The Royal Navy and Anti-Submarine Warfare, 1917-49

Malcolm Llewellyn-Jones

• Publisher: Routledge
• Year: 2005
• ISBN:
  9781134172726
• Edition: 1
• Topic: History
• Subtopic: Military & Maritime History
• Index: History

An essential new account of how anti-submarine warfare is conducted, with a focus on both historic and present-day operations. This new book shows how until 1944 U-boats operated as submersible torpedo craft which relied heavily on the surface for movement and charging their batteries. This pattern was repeated in WWI

1 Echoes from the past, 1917–40

British anti-submarine warfare, 1917–40

The U-boat of the First World War relied on surface travel for tactical mobility and for searching for its targets, where using their diesel engines they could achieve 18½ knots in good conditions. They would submerge when threatened by antisubmarine (A/S) forces or, sometimes, in rough weather. Once underwater, the Uboats relied on their batteries and electric motors for propulsion, which gave them a top speed of about eight and a half to nine and a half knots but only for an hour. At, say three and a half knots, however, the U-boat’s underwater endurance was about 24 hours. Charging the batteries could only be done on the surface when the diesel engines could be run, so these boats were in every sense ‘submersibles’.¹ While submerged the U-boat’s range of vision through the periscope was restricted. The type and rough course of a victim could be distinguished at about six or seven miles (in reasonable visibility by day) but this required a considerable exposure of the periscope.² The periscope was, of course, normally used only intermittently. It is not surprising, therefore, as the Naval Intelligence Division (NID) noted, Uboats preferred to remain on the surface, unless forced to dive by anti-submarine patrols. On the surface their visual horizon was much extended, they could keep their batteries fully charged, and they were able to manoeuvre into an attacking position ahead of a convoy before diving to attack unseen. The search was considerably extended when two U-boats operated in concert, spread at rightangles to the convoy’s track. Such tactics had been noted, although there was no positive evidence of greater numbers working together. With the U-boats widely spread, they had to rely on wireless telegraphy (W/T) for passing sighting information, though, even with this advantage, it was not supposed that U-boats could easily deliver simultaneous attacks. The disadvantage of the method was that the British were able to intercept the radio transmissions, and by applying direction-finding (D/F) techniques, warn the convoy of a U-boat in its vicinity. The shore authority (especially when the enemy’s signals were decrypted) might also be able to direct an anti-submarine patrol vessel to the U-boat’s rough location.

The U-boat made frequent use of her periscope to assess the relative movement of her target during the final dived approach. Inside about two miles she would be wary of over-exposing her periscope, for fear of it being seen by an escort and the U-boat counter-attacked. The ideal submerged torpedo attack was aimed at a specific ship in the convoy at a range of 500–1,000 yards.³ With her restricted underwater endurance, a U-boat could only get into a firing position from a relatively narrow angle ahead of the convoy. If the initial sighting was made on the beam or quarter of the convoy, the U-boat had to race on the surface around the convoy until she gained a position ahead of the convoy where she could dive to make a covert final approach. The further ahead she was, the further off the convoy’s track the U-boat could afford to be. Subject to the restricted underwater speed and endurance of the U-boat, she could therefore start from positions within what were known as ‘limiting lines of submerged approach’ (LLSuA), which for a seven and a half knot convoy were angled about 40º off each bow of the convoy. Dived attacks could only be made in daylight, or on nights were there was strong moonlight, for on a dark night, when a U-boat was practically blind, she would at risk of being run down by a convoy. However, the Admiralty’s Anti-Submarine Division noted by October 1917, that on moonlit nights submarines would probably operate on the surface with very little buoyancy, which would make them very difficult to see, even as close as 400 yards. When night attacks were made, it was thought unlikely that the U-boat captain would get between the lines of the convoy, for fear of being run down. Ideally, he would fire from the flank, aiming at an individual ship in the convoy. More often attacks were made at long-range from outside a strong escort (and known as ‘browning’ shots fired in the hope of hitting any ship in the convoy).⁴ The general concept for convoy escort dispositions was, where possible, a line of escorts, spaced a mile apart, across the front of the convoy at a distance of 600–800 yards. By zig-zagging, these escorts would provide a physical obstruction to U-boats about to fire at close range. Escorts would also be stationed on the flanks and, where sufficient forces were available, one or more were placed astern where they could respond to a torpedoing with a broadcast barrage of depth-charges, as the remainder of the convoy cleared the area. An escort astern was also used to deter a shadowing U-boat by forcing it to submerge as the convoy made an evasive turn just after dusk.⁵

