Although the Battle of Britain had been lost, Germany continued to pressure Great Britain. While the aerial bombardments persisted, the German navy intensified its campaign of submarine warfare against the Allied naval convoys, which endeavoured to supply a besieged Britain.
Under Admiral Karl Dönitz, Commander-in-Chief of U-boats (submarines), the German navy employed the "wolf pack" strategy of attack. German wolf packs of submarines were to be stationed at right angles to known Allied convoy lanes in the North Atlantic. The first U-boat captain to spot the target was to radio headquarters and then mirror the convoy until the remaining U-boats could zero in. Once assembled, the wolf pack executed a surface attack under cover of darkness. Daring U-boat commanders even surfaced in the middle of the convoy before launching their devastating assault, bringing both fore and aft torpedo tubes to bear on the unfortunate merchant ships.
Germany initiated its wolf pack assaults in August and September of 1940. The odds in these opening stages of the Battle of the Atlantic were firmly in favour of the German submarines. After the fall of France in June 1940, German U-boats were able to range from French ports farther west into the mid-Atlantic, beyond the reach of Royal Navy (RN) escorts. There, they ravaged Allied shipping. Over the winter of 1940-1941, German submarines sank roughly 250,000 tonnes of British shipping per month.
By March 1941, the British prime minister, Winston Churchill, attempted to counter the U-boat threat by ordering the RN to establish a supply base in Iceland. RN warships were to escort vulnerable supply ships from Ireland to Canadian waters. In early 1941, the Royal Canadian Navy (RCN) took on increased responsibility for escorting the convoys.
The Canadian navy, which had only 11 fighting ships and 3,000 men in 1939, grew in size and responsibility during the war. By the end of the conflict, Canada's navy possessed some 400 fighting ships and 100,000 men. The RCN served in a variety of capacities during the war. For example, in May 1941, the Admiralty (Britain's naval command) asked the RCN to escort supply ships around the British colony of Newfoundland. The RCN accepted and located its base of operations at St. John's. It played a crucial role in escorting convoys of merchant vessels through the treacherous waters of the North Atlantic from Halifax, Nova Scotia, to Iceland. As the war progressed, the RCN would assume additional escort duties, and, by late 1944, would provide escorts for Allied convoys along the entire convoy routes to Britain.
Initially, the only submarine detection equipment available to the RN and RCN was ASDIC (Allied Submarine Detection Investigation Committee), an early type of sonar. However, ASDIC was most effective in detecting submerged U-boats at long range, and submarines on the surface were virtually undetectable with ASDIC. As the sea war intensified, escorts were equipped with radar, which could detect surfaced U-boats even in dense fog. In addition, the RN and RCN perfected the techniques of Radio Directional Finding (RDF). The location of wolf packs could be accurately determined using U-boat radio transmissions. As a result of these technological innovations, RCN escorts were able to hold their own against marauding German submarines.
At the same time, the RCN assumed additional escort duties. Canadian corvettes and destroyers protected merchant ships on the Murmansk Run as they brought crucial supplies to a beleaguered Soviet Union. RCN escorts provided protection for Allied task forces as they landed in North Africa, Sicily, and Italy. As the demands for escort protection increased, shipyards in Canada and Britain turned out additional escorts at an astounding rate. By the summer of 1943, the RCN and RN had gained the upper hand in the North Atlantic, and the German stranglehold on vital shipping lanes had been broken. Soon, a steady stream of men, supplies, and equipment flowed from Halifax and Sydney to Britain, and the Allies were finally able to assemble sufficient forces to invade France on 6 June 1944.
With the aid of the RCN, warships now guarded supply convoys, laden with vital food supplies and other war material, throughout the entire perilous journey. RAF Coastal Command and the RCAF provided air cover for most of the voyage; however, a significant 300-mile gap, known as the "Black Pit," existed in the mid-Atlantic. This gap, which was located within the RCN's zone, allowed Germany to maintain the upper hand in the Battle of the Atlantic for the years 1941-1943. The introduction of escort carriers and the long-range B-24 Liberator bomber finally closed the gap by mid-1943, and Portugal made a big contribution by allowing the use of airfields in the Azores. Nevertheless, the convoys succeeded, albeit at a very high cost in lives and ships, in transporting the supplies that Britain needed to continue the fight.
The convoy system offered some protection for merchant shipping, but sea duty for the RCN sailor, whose job it was to protect the convoys, was still enormously perilous. Death came quickly in the frigid North Atlantic, and rescue was often extremely difficult.