Editing Battle of the Atlantic (section)

==Anti-submarine warfare==
With the introduction of ASDIC, the British Admiralty believed to have effectively neutralised the submarine threat. Hence the number of destroyers and convoy escorts was reduced and the anti-submarine branch was seen as third rate. Destroyers were also equipped with ASDIC, but it was expected that these ships would be used in fleet actions rather than anti-submarine warfare, so they were not extensively trained in their use. Trials with ASDIC were usually conducted in ideal conditions and the British Admiralty failed to appreciate the limitations of ASDIC: range was limited, ASDIC worked only well if the speed of the operating vessel was below eight knots, ASDIC was hampered by rough weather and it took a very skilled operator to distinguish echoes from [[thermocline]]s, whales, shoals of fish and wrecks.{{sfn|Costello|Hughes|1977|pp=31-33}} Also, early versions could not look directly down, so contact was lost during the final stages of a depth charge attack. The basic set could detect range and bearing, but target depth could only be estimated from the range at which contact was lost.{{efn|The Q attachment was a later addition to ASDIC, providing an extra beam that looked directly down. This gave an accurate depth of the target}}{{sfn|Burn|1993|pp=20, 326-328}}
[[File:Leaning against a depth charge thrower, the quarterdeck lookout on board HMS VISCOUNT searches the sea for submarines, 1942. A13362.jpg|thumb|A lookout of a convoy escort, posing his binoculars on a depth charge thrower with which depth charges were launched to the sides of the escort]]
An escort swept its ASDIC beam in an arc from one side of its course to the other, stopping the transducer every few degrees to send out a signal. On detection of a submarine, the escort would close in at moderate speed and increase its speed to attack. The intention was to pass over the submarine, rolling depth charges from chutes at the stern, while throwers fired further charges to either side, laying a pattern of depth charges. To disable a submarine, a depth charge had to explode within about {{convert|20|ft|m|abbr=on}}. Since early ASDIC equipment was poor at determining depth, it was usual to vary the depth settings on part of the pattern.

When in early 1939 the threat of war became more clear it was realised that Britain could not rely on the London Naval Treaty that outlawed unrestricted submarine warfare.{{Sfn|Blair|1996a|pp=73-74}} The organisational infrastructure for convoys had been maintained since World War I, with a thorough and systematic upgrade in the second half of the 1930s,{{sfn|Milner|2011|p=13}} but not enough escorts were available for convoy escorting, and a crash programme for building [[Tree-class trawler]]s, [[Flower-class corvette]]s and [[Hunt-class destroyer]]s was initiated.{{sfn|Costello|Hughes|1977|pp=31-33}} Merchant ships that were either too fast or too slow for convoys, were to be equipped with a [[Defensively equipped merchant ship|self-defence gun]] against surfaced submarine attacks, thus forcing an attacking U-boat to spend its precious torpedoes. This removed these ships from the protection of the cruiser rules under the [[Prize (law)|prize law]].{{Sfn|Blair|1996a|pp=73-74}}

Despite this lack of readiness, in 1939 the Royal Navy probably had as many ASDIC equipped warships in service as all the other navies of the world combined.{{sfn|Milner|2011|p=11}}

Similarly the role of aircraft had been neglected; the [[Royal Air Force]] had organised a [[RAF Coastal Command|Coastal Command]] to support the Royal Navy, but it possessed insufficient aircraft, had no long range aircraft nor were aircraft crew trained in anti-submarine warfare. The only weapon against submarines was inadequate bombs.{{sfn|Costello|Hughes|1977|pp=33-34}}{{Sfn|Blair|1996a|pp=72}} Finally, it was not forgotten that in World War I, mines had sunk more U-boats than any other weapon. Plans were drafted for mine fields in the Channel and along the east coast in defence of shipping lanes, and also offensive mine barrages on the German U-boat lanes towards the Atlantic Ocean.{{Sfn|Blair|1996a|pp=71}}