Anti-submarine escorts fitted with hydrophones were able to listen for the hydrophone effect (HE) from the U-boat’s propellers, principally caused by the collapse of the cavitation bubbles created by a rapidly spinning propeller, and, to a lesser extent, by the noise made by the flow of water past the hull and from internal machinery.⁶ Listening in the vicinity of surface ships was unproductive, since their hydrophone effect was likely to drown any noise from the U-boat. When a convoy was attacked, escorts had no means of detecting submerged submarines other than by sighting their periscopes, the trail of oil often left by Uboats, or the water disturbance from a torpedo’s discharge and subsequent track.⁷ But given a sighting, retaliation could be instant on the part of the escort, and had to be if they were to stand any chance of getting a kill because the U-boat would inevitably move away from its last reported, or datum, position. Depending on the depth of water, the U-boat would either go deep at slow speed, or rest on the bottom, to avoid anti-submarine forces hunting them with hydrophones.⁸ The blind barrage attacks by the escorts would not be improved until the introduction of the technique of transmitting and receiving a beam of ‘supersonic’ sound pulses from an underwater acoustic projector (known as the transducer or oscillator). This equipment was known in Britain as ‘Asdic’, and ‘Sonar’ in America. This seemed the most promising device of several being experimented with. When the pulses struck the hull of a U-boat an echo was returned. By timing the interval between the transmission and reception of the sound, an accurate range could be calculated, and by noting the direction of the oscillator, a rough bearing was achieved. Experiments with this method had been carried out since the middle of 1917, but it was only towards the war’s end, that seven RN ships were fitted with this gear. In the meantime, several anti-submarine ships working as a team would lay a blind barrage of depth-charges over the suspected position of the U-boat.⁹ Initially, escorts were equipped with only four depth-charges, though this outfit was later increased, with individual vessels carrying 30 depth-charges. Up to 40 depthcharges were expended by hunting groups against individual submarines. Escorts were also used on ‘extended patrols’ at, say, 11 miles, specifically designed to interfere with U-boats on the surface trying to overhaul the convoy. With their greater range of vision and speed, aircraft often replaced surface escorts in this role.¹⁰ Experiments were made with the fitting of searchlights and parachute flares to aircraft in order to detect U-boats on the surface at night. These searchlights were, however, the primary means of detection (not of final attack, as with the later development of the Leigh Light in WWII). Use of the light, therefore, was more likely to warn the U-boat, which could dive before the aircraft had any chance of making contact. The lethality of aircraft attacks, whether by night or day, left much to be desired.¹¹ Although attacks on convoys had occurred when aircraft were present, U-boat operations were seriously hampered by the constant fear of being sighted by aircraft, for apart from the U-boat, or its periscope being seen, the tracks of its torpedoes were clearly visible from the air. With their low underwater mobility, the U-boats might not be able to get away from the tell-tale beginning of the track before surface escorts arrived to counter-attack. As a result U-boats refrained from attacking convoys with air escort.

The Admiralty’s small Historical Section had only been able to narrate the first six months of the main German First World War unrestricted U-boat campaign by 1939. Nevertheless, they noted that defeat by the U-boat was averted principally by the introduction of convoy. It was expected that delays due to convoy assembly, and sailing at the speed of the slowest ships, would reduce the carrying capacity by 12 to 20 per cent. However, ‘… if the situation was serious enough to require a convoy system’, the Admiralty realized, after the war, ‘no reduction in carrying capacity might be involved, compared with other methods of trade protection.’¹² Once the decision to institute convoy had been made, its implementation was delayed by administrative difficulties. By July 1917, 90 per cent of the losses continued amongst independent shipping, although gradually the ‘… proportion of ships in convoy was increased until practically the whole of the traffic was included.’¹³ One advantage of convoy that was immediately obvious was that ships in convoy ‘… could be kept in touch with the latest intelligence.’¹⁴ Both the Official and Staff Histories wondered whether the efficacy of convoy lay ‘… rather in its power of evasion and its greater power of control than in its power of actual protection by escort?’¹⁵ The imposition of convoy faced the U-boats with a conundrum. By concentrating the shipping into a small area, convoy made it more difficult for the U-boats to find their targets. This was accentuated in the open ocean, where individual convoy routes could be widely separated, and even if the U-boat made a sighting, getting into position to attack without being sighted himself was difficult. Yet if the enemy moved inshore, where convoys would be easier to find, the U-boats would be faced with heavy air and surface patrols, which forced them to operate while submerged for considerable periods and thereby lose their mobility and search capability. These patrols, especially those by hydrophonefitted trawlers, would also reinforce convoy escorts as they passed through the patrol areas. At least some officers considered that these operations should be combined with ‘… bold measures to strike at the U-boats at source.’¹⁶ Although evasion by convoys was the priority, it was still the convoys that brought about more actions between the contending forces than any other cause. Overall, about 250 vessels were employed directly on convoy work, and a further 500 were intermittently on convoy duties, escort or support work. These vessels represented about 15 per cent of the ships in commission in the Royal Navy.

