Allied Countermeasures against the snorkel-equipped U-boat II

Allied Surface Operations

From the start of the war until mid-1944 BdU did not believe that U-boats could patrol effectively along an enemy’s coast. This self-imposed operational limitation was due to the belief that Allied aircraft combined with the critical limiting factor of a U-boat’s requirement that it had to surface in order to recharge its batteries simply prevented extended coastal operations. Just prior to the Allied invasion of Normandy the Admiralty conducted an assessment of the potential for U-boats to operate in the shallows, titled ‘Inshore Operations by U-boats’, where they concluded that once a U-boat conducted an attack in the shallows it would immediately proceed to deeper water in order to withdraw. The idea that a U-boat would ‘bottom’ at the scene of the attack, or move closer inshore, was ‘considered unlikely’ by the Admiralty. This view changed before the end of the war.

The Royal Navy’s ‘support group’ was the corollary to the US Navy’s ‘hunter-killer group’. The number of support groups in the North Atlantic grew to seventeen Royal Navy and seven Royal Canadian Navy by November 1944, the predominant number being assigned to the critical area of the United Kingdom’s coast. Despite the advantage held by the Allies in the number of anti-submarine surface vessels operating in familiar home waters equipped with state-of-the-art search gear, they found that hunting a snorkel-equipped U-boat in the shallows was difficult.

The first Allied experience against a snorkel-equipped U-boat in coastal waters occurred in the English Channel in the days immediately following the Normandy invasion. One particularly graphic account is provided by Lieutenant Commander Allan Easton (Distinguished Service Cross recipient), who commanded the destroyer HMCS Saskatchewan at the time. Operation Neptune was the code name for the Allied protection of the invasion area from U-boats operating in the English Channel. HMCS Saskatchewan was operating as part of 12th Escort Group in that operation. On 7 June, one day after the Normandy invasion, Easton’s ship was on escort duty accompanying four other vessels when a ‘low rumble was heard, the unmistakable sound of an underwater explosion’ at around 8pm. No one knew exactly what it was. The flag went up from the other escort communicating that it was searching for a contact it made via ASDIC. ‘Action stations’ was now called onboard HMCS Saskatchewan. A search began and a salvo of hedgehogs was fired that exploded without hitting any target. Three destroyers were now searching for the original contact with active ASDIC. The ASDIC officer onboard Easton’s destroyer, Sub-Lieutenant Coyne, noted ominously that: ‘I think the Channel is not going to be an easy place to locate submarines.’ He elaborated on his statement to Easton: ‘In the few days we’ve been in these waters, the H.S.D. [Higher Submarine Detector] and I have been watching the sound effects and so on of the ASDIC and they’re very confusing. Fish appear to be very plentiful. The tides may have something to do with it. To my mind the outlook isn’t particularly bright.’ He concluded his observation, while observing four destroyers now searching for a U-boat, that: ‘It’s not like the clear water of the mid-Atlantic, sir. There’s the place for good ASDIC condition!’ At that very moment an explosion was heard, immediately followed by a column of water that shot up into the air a mere 100 yards abaft Eaton’s destroyer.

As Easton observed the area of the explosion there were no track marks left by a typical torpedo’s stream of bubbles in the water. He correctly concluded that a defective electric-drive acoustic torpedo had self-detonated just four seconds before hitting their stern. As was the custom in a U-boat hunt, the destroyer’s loudspeaker was switched on as it was connected to an ultra-short range telephone on the bridge. It was assumed that the acoustic torpedoes had a short range so everyone was alert searching the immediate area. Eight minutes after the first explosion a report came over the loudspeaker from one of the other destroyers: ‘Torpedo just passed down starboard side running very shallow leaving slight swirl behind it. Heard on hydrophones. No explosion.’ The ASDIC operator on a nearby destroyer picked up multiple contacts, then almost immediately dismissed them as fish. The same false signals were heard by Easton’s ASDIC operator. Then a third destroyer picked up something slightly different and turned slowly to starboard to investigate when the slender pipe of a periscope broke the surface, just abaft to starboard.

