to employ a range of counter-measures to either thwart the penetrative gaze of radar and the sono-buoy or reduce the amount of time the individual U-boats spent on the surface to make it far more difficult for reconnaissance planes to discover their whereabouts. Unfortunately for the Germans none of the main radar warning receivers they developed (Borkum, Metox, Naxos, Tunis and Wanze) operated flawlessly against aircraft; their own active radar sets (Gema, Hohentwiel and Lessing) though effective went into service tardily; what radar and sonar decoys (Aphrodite and Thetis for the former and Bold, Sieglinde and Siegmund for the latter) they produced failed to achieve any lasting success; and the anti-sonar synthetic rubber Oppanol coating they used on the hulls of the U-boats to disguise their acoustic signature (known by the codename Alberich) had major adhesive problems that restricted its application. In addition, it was soon discovered that the use of schnorchel air induction tube, which enabled submarines to run their air-breathing diesel engines and recharge their batteries while still submerged, was not quite all that it seemed at the outset. Quite apart from the health risks (specifically, oxygen-deprivation) that the early non-fully automated models posed for the U-boat crews, a schnorchel boat could also be detected even if its mast was coated with the camouflage antiradar coating of synthetic rubber and iron oxide powder (Tarnmatte).


This was a textured synthetic rubber called OPPANOL. The idea behind this 4MM thick rubber was to cover the entire U-boat in this textured rubber. In reality this OPPANAL only reduced the sonar pulse by about 15 per cent when the boat was at periscope depth. Absorption varied with depth, temp and salinity. The big problem with this system was that of adhesion. There was just no glue at the time that would keep the rubber panels in place. Over time wave action etc. made the rubber panels come lose and actually create more noise than a boat that did not have the coating. It was also found to decrease the speed of the boat by about 1.1/2 knots. The hull and tower were also occasionally covered. For this purpose a black, rubber-like material was used “Alberich”. It served however only for the camouflage against ASDIC, not against radar.

This material reduced the detectable engine noise from the powerplant, and also the sound-echo reflected from the submarine by some 15 per cent. The patented material was a polyisobutene known as Oppanol; in the form used by the U-Bootwaffe it was made in 4mm-thick sheets, to be glued to the steel structure of the submarine. Initial tests on a Type II boat seemed promising, and it was decided to apply the coating to a new Type IXC, U-67, before she entered service.

Although the concept was sound (and is widely used today), the problem in 1941 was that a suitable adhesive had not been perfected. During U-67’s short voyage to her first operational base with 2. Unterseebootsflotille at Lorient that August, it is estimated that at least 60 per cent of her Alberich coating was lost. Once the Alberich `tiles’ had loosened, the turbulence caused by the loose ends of partially detached panels flapping around in the current caused increased drag, and in fact a treated and `peeling’ submarine could end up more `noisy’ than an untreated one.

A suitable adhesive was only found in 1944, and when this was used to coat the Type VII boat U-480 with Alberich it was judged to be effective. On 25 August 1944, U-480 (Olt z S Hans-Joachim Förster) made an attack on convoy BTC-78 off the English coast near Lands End, sinking the freighter Orminster. By this late in the war very few boats would escape once detected by Allied surface ships, but although the escorts began a determined hunt they had to give up after seven hours, and the Alberich-treated U-boat escaped unscathed. Of course, it was not possible to determine to what degree her escape was due to the Alberich rather than simply to the skill of her commander, but it was ordered that in future all new Type XXIII and Type XXVI boats should be treated with Alberich. In the event, only one Type XXIII, U-4709, had been treated before the war ended, and she was scuttled before undertaking any combat patrols.

Type XXIII, U-4709

Type XXI

Anti-Radar Coatings

Tarnmatte (camouflage mat)

A more successful attempt at improving stealth characteristics was the use of a synthetic rubber coating for the exposed head of the U-boat schnorkel or air intake/exhaust tube. The installation of this device was one of the German responses to the introduction in spring 1943 of Allied centimetric radar capable of detecting boats running on the surface day or night, which thereafter forced them to spend most of their time `in the cellar’. This not only seriously degraded their ability to intercept Allied targets, but made the unavoidable periods spent running the diesel engines on the surface to recharge the electrical batteries extremely hazardous. The retractable schnorkel came into widespread use only in May 1944; it provided an air intake and exhaust for the diesels, so that boats could theoretically stay underwater 24 hours a day, not only charging batteries but cruising submerged (very slowly) on diesel power. However, not only was it difficult and even dangerous to operate, but its head above the surface could easily be detected by radar-equipped Allied aircraft.

The synthetic material used to coat the head was known as Buna; the thickness of the coating was dependent on the wavelength of the specific Allied radar emission, and to defeat the 9.7cm-wavelength ASV Mk III set the Tarnmatte was applied 2cm thick. It was very flexible and used on KUGEL-SCHWIMMER SNORKELS etc. Its thickness dictated the frequency of radar radiation that was absorbed. It was much more successful than Alberich, and was reported to be 90 per cent effective.

Despite claiming that Tarnmatte could absorb 90% of the waves emitted by the Allied airborne Mark III radar sets, schnorchel boats could be let down on occasion by the wake left by their mast on the surface of the sea or by a telltale cloud of diesel exhaust fumes that revealed their presence to the eyes of a trained aerial observer hunting for them.


This was far better at absorbing radar radiation but due to difficult manufacturing it was not flexible and could only be made in flat sheets or in round forms (it could not be made to cover the various shapes of a U-boats hull). It was made up of 7 layers of conductive material (paper or plastic with carbon black) separated by layers of di-electric material (rigid synthetic called IGELIT, which is a polyvinylchloride foam that was 70 per cent air by volume). The total thickness was about 8CM thick. This material absorbed radiation between 2 and 50 CM.