These lessons were emphasized in post-war histories and staff papers. The interwar years have been portrayed as a period of stagnation in anti-submarine development, both tactically and technically. However, recent research has begun to prove that this was not the case, especially after 1932 when British anti-submarine policy was reviewed. During this period, Germany was not seen as the major threat to British trade. That would come later with her development of ocean raiders and finally U-boats.¹⁷ At the detailed tactical level, attention was paid to increasing the weight of depth-charge attacks to make them more effective. By 1935 it was also recognized that if aircraft were to be effective U-boat killers, they would have to be armed with depth-charges. However the post-war Naval Staff History claimed that, as late as 1937, the Admiralty had had no intention of introducing:

… A/S bombs larger than 100 lb into the Naval Service, since it is considered that a stick of 100 lb bombs is far more likely to sink or damage a submerged submarine than an equal weight of larger bombs … .¹⁸

The key here seems to be the emphasis on attacks on submerged submarines. The aircraft of the inter-war period were all slow and were only able to carry relatively small bomb-loads, except for Royal Air Force (RAF) Sunderland flying-boats, itself a slow aircraft. Speed was important to convert a sighting into an effective attack, otherwise an aircraft would not be able to attack before a submarine had submerged for long enough to make the aiming point uncertain. It was thought that there was a greater chance of one bomb bursting close to the submarine if a large stick straddled the aiming point. There seem to have been no rigorous tests of the anti-submarine bombs at sea, which might have exposed the poor effectiveness, either when bursting close to a submerged target or even from a direct hit. Attacks by escorts could, however, be deadly, but these too had to be started as soon as possible after contact was gained, normally without waiting for a consort to complete the hunting unit. This was designed to throw the U-boat onto the defensive, to avoid it being able to complete an accurate torpedo shot. Attacks were then to continue with two ships cooperating until the U-boat was destroyed, if this was considered to be expedient. It was recognized at the time that lessons drawn from exercises had to be treated with some caution. It was difficult to divert merchant ships from trade, so convoy exercises had to use naval and auxiliary vessels acting as a convoy. The Admiralty repeatedly noted that the results were also devalued by artificialities imposed by peacetime safety rules, and the desire to get maximum training benefit, which lead to an unrealistic number of antisubmarine units being involved during actions. The analysis of sea exercises was compared with the results from strategic board games at the War College at Greenwich.

Convoys and striking forces

The central role of convoy was firmly established both theoretically and in exercises, though the threat from U-boats was not the only, nor even the main, threat. Combined attacks by surface raiders and U-boats was seen as the critical threat, and was known to be a tactic being explored by the Germans.¹⁹ If British heavy ships were required to be part of the escort, convoys would be sailed infrequently and may have to be large, varying between 40 and 90 ships. And as the inter-war years passed, the Admiralty also had to consider the increasing threat of air attack on convoys. Here the focus will be on measures adopted to counter the U-boat, though it was not treated in isolation by the Admiralty during the 1930s. Britain’s geographic position vis-à-vis Germany forced commerce raiders to make long, hazardous passages to their hunting grounds. The Scandinavian convoys of the First World War showed the danger of an enemy able to sortie from the flank of a convoy route (as the German possession of the Biscay ports was to demonstrate again in the Second War). Principal among the anti-submarine measures was that evasion was the best defence for convoys, particularly if the enemy adopted unrestricted U-boat warfare from the outset. Of course, diversion of a convoy had its limits, if both elements of the ‘safe and timely arrival’ dictum were to be met. From wartime experience, and peacetime exercises, it was ‘… not envisaged that the escorting vessels will be able to prevent a submarine attacking the convoy, but it is hoped that they will be able to destroy the submarine after it has made its attack.’

Yet, while a successful defence might be the primary consideration, ‘… in general’, it was thought, ‘the most certain means of obtaining security from enemy submarines is by carrying out a vigorous offensive against them.’ Furthermore:

The moral effect of early success against enemy submarines is likely to militate heavily against the value of his subsequent operations. It is, therefore, of great importance that organization and training should be such as will allow of the full development of offensive A/S measures immediately on the outbreak of war.²⁰

Such protestations were not merely the product of bravado, or an overly optimistic view of the technical progress in asdic development. During the 1930s (when Burnett, Ormsby and Mo...

Table of contents

• Cover Page
• The Royal Navy and Anti-Submarine Warfare, 1917–49
• Cass series: Naval policy and history
• Title Page
• Copyright Page
• Preface
• Abbreviations
• Introduction
• 1 Echoes from the past, 1917–40
• 2 Mastering the submersible, 1939–43
• 3 Elusive victory: Countering the schnorkel, 1944–45
• 4 The dawn of modern anti-submarine warfare, 1944–46
• 5 Short-term problems, long-term solutions, 1946–47
• 6 New problems, old recipes, 1947–48
• 7 Future uncertainties, 1948–49
• Conclusion: Joining up the dots, 1944–49
• Notes
• Bibliography