The periscope was indeed from a U-boat operating within 20m of the surface among four searching destroyers. Seeing the destroyers, the U-boat retracted its periscope and began evasive manoeuvres that quickly broke contact. Twenty minutes later a hydrophone operator noted that he distinctly heard blowing tanks. HMCS Saskatchewan quickly turned and gave chase, letting loose a salvo of depth charges, but they did not hit the mark. After the first detected torpedo the crew had dropped a trailing noise maker into the water that was designed to detonate acoustic torpedoes. It was a good thing, because no sooner had the depth charge explosions stopped echoing over the hydrophones than another explosion abaft of Easton’s destroyer was heard, accompanied by a column of water 100ft high. Another acoustic torpedo had been detonated by the noise maker.

Oberleutnant zur See Heinz Sieder (1920-1944), Kommandant Unterseeboote “U 984”, Ritterkreuz 08.07.1944

The U-boat Eaton encountered was U-984 (VIIC), which reported firing two T5 torpedoes and a LUT acoustic homing torpedo at a group of three destroyers on that day. Oberleutnant Heinz Sieder claimed ‘one missed, one detonated too soon, one probable hit after 7 minutes’. On both attacks against HMCS Saskatchewan Sieder set the running depth of the T5s at 4m. Both were launched at periscope depth. After the third torpedo, Sieder ordered the U-boat to bottom in order to reload the tubes. He believed that an hour later he was accurately located on the bottom by two destroyers that bracketed him with depth charges. While the destroyers had detected an ASDIC return they did not know it was U-984. The lights flickered onboard the U-boat and sea water began to leak into the bow compartment, but the boat withstood the concussions.

U-984 remained bottomed until the afternoon of the 8th as the destroyers criss-crossed the area. Sieder knew the destroyers were utilising a well-drilled tactic to starve the U-boat of oxygen and battery power in order to force it to the surface, where it was vulnerable. However, U-984 was a snorkel-equipped U-boat. At noon, Sieder decided to lift off from the bottom and move closer to shore, to the area of Ushant, France, where he planned to snorkel to recharge batteries and refresh oxygen. He noted in his KTB that ‘the air condition was exceptionally bad in the last 12 hours. The men literally gasped for air. A certain amount of relief was provided by breathing with potash cartridges. (Mouthpiece of the escape lung connected).’ By 9pm U-984 was snorkelling close to the coast. Sieder then decided to return to Brest, where he could recharge the batteries in a long snorkelling run, instead of the two it would require in the operational area.

U-984 returned to Brest on the 10th and after some minor repairs went back out to the operational area two days later. Sieder’s second foray into the English Channel was an extraordinary example of the snorkel’s new ability. He penetrated the Allied defensive screen and reached the invasion area off the Cotentin Peninsula. In the five days from 25 to 29 June he sank a British frigate, HMS Goodson, and subsequently sank three vessels and damaged one off the invasion beaches of Omaha and Utah. The Allied vessels were from Convoy ECM-17 and carried military equipment and troops for the invasion. His total tonnage for this patrol was 30,090 GRT. BdU noted in Sieder’s KTB after it was turned in: ‘Exemplarily executed enterprise carried out with high attack spirit. The Commander carried himself on 7.6 and 25.6 with iron calmness and toughness towards the destroyer and took advantage of every opportunity for attack. The attack on the convoy on 29.6 was a tactical masterpiece with magnificent success. The experiences of the boat are valuable.’ Sieder was awarded the Knight’s Cross on 8 July 1944 for his patrol.

Back on the surface, the rest of the night and following day were quiet. Then at around 7.30pm on the 8th a deep underwater explosion was heard and the previous night’s attack and counter-attack with a U-boat was repeated. Torpedoes were fired, missed their target or prematurely exploded, followed by ASDIC contacts, depth charges, and in Easton’s words, ‘dead or unconscious cod rising to the surface’. This U-boat, if it was an actual U-boat, was not U-984 as Sieder had already departed the area. If the encounter had occurred in the mid-Atlantic in 1943 the U-boat would have probably been destroyed in short order as clear ASDIC returns would have been acquired. Here in the shallows, four destroyers could not zero in on a U-boat due to the benefits gained from the rocky bottom, thermoclines and current.

