Anti-submarine Warfare - History - World War II - Battle of The Atlantic

Battle of The Atlantic

During the Second World War, the submarine menace revived, threatening the survival of island nations like Britain and Japan which were particularly vulnerable because of their dependence on imports of food, oil, and other vital war materials. Despite this vulnerability, little had been done to prepare sufficient anti-submarine forces or develop suitable new weapons. Other navies were similarly unprepared, even though every major navy had a large, modern submarine fleet, because all had fallen in the grip of Mahanian doctrine which held guerre de course could not win a war.

At the beginning of the war, most navies had few ideas how to combat submarines beyond locating them with sonar and then dropping depth charges on them. Sonar proved much less effective than expected, and was no use at all against submarines operating on the surface, as U-boats routinely did at night. The Royal Navy had continued to develop indicator loops between the wars but this was a passive form of harbour defense that depended on detecting the magnetic field of submarines by the use of long lengths of cable lain on the floor of the harbour. Indicator loop technology was quickly developed further and deployed by the US Navy in 1942. By then there were dozens of loop stations around the world. Sonar was far more effective and loop technology died straight after the war.

The use and improvement of radar technology was one of the most important proponents in the fight against submarines. Locating submarines was the first step in being able to defend against and destroy them. Throughout the war, Allied radar technology was much better than their German counterparts. German U-Boats never had the proper radar detection capabilities and the Allies took advantage of this by equipping aircraft with micro and macro radar technology. Allied planes were also eventually fitted with the proper weaponry to sink submarines and from 1943-1945 they would account for the bulk of Allied kills against U-Boats. Allied anti-submarine tactics developed to defend convoys (the Royal Navy's preferred method), aggressively hunt down U-boats (the U.S. Navy approach), and to divert vulnerable or valuable ships away from known U-boat concentrations.

During the Second World War, the Allies developed a huge range of new technologies, weapons and tactics to counter the submarine danger. These included:

Vessels

• Allocating ships to convoys according to speed, so faster ships were less exposed.
• Adjusting the convoy cycle. Analysis of convoy losses over the first three years of the war showed that the overall size of a convoy was less important than the size of its escorting force. Therefore, escorts could better protect a few large convoys than many small ones.
• Huge construction programmes to mass-produce the small warships needed for convoy defense, such as corvettes, frigates, and destroyer escorts. These were more economical than using destroyers, which were needed for fleet duties.
• Ships that could carry aircraft, such as the CAM ships, the merchant aircraft carrier, and eventually the purpose-built escort carriers.
• Support groups of escort ships that could be sent to reinforce the defense of convoys under attack. Free from the obligation to remain with the convoys, support groups could continue hunting a submerged submarine until its batteries and air supplies were exhausted and it was forced to surface.
• Hunter-killer groups, whose job was to actively seek out enemy submarines, as opposed to waiting for the convoy to come under attack. Later hunter-killer groups were centered around escort carriers.
• Huge construction programmes to mass-produce the transports and replace their losses. Once shipbuilding had ramped up to full efficiency, transports could be built faster than U-boats could sink them, playing a crucial role in the Allies winning the "Tonnage war".

Aircraft

• Air raids on the German U-boat pens at Brest and La Rochelle.
• Long-range aircraft patrols to close the Mid-Atlantic gap.
• Escort carriers to provide the convoy with air cover, as well as close the mid-Atlantic gap.
• High frequency direction finding (HF/DF), including shipborne sets, to pinpoint the location of an enemy submarine from its radio transmissions.
• The introduction of seaborne radar which could enable the detection of surfaced U-boats.
• Airborne radar.
• The Leigh light airborne searchlight, in conjunction with airborne radar to surprise and attack enemy submarines on the surface at night.
• Magnetic anomaly detection
• Diesel exhaust sniffers
• Sonobuoys

Weaponry

• Torpedo countermeasures such as the Foxer acoustic decoy.
• The development of forward-throwing anti-submarine weapons such as Hedgehog and the Squid.
• The FIDO (Mk 24 'mine') homing torpedo.
• Pattern running torpedoes

Intelligence

• One of the best kept Allied secrets was the breaking of enemy codes including some of the German Naval Enigma codes (information gathered this way was dubbed Ultra) at Bletchley Park in England. This enabled the tracking of U-boat packs to allow convoy re-routings; whenever the Germans changed their codes (and when they added a fourth rotor to the Enigma machines in 1943), convoy losses rose significantly. By the end of the war, the Allies were regularly breaking and reading German naval codes.
• To prevent the Germans from guessing that Enigma had been cracked, the British planted a false story about a special infrared camera being used to locate U-boats. The British were subsequently delighted to learn that the Germans responded by developing a special paint for submarines that exactly duplicated the optical properties of seawater.

Tactics

Many different aircraft from airships to four-engined sea- and land-planes were used. Some of the more successful were the Lockheed Ventura, PBY (Catalina or Canso, in British service), Consolidated B-24 Liberator (VLR Liberator, in British service), Short Sunderland, and Vickers Wellington. U-boats were not defenseless, since their deck guns were a very good anti-aircraft weapon. They claimed 212 Allied aircraft shot down for the loss of 168 U-boats to air attack. At one point in the war, there was even a 'shoot back order' requiring U-boats to stay on the surface and fight back, in the absence of any other option.

The provision of air cover was essential. The Germans at the time had been using their Focke-Wulf Fw 200 "Condor" long range aircraft to attack shipping and provide reconnaissance for U-boats, and most of their sorties occurred outside the reach of existing land-based aircraft that the Allies had; this was dubbed the Mid-Atlantic gap. At first, the British developed temporary solutions such as CAM ships and merchant aircraft carriers. These were superseded by mass-produced, relatively cheap escort carriers built by the United States and operated by the US Navy and Royal Navy. There was also the introduction of long-ranged patrol aircraft. Many U-boats feared aircraft, as the mere presence would often force them to dive, disrupting their patrols and attack runs.

There was a significant difference in the tactics of the two navies. The Americans favored aggressive hunter-killer tactics using escort carriers on search and destroy patrols, whereas the British preferred to use their escort carriers to defend the convoys directly. The American view was that defending convoys did little to reduce or contain U-boat numbers, while the British were constrained by having to fight the battle of the Atlantic alone for the early part of the war with very limited resources. There were no spare escorts for extensive hunts, and it was only important to neutralize the U-boats which were found in the vicinity of convoys. The survival of convoys was critical, and if a hunt missed its target a convoy of strategic importance could be lost. The British also reasoned that since submarines sought convoys, convoys would be a good place to find submarines.

Once America joined the war, the different tactics were complementary, both suppressing the effectiveness of and destroying U-boats. The increase in Allied naval strength allowed both convoy defense and hunter-killer groups to be deployed, and this was reflected in the massive increase in U-Boat sinking in the latter part of the war. The British developments of ASDIC, Centimetric Radar and the Leigh Light also reached the point of being able to support U-Boat hunting towards the end of the war, while at the beginning technology was definitely on the side of the submarine. Commanders such as F. J. "Johnnie" Walker of the Royal Navy were able to develop integrated tactics which made the deployment of hunter-killer groups a practical proposition.

The earliest recorded sinking of one submarine by another while both were submerged occurred in 1945 when HMS Venturer torpedoed U-864 off the coast of Norway. The captain of Venturer tracked U-864 on hydrophones for several hours and manually calculated a three-dimensional firing solution before launching four torpedoes.