The antenna of the FuMO–61 Hohentwiel; Type VIIs began to receive this in March 1944.
Type VIIC/41 with its snorkel and periscopes (the attack periscope is the taller one). There is no 88mm deck gun – by the end of the war Type VIIs only carried AA guns.
In 1857 Rudolf Hertz succeeded in generating electromagnetic waves from sparks, which in 1898 allowed Marconi to transmit the first wireless message to a distance of 14.5km (9 miles) – radio communications had been invented. But the concept of radar was of equal age. Hertz had already pointed out that radio waves were reflected by objects they encountered. This phenomenon was the subject of extensive research in military and governmental laboratories. Further inventions, such as the directional YAGI antenna and the electron tube, appeared, and when Dr Rudolf Kühnold combined these two devices to obtain an echo on the screen of an oscilloscope, the era of radar had begun.
In 1938 the Germans already had an effective radar system, codenamed Freya, which could detect a Junkers Ju 52 aircraft at 92km (50nm). In July 1939 the German antiaircraft artillery received Telefunken’s Würzburg A radar, a very advanced device with a power of 8 kW operating at a wavelength of 50cm. An additional rotating dipole increased its bearing accuracy to half a degree, and it gave a range accurate to within 100m (110yds). Later however, the British (in cooperation with the Americans) took the lead in this technological race and introduced radars superior to those of the Germans.
Originally U-boats were not equipped with radar. During the Battle of Atlantic, the convoy escorts, which had priority for radar equipment, were able to detect their opponents on the surface at night or in fog, whilst the U-boats were effectively blind. U-boat commanders initially punished their lookouts who ‘failed to spot’ attacking aircraft, but the men were not guilty as the aircraft could ‘see’ their targets through cloud cover with their on-board radars – initially the ASV Mk II type, later superseded by the Mk III and finally by the Mk XII, which initially could not be detected by the German Metox radar warning devices. It was a technological race – the Allies kept developing improved radars, while the Germans constantly worked on improved countermeasures. The Germans were lucky in this area, though it took some time until they realised this. Before the war a French inventor, Metox Grandin, developed a device for monitoring radio signals with wavelengths between 1.3m and 2.6m, known after him as the Metox, which proved capable of detecting radar signals with the same wavelengths. The device was simple and had a wooden cross-shaped support for the antenna. This antenna, combined with the bad reputation of the Bay of Biscay – the U-boats’ graveyard – earned it the nickname ‘Biscaya-Kreuz’ (‘Biscay Cross’). It was a simple passive radar of the FuMB NVK R600 A type, produced by the French companies Grandin and Metox, fitted with the FuMB Ant 2 antenna. However, extensive tests showed that it could actually betray the position of a U-boat fitted with it, as it itself emitted some radiation. An aircraft flying at an altitude of 500m (1650ft) could detect a Metox at 22km (12nm), at 1000m (3300ft) at 33.4km (18nm) and at 2000m (6550ft) as far away as 46.3km (25nm).
The Metox was liked by the U-boat crews as it helped them to survive – some commanding officers would even abort their patrols if the Metox malfunctioned – but it was disliked because it was terribly cumbersome to use. After the boat surfaced the antenna – connected by a long cable to the receiver – had to be set on the bridge, and then regularly rotated by 90° to eliminate ‘blind’ sectors. It also had to be dismantled before the boat dived, as the cable would prevent the hatch from closing. The Metox could detect radar signals in the 62 to 264 HZ band at a range of up to 80km (45nm). Changes in signal amplitude gave the source’s bearing – it increased if the aircraft was closing and decreased if it was moving away.