The next afternoon Easton’s squadron fell in with six British destroyers that formed a line of ten abreast that began to search for U-boats. This was a formidable defensive line at the western end of the English Channel. The weather was overcast, with a rain squall, though visibility was still good at 3 miles. At some time after 5pm a puff of smoke was seen in the distance. Two destroyers began to turn towards the smoke, with one opening fire from its deck gun. Easton recalled:

On that instant I knew exactly what she was firing at. I altered course towards the smoke and rang for twenty-five knots as I saw our neighbour turning, too. A minute later the smoke had disappeared and nothing whatever was to be seen. What we had observed was a new German invention in operation, an invention with which we were acquainted but not familiar. It was the snorkel …

Easton and his fellow commanders were briefed on the German secret invention before the invasion. He recalled that in his briefing it was noted that the snorkel was to allow U-boats to operate in British coastal waters by allowing it to recharge batteries without surfacing. The report went so far as to note that it was equipped with a radar antenna on the snorkel head that would allow it to identify aircraft and surface ships. However, Easton noted: ‘It did not occur to us that a [snorkel] would smoke.’ This fact was kept as an Allied secret, as previously noted. Not surprisingly, the U-boat evaded all ten destroyers.

Improved detection of a snorkel-equipped U-boat was the Allies’ first priority. The snorkel evolved U-boat tactics to a degree that traditional detection techniques were rendered nearly irrelevant overnight. Usual detection sources such as the HF/DF of wireless signals, surface radar returns, or visual sightings of a surfaced U-boat could not be relied upon. ASDIC was only effective at short range and when used in shallow coastal water could not effectively discern rock from wreck from U-boat. During the convoy battles of previous years ASDIC had proved effective in maintaining contact with a U-boat after an initial detection, often from a visual source such as a surface vessel or aircraft that subsequently vectored in an escort vessel. From mid-1941 until mid-1943 most U-boats were initially detected through radar or visual identification on the surface, not by a chance ASDIC contact.

As BdU shifted focus from open-ocean convoys to specific coastal embarkation/debarkation routes, so did the support groups. Support groups were assigned coastal patrol areas with the intent that they learned all the sub-surface ASDIC signature returns given off by the shallow sea floor and various wrecks. This was believed to be a critical factor in the future success of hunting U-boats as the knowledge gained of the bottom conditions made the difference in distinguishing a U-boat sonar signature from a known sub-surface anomaly.

In August the Admiralty conducted a preliminary analysis of the problem that continued to hold true for the remainder of the war. Through the use of Ultra intercepts, analysis showed that search groups had probably passed over U-boats ten times, but made only one contact. The U-boat on the other hand, had detected 65 per cent of the escorts that passed within 3 miles of them by using their own hydrophones. At this range a U-boat had an excellent chance of slipping through the search screen or manoeuvring around their flank.27 Captain R Winn came to the judgement at the end of August that a U-boat was able to remain ‘submerged for up to ten days without presenting any target detectable by radar or visually except at short range’.

U-boats operated with two main weaknesses prior to the introduction of the snorkel. First, a U-boat could not remain submerged for more than twenty-four hours without surfacing completely to recharge its batteries. Second, when operating in the deep open ocean of the North and Central Atlantic, a U-boat’s sonar signal was often clear and distinct. Allied escort vessel tactics prior to the introduction of the snorkel were to leverage these two weaknesses in a ‘hunt to exhaustion’ whereby the U-boat, once identified through ASDIC, would be pursued until it was compelled to the surface after a loss of electric power caused by drained batteries. Once on the surface, the U-boat could be destroyed by surface gun fire or ramming. This tactic worked only when a U-boat was identified by ASDIC and contact maintained. In the shallows, density layers, thermoclines, currents, rocky bottoms, other wrecks as well as the tactic of bottoming for long periods, significantly diminished the effectiveness of ASDIC. Additionally, a U-boat only had to raise its snorkel mast just a few metres above the surface to recharge its batteries or suck in fresh air required for it to run full diesels and escape at high speed, thereby presenting the Allies with a very difficult target. Allied tactics had to change.