D-Day caused U-Boat Command to order an evacuation. Within the French bases, eight U-boats newly fitted with schnorchel gear were approaching readiness as the evacuation gathered pace. With the decision to stop sending boats against the D-Day traffic in the Seine area, they instead were despatched to the Bristol and North Channels on Britain’s west coast. U667 had been operational within the Bristol Channel since the end of July and had reported sinking a destroyer and 15,000 tons of enemy shipping.3 Kapitänleutnant Hardo Rodler von Roithberg’s U989 also scored surprise successes in late August when he damaged the American freighter SS Louis Kossuth and sank the British SS Ashmun J Clough southwest of the Isle of Wight, before being ordered to head for Norway as part of the general evacuation. Another 9th U-Flotilla boat U480 that had left Brest in early August for the English Channel sank corvette HMCS Alberni, minesweeper HMS Loyalty and badly damaged SS Fort Yale northeast of Barfleur, before moving on to attack convoy FTM74 on the afternoon of 25 August. The convoy had overrun the submerged U-boat, the din of propellers easily audible throughout the German hull. The 5,712-ton straggler SS Orminster was torpedoed thirty-five miles northwest of Cap d’Antifer by Oberleutnant zur See Hans-Joachim Förster after which U480 was hunted for seven hours but escaped, his ability to avoid detection enhanced by the Alberich covering – an early form of stealth technology – that had been applied during the previous May:

25 August 1944. 1508hrs. Am being pursued by four anti-submarine vessels, two of which are operating ASDICs; the third, which apparently acts as depth-charge dropper, approaches at intervals of from five to ten minutes and drops charges; the fourth can be heard to be running her engines at very low speed. Listening conditions are particularly good.

2140hrs. Beginning of dusk. Pursuit lasts until 2200 during which time we have covered five miles over the ground … I maintain my depth by shifting the crew. One of the A/S vessels frequently lies directly above us with her engines just ticking over, when the least sound aboard her is clearly audible and ASDIC impulses are extremely loud … The depth-charge dropper, which has lately been lying stopped, approaches and drops five or six depth charges at intervals. These cause such trivial damage that I am convinced that the enemy is unable to locate us with ASDIC … I attribute the enemy’s failure to locate me mainly to the protection afforded by Alberich …

Alberich, named after the guardian of the Rhinegold treasure from Wagner’s Der Ring des Nibelungen, consisted of 4mm-thick sheets of synthetic rubber, Oppanol, which possessed sound absorbing properties. The sheets were secured to the outer hull with adhesive, claiming a 15 per cent reduction in sonar echo reflection, as well as acting as sound insulation for the internal machinery of the U-boat. While Alberich itself was reliable, the adhesive used to secure its place on the hull was less so and experiments continued until the war’s end to prevent sheets from being partially washed off and so flapping in the water stream creating both drag and noise.

The first U-boat to receive Alberich was the Type II U11, covered in the sheeting for initial trials with the 5th U-Flotilla during 1940. In 1941 a larger boat, the Type IX U67 and then UD4 were similarly tested, though the adhesive problem prevented its widespread use amongst combat boats. Not until the dawn of the ‘schnorchel war’ was Alberich used on patrol, U480 being the first U-boat to enter combat clad in the rubber sheeting. After Förster’s enthusiastic appraisal of the material the decision was made to attempt to cover numerous Type XXIII U-boats with Alberich, though the first was not ready for service until February 1945 when U4709 was commissioned. It was suggested that the huge unfinished ‘Valentin’ bunker in Bremen be given over to Alberich fitting, though the plan was shelved. In total there were more Type VIICs that ended the war with Alberich coatings, despite the fact that for every one of its type covered, two and a half Type XXIIIs could be so treated.

More commonly used was Tarnmatte, a compound synthetic rubber and iron-oxide powder that coated the head of a U-boat’s schnorchel to absorb enemy radar waves, and which was claimed to absorb 90 per cent of waves emitted by the ASV Mk III airborne radar.

Förster and U480 would not make landfall until October, and his War Diary provides an interesting glimpse at the difficulties faced by U-boats compelled to remain submerged for long periods of time in transit to, from and within the combat zone:

12 September: 0511hrs. 300 miles west of Ireland. Surfaced for the first time in 40 days. The boat stinks. Everything is covered with phosphorescent particles. One’s footmarks on the bridge show up fluorescently … Schnorchel fittings and flooding slots also glow brightly in the darkness. Because of a high stern sea the bridge is constantly awash and the men cannot stand up on the slippery wooden deck; it is therefore impossible either to change or to dismantle the AA guns. The shields of the twin AA guns cannot be opened; the hinges appear to have rusted up and cannot be attended to in the dark. The 3.7cm gun is out of action; so shall first transmit my situation report and then proceed on schnorchel until the state of the sea permits me either to change the AA guns or dismantle them for overhaul below.

2nd October: 1710hrs. Off the west coast of Norway. Surfaced. The whole flak armament is unserviceable. The gun shields have been torn away from their mountings and are fouling the guns. Everything, including the 3.7cm gun, is corroded and covered with growth.

Balkon-Gerät – The Balkon-Gerät (Balcony Apparatus) was an improved version of (‘’Gruppenhorchgerät’’) GHG. The GHG fitted to early U-boats could not be used effectively at periscope depth. To solve this, a new listening device, known as ‘balkon’ (balcony) fitted to a second, lower hull, was successfully tested on U-194 in January 1943. Where the previous had 24 hydrophones, the Balkon had 48 hydrophones and improved electronics, which enabled more accurate readings to be taken. It was standard on XXIs but was fitted to some VIICs and VIIC/41s in 1944 and 1945.