Metox sets began to be installed aboard U-boats in August 1942, but they were rendered useless when the Allies began to use centimetric-band radar, which they could not detect. It was proved that the Allies were using these short-wave radars when a petty officer aboard U 124 installed an oscilloscope aboard his boat that could detect these wavelengths when the Metox could not. New radar-warning receivers were therefore introduced. From autumn 1942 U-boats were equipped with the FuMB4 Samos with a wavelength of 157–333cm in the 87–70 MHz band. In April 1943 this was replaced by Bali with a FuMB Ant–3 dipole antenna. August 1943 brought the new detectors FuMB8 Zypern (W. Anzg. 1), FuMB9 Zypern 2 (wavelength 118–192cm, frequency 156–254 MHz, with the Fu MB Ant 3 Bali and Timor antennae) and FuMB 10 Borkum (wavelength 75–300cm, frequency 100–400 MHz with the Bali and FuMB Sumatra antennas and the FuMZ 6 amplifier). Initially German scientists claimed that it was impossible to construct a radar operating on a 10cm wavelength, but analysis of U-boat logbooks by Konteradmiral Ludwig Stummel, head of the naval radar department, lead him to deduce that such a device had to be being used by the enemy. A British radar retrieved from an aircraft shot down over Rotterdam – and therefore nicknamed by the Germans the ‘Rotterdam Gerät’ – confirmed his deduction. Therefore a new detector, the Naxos FuMB7 was introduced. Its range was 5000m (27mm) and it was equipped with the manually-rotated FuMG Ant 1X antenna. The equipment was supplied by NVK Telefunken and installed on U-boats from October 1943. In March 1944 the Naxos was combined with the FuMB antenna into a device codenamed Cuba 1 or Fliege. Its range was 20km (11nm) and it could detect radars operating on 8–12cm wavelengths. A German response to Allied 3 cm radars was the Mücke antenna providing a detection range of up to 50km (27mm). In 1944 the newly developed FuMB 35 Athos antenna was added to the Bali device resulting in a new radar warning device called FuMB 37 Leros.
These radar-warning devices certainly reduced the effectiveness of enemy ASW forces, but they did nothing to help the U-boats in their own attacks. Although they kept improving existing equipment, the Germans neglected development of radar and fell well behind the British, who became the leaders in this field. Both jammers and decoys were developed in the sonar field, but only radar detectors. The German navy was particularly poorly equipped. It was not until Dönitz and his chief of signals met with representatives of industry and scientists that he realised that they had no idea about requirements of the navy in general and the U-bootwaffe in particular, and he took this opportunity to make them aware of what the Kriegsmarine was actually doing and what it needed. As we have seen, it was not until January 1944 that the Germans realised that the Allies were operating on ‘impossible’ centimetric wavelengths, and this produced the Naxos detector, but there were still no search radars available for the U-boats. There had in fact been some development of naval radar and several types of search radar had been designed, but these were all fitted aboard large ships, U-boats not coming into the programme until much later. As we know, no earlier than in January 1944 did the Germans realise that the enemy radars were operating on ‘impossible’ centimetre wavelengths. The detector response was Naxos, but search radars remained unavailable for U-boat commanders. That said, it cannot be stated that there was no development at all in this area. Indeed, there were radar-related experiments in the Kriegsmarine, and several search radar models were actually designed, but all those were tested aboard large warships. U-boats were only included in the programme quite late. The first to carry Dete Gerät radars supplied by the Gema company were the larger Type IXA boats. This set was quite bulky and heavy, provided only a 30° sector of observation, had a range of 7000m (3.7nm) and was rather inaccurate (bearing error ± 2°). After tests with the Dete Gerät aboard U 39 and U 41 (both Type IXAs) in the autumn of 1941 similar set were installed aboard on other Type IX submarines. In 1942 some Type VIIs finally got them as well. The model used in this case was Dete Gerät 500, now redesignated FuMO29, operating on a 82cm wavelength. The first boats to carry this radar were U 623 and U 231.
The FuMO29 used an array composed of two rows of dipoles, six in each, installed in the forward part of the conning-tower. Therefore in order to scan the entire horizon the boat had to carry out a full 360° turn. In 1942 a new 1000 × 1400mm (39 × 55in) antenna with two rows of four dipoles each was introduced – this version was called FuMO30.
In 1943 new parabolic antennas installed in the after part of the tower were introduced. In 1942, despite opposition from the Kriegsmarine designers, the FuG 200 Hohentwiel aircraft radar also started to be installed aboard U-boats. This set was developed by Dr Christ and Dr Müller from the FMG 40 Kurmark model. It operated at a frequency of 550 MHz, wavelength 56cm, and had a power of 30 kW. The naval version of this device was called FuMO61 Hohentwiel U. Its range was 10km (5.4nm) and distance accuracy was 300m (1000yds). It was tested aboard U 763 (Type VIIC).
The process of equipping Type VIIs with radar proceeded very slowly due to the difficulty German industry had with manufacturing radar equipment due to damage done by air-raids. By September 1944 only sixty-four U-boats had been equipped with the FuMO64 set. New variants of this radar set were successively introduced, e.g. FuMO65 Hohentwiel U 1 (frequency 110–160 MHz or 25–575 MHz) which provided sectoral or panoramic displays. In late 1944–early 1945 testing of the FuMO84 Berlin U 1 began – the new design operated at 9cm wavelength and a frequency of 3300 MHz. It had power of 20 kW and its antenna was installed on a watertight mast which could be raised from the conning-tower.
The air aboard a U-boat was consumed both by the diesel engines and the crew themselves. Carbon dioxide could be removed by special absorbers and some of the compressed air could be released inside the boat, but this was only done in dire emergencies and the amount of air released was relatively small. The boat could only be properly ventilated on the surface, until the development of the snorkel allowed the boat to ‘breathe’ while submerged.
The snorkel was invented in 1933 by a Dutch Lieutenant, Jan Jacob Wichers, who later collaborated with Lieutenant J C van Pappelendam. It was first installed for trials aboard the Dutch submarine O-21 in February 1940. When the Germans conquered the Netherlands later that year, the O-21 fell into their hands, although the snorkel did not enter service aboard U-boats for several years. Development of the German snorkel (in German: ‘Schnorchel’) was inspired by Professor Walter with Dönitz’s support and carried out by the engineers Ulrich Gabler and Hepp. The original design consisted of two concentric pipes which could be raised and lowered like periscopes. The intake pipe was equipped with a valve with a ball-shaped float to prevent accidental flooding. The other was used to vent the exhaust gases, and had no such safety device. By June 1943 the blueprints were ready and the first boats were selected for testing – U 57, U 58 and U 236. Trials revealed that the cut-off valves did not operate properly, so U 235 and U 237 – the next boats earmarked for the snorkel development programme – were equipped with an improved model. The new device had a diameter of 220mm (8.66in) and a total length of 8300mm (326.77in). The exhaust was located 1300mm (51.18in) lower than the air intake, to prevent the gases just being sucked back into the boat.
By July 1944 only 120 snorkels had been installed aboard Type VII and Type IX U-boats. The commanders of the upgraded submarines invented a quite interesting, non-combat application for their equipment. Running on diesels, they would order the air intake valve to be partially closed, so the engines quickly sucked the air from the boat. Then the valve was fully opened again and the fresh air rapidly filled the entire interior of the submarine, thus ventilating the boat much more quickly.
The snorkel turned U-boats from submersibles into true submarines. They no longer had to surface to run the diesels and charged their batteries or to ventilate the boat. But the fitting of snorkels did not begin until 1943 and was painfully slow thereafter, as the equipment was not available in sufficient quantities. Even if it had been, fitting it required a major reconstruction of the boat, meaning it would have to be withdrawn from service for a considerable time and also increasing the workload in the already overburdened shipyards, which were barely able to keep up with ordinary maintenance requirements
The Type VII U-boats were equipped with a snorkel composed of two concentric pipes with a hinged base, allowing it to be lowered and laid flat in a special cradle, forward of the conning tower. Its use was restricted by the height of the waves and the rolling of the boat when cruising at periscope depth. Rough seas could close the intake valve, causing the diesels to start to suck the air out of the interior of the boat, the drop in pressure causing discomfort to the lungs and ears of the crew. The speed of the boat was also restricted. Theoretically the snorkel should have allowed the boat to achieve the same speed as it would have done on the surface, but in practice it could make only 6 knots, just what it would have done on the electric motors. But it did provide the undisputed advantage that the boat could now remain at periscope depth for prolonged periods, without being forced to surface.