In September 1944, after four months of hunting U-boats in the English Channel, the Admiralty changed their conclusion on U-boat tactics. They now put out guidance that when a ship was torpedoed in waters where a U-boat could bottom effectively, that it would do so if immediate anti-submarine attacks occurred.29 They began to realise that a U-boat no longer attempted to run for deep water; its best defence was to hide in the shallows. By October 1944 the Admiralty issued new guidance:

U-boats can now operate inshore and are likely to adopt static tactics in place of the mobile tactics which we have been used to dealing with. Static tactics involve the use of curly and gnat torpedoes fired from U-boats which endeavour to lie in wait on the course of convoys. When no targets are available U-boats are likely to move with great caution and charge by snort [i.e. snorkel] mainly by night. On approach of a hunting force [the U-boat] will probably bottom or may drift with tide near bottom.

In addition to ASDIC, another source of submerged U-boat identification was through the sound emitted from inside the vessel. The Admiralty was interested in learning if there were ways to improve U-boat identification through hydrophones. The Admiralty Research Laboratory in Teddington, Middlesex, published a classified study on 22 February 1945 titled Supplement to the Detection of Schnorkel by Hydrophone. This was part of a series of reports designed to show the ‘probability of recognition of Schnorkel over a yearly cycle on various hydrophone systems’. As a baseline the probability model was based on a snorkel-equipped U-boat charging its batteries at a regular snorkel depth cruising at 2–3 knots. A subsequent report considered propeller noise in the calculus among other operational conditions, as well as practical results from test trials being carried out. What is interesting is how limited the report was given that it was nearly one year since the first snorkel-equipped U-boat became operational. The analysis factored in the rule that pressure is inversely proportional to distance, as well as the impact of temperature gradients based on the report ‘Sound Beam Patterns in Sea Water’ issued by the Woods Hole Oceanographic Institute, Massachusetts, on 10 October 1944. This latter point is interesting as it shows how little the Admiralty understood the impact of temperature gradients, especially now that the U-boats had moved back to operating inshore.

The study looked at all hydrophone and sonar systems across the spectrum of distances. It was based on depths of 150ft and 900ft, with the former having a bottom reflection coefficient of .3 and the latter being evaluated with a bottom reflection coefficient of both .3 and .7. All probabilities were modelled across sea states 1–4. Looking at a sea state 2, in 150ft of water with a bottom reflection coefficient of .7, we see that in order to have a 60 per cent chance of picking up a snorkelling U-boat that is charging its batteries, a 50ft vertical rod hydrophone has the best chance at 14,000 yards, followed by 15ft ASDIC at 10kc/s being 6,000 yards, 15ft ASDIC at 20kc/s being approx. 4,200 yards, a non-directional hydrophone suspended from a sonobuoy being 3,900 yards, and finally a non-directional hydrophone suspended from a ship being less than 2,000 yards. A calm sea would give greater distances, while a sea state 4 would reduce them significantly. These averages were compounded in shallow water by the extensive noises heard from a wide range of other vessels and differing gradients. The study was based on British submarine diesel noise trials. The results were not promising and confirmed the difficulty of submerged U-boat detection in coastal waters, already well known by support group commanders.

In addition to the use of technology, new search patterns were adopted by the search groups designed specifically to find a bottomed U-boat. These patterns were ‘Scabbard’ and ‘Artichoke’. Both made the assumption that a U-boat would in fact bottom near the torpedoed vessel, bow pointed in the direction of tide. The search patterns were designed to improve the chances of ASDIC detection by hopefully encountering the bottomed U-boat abeam. However, it is not clear if these patterns were routinely employed in practice as the tactical conditions of a U-boat encounter were often highly varied.

While the British wrestled with U-boat detection in coastal waters they also introduced a series of defensive measures designed to blind and confuse U-boats, which now operated almost exclusively in an underwater world of sound. Code-named ‘Foxer’, this noise maker was towed behind an Allied ship and was originally intended to confuse the German G7 acoustic homing torpedoes introduced in 1943–44. These were turned into buoys in the autumn of 1944 and employed in a static fashion around harbours and close-in waterways where Allied vessels manoeuvred to blind U-boats operating in these areas. BdU issued guidance regarding this Allied tactic to all U-boats on 3 January 1945:

It has been determined that the enemy has noise-producing buoys which reproduce the noise of a buzz saw. Apparent purpose: (1) Attempt to deflect Zaunkönig (2) Blocking our hydrophones. Details on the buoy not yet known. The following observations are important and are to be reported: (1) Appearance of buoy, if it can be determined by chance observation through periscope. (2) Length of the towing connection. (3) Does the buzz saw noise also occur in noise-producing buoys? (4) Is the noise produced by the movement of the current or by a special mechanism?

