'By the end of July 1944, U-boat command concluded that operations in the eastern Atlantic, or English Channel would be unlikely to provide any kind of success'

'the autumn of 1944 saw the beginning of a series of waves of U-boats moving into the Bristol Channel and the Irish Sea'

In the aftermath of the Normandy invasion in June 1944, there was an increase in Bristol Channel traffic as men and materials were despatched to France from the western ports. This increase had been predicted by U-boat command and they despatched a boat to the Channel to gather intelligence about supply traffic. Using her schnorchel to good effect, U 667 patrolled along the north Cornish coast in July, sinking a number of ships including the Canadian corvette Regina. U-boat command were expecting valuable intelligence from the boat on its return to La Pallice. However, the boat struck a mine on its return to base and was lost before it could pass on any information. Nevertheless, U-boat command made the assumption that Allied anti-submarine activity in the Bristol Channel was low offering reasonable opportunity for effective operations.

Tactically, Allied air supremacy made it extremely difficult if not impossible for U-boats to travel on the surface in the English Channel or British coastal waters. The aggressive use of naval hunter killer groups with effective anti-submarine weapons practically ensured the destruction of any conventional (non-schnorchel ) U-boats unlucky to be discovered. By the end of July 1944, U-boat command concluded that operations in the eastern Atlantic, or English Channel would be unlikely to provide any kind of success.

However, by August increasing numbers of boats were being fitted with schnorchel devices which enabled boats to use their diesel engines whilst submerged. Boats so equipped could evade aircraft patrols or present smaller targets for radar and travel at up to six knots submerged (as opposed to only two knots on electric propulsion). Thus, the boats came a step closer to being true submarines, being able to spend extended periods of time at periscope depth making their detection much more difficult. The crews of the schnorchel boats accepted the development with mixed feelings. Operating the schnorchel was a difficult business, with extra effort needed to keep the boat at the correct depth for extended periods. Using the diesels whilst submerged was dangerous and the danger of asphyxiation was always present. It is now believed that a number of boats came to grief as a result of faulty schnorchels. Despite misgivings many crews quickly adapted to the new way of working and the operational advantages it gave them. The success of U 667 off Cornwall in July 1944 was due in large part to effective use of the schnorchel, enabling the boat to attack a convoy and avoid the ensuing search by escorts.

A number of boats sent into British coastal waters in August 1944 reported that the schnorchel helped considerably in enabling the boats to avoid naval escort vessels. In addition, a number of commanders discovered that the Allied ASDIC system was often compromised by false echoes and readings in the relatively shallow waters. Correspondingly, Dönitz reasoned that the U-boat offensive should be resumed in British coastal areas, heralding the return of U-boats to the Bristol Channel and the Irish Sea. The stakes were further raised by the Allied armies' successes in France creating a need for increased supply traffic to enter British and French coastal waters using routes to the south of Ireland and around the tip of Land’s End. The waters off Milford Haven were identified as a likely bottleneck of shipping traffic offering both good targets and sufficient room for evasion and escape from Allied ASW patrols.

Thus, the autumn of 1944 saw the beginning of a series of waves of U-boats moving into the Bristol Channel and the Irish Sea. The schnorchel was followed by a series of technical advances that further eroded Allied command of the sea. By November, most schnorchel boats were equipped with a variety of active and passive radar devices that gave early warning of the approach of aircraft. Allied commanders even had to acknowledge that U-boats could operate free of interference from detection or attack in coastal waters. The anti-submarine aircraft that hitherto had claimed so many victims were now reduced to the role of ‘scarecrows,' keeping the U-boats submerged and limiting their mobility.

Working in coastal waters was not without disadvantage. Shallow water provided a poor listening environment for hydrophones. On a number of occasions boats lying on the bottom or moving at depths of 30 to 40 metres, found themselves being run over by ships whose propeller noise went unheard. In such instances it was extremely difficult for boats to obtain a satisfactory torpedo firing solution and the ships carried on unhindered. In this period a number of Bristol Channel and Irish Sea patrols were characterised by long cruises without being able to attack any vessels. By January 1945, most U-boats remained at periscope depth in daytime relying on their detection devices to give early warning of aircraft. Another characteristic of this stage of the War was radio silence. The best U-boat commanders made no messages at all on their patrols, and usually survived to tell the tale. This was in stark contrast to the period 1941 to 1943 when regular radio contact was the norm, enabling Allied direction finding to locate and destroy U-boats with great regularity.

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