One of the main assets of the German U-boat (submarine; German: U-boot, shortened from Unterseeboot, literally “underseaboat”) was stealth, and the first naval casualty of the war, the British ocean liner, SS Athenia, was attacked and sunk by a U-boat (which it mistook for a British warship) on September 3, 1939, with 128 lives lost. Also in September 1939 and just a few days apart, two British aircraft carriers, the HMS Ark Royal and HMS Courageous, were both attacked by a U-boat, with the former narrowly being hit by torpedoes, while the latter was hit and sunk. Then in October 1939, another U-boat penetrated undetected near Scapa Flow, the main British naval base, attacking and sinking the battleship, the HMS Royal Oak. On November 13, 1941 off Gibraltar, a U-boat fired one torpedo on the HMS Ark Royal, which sank the next day.
(Taken from Battle of the Atlantic – Wars of the 20th Century – World War II in Europe)
At the start of the war, the British military was hard-pressed on how to deal with the U-boat threat. During the interwar period, prevailing naval thought and budgetary resources, both Allied and German alike, focused on surface ships, and the belief that battleships would play the dominant role in naval warfare in a future war. German U-boats had proved highly effective in World War I, causing heavy losses on merchant shipping that nearly forced Britain out of the war, before the British introduced the convoy system that turned fortunes around.
However, the British Navy’s implementing the ASDIC system (acronym for “Anti-Submarine Detection Investigation Committee”; otherwise known as SONAR), which could detect the presence of submerged submarines, appeared to have solved the U-boat threat. Naval tests showed that once detected by ASDIC, the submarine could then be destroyed by two destroyers launching depth charges overboard continuously in a long diamond pattern around the trapped vessel. The British concept was that the U-boats could operate only in coastal waters to threaten harbor shipping, as they had done in World War I, and these tests were conducted under daylight and calm weather conditions. But by the outbreak of World War II, German submarine technology had rapidly advanced, and were continuing so, that U-boats were able to reach farther out into the Atlantic Ocean, eventually ranging as far as the American eastern seacoast, and also were able to submerge to greater depths beyond the capacity of depth charges. These factors would weigh heavily in the early stages of the Battle of the Atlantic.
In December 1939, hostilities were suspended by the harsh Atlantic weather, and German surface ships and U-boats returned to their bases in Germany. In May 1940, the eight-month “Phoney War” period of combat inactivity in the West was broken by the German invasion of France and the Low Countries, which had been preceded one month earlier, April 1940, with the conquest of the Scandinavian countries of Denmark and Norway. By late June 1940, these campaigns were complete, Italy had joined the war on Germany’s side, and Britain remained the sole defiant nation in Western Europe.
These triumphs in Scandinavia and Western Europe were important for the Kriegsmarine: in the Norwegian campaign, the German Navy, which played a major role by transporting the troops and war supplies to the landing