It was clear that BdU did not completely understand the buoy’s tactical use. This tactic was employed some six months after the introduction of the snorkel and the inshore campaign, highlighting how long it was taking the Allies to adapt to the new U-boat tactics.

A post-war analysis of the inshore campaign revealed the Allied struggle with locating and destroying U-boats. In the period from July to mid-September 1944 twenty-six ships were torpedoed. During that time only three U-boats were found and located immediately by Allied support groups after a torpedoing. From the period mid-September to December 1944, where U-boats were completing their evacuation to Norwegian bases and snorkel retrofittings were being completed on the existing operational boats, the number of U-boat patrols against the British Isles decreased. However, three Allied ships were sunk with no U-boats located after the attacks. From mid-December 1944 until February 1945, twenty-three Allied ships were torpedoed with only three U-boats sunk immediately after the reported attack. This period of the inshore campaign proved the most effective for snorkel-equipped U-boats and the most concerning for the Allies. The final period was from mid-February until May, 1945. This period saw a marked increase in operational U-boats sent against the British Isles, which resulted in thirty-five Allied ships torpedoed and eight U-boats identified and sunk shortly after the attack. This suggested that Allied tactical improvements were proving effective, though the number of operational U-boats deployed also increased at the same time. During these four periods the number of U-boats sunk by aircraft proved dismal, with only one each for the first three periods and six in the fourth.

There were other U-boats sunk around the British Isles during these four periods of analysis, however, they were identified by chance. In the first period there were eight sunk, followed by two, one and twelve. These numbers reflected a combination of contributing factors that ranged from some U-boats transiting British waters without a snorkel in July–September on their return to Norway to a large increase of U-boats sent on patrol at the end of the war.

The Admiralty understood that the chance encounter provided the best opportunity to locate and sink a U-boat. This is revealed by the fact that out of thirty-seven U-boats sunk during the inshore campaign by surface vessels, only eleven were destroyed by surface ships escorting or supporting a convoy, while the other twenty-six were targeted by patrolling support groups. Most U-boats sunk in the inshore campaign were encountered en route to their patrol areas. Put another way, by April 1945 U-boats that reached the British coast found themselves in a very effective position. The Admiralty assessed that their escorts only had an 8 per cent chance of detecting a U-boat as it approached a convoy in coastal waters.35 The key was interdicting the U-boat before it reached the coast.

Acoustic Camouflage

The Admiralty did take note of the use of rubber coating of U-boats during the war. Survivors of U-485 were interrogated about this technology. The Royal Navy interrogator noted that the: ‘Rubber covering was against our ASDIC. His was one of the first boats to be so fitted, and he personally had had insufficient experience to be able to assess its value.’

In the autumn of 1944 the Admiralty took serious note of the development. In a report dated 21 November 1944, it reviewed the possibilities of what the rubber was used for and concluded: ‘In general, it is considered that there is a possibility of reducing echo strength from a submarine by protective coatings, and that any development along these lines must be carefully watched.’

The November assessment was indeed correct that developments had to be ‘watched’ as Alberich reduced the ASDIC effectiveness and gave U-boats a marked advantage when operating in coastal waters.

Strategic Countermeasures

Air raids were employed as a strategic countermeasure to U-boats. Starting in the spring of 1943 and lasting through to the end of the war, Allied air raids hit German shipyards and production centres. The result was that most shipyards experienced work stoppages of three to four weeks that delayed the VIIC construction and operational readiness of existing boats. In the last twelve months of the war air attacks reportedly contributed to preventing some 150 Type XXIs from being built. Diesel production was hit hard when a raid on the factory in Augsburg delayed work for nearly four weeks in June 1944. Battery production at Hagen and Hanover was paralysed by air attacks, causing a scarcity in them after February 1945. It was determined that due to this shortage, only half of the ordinary number of batteries required for the Type XXIs would be installed after December.

Despite the strategic bombing campaign, more U-boats were at sea or operationally ready in May 1945 than at any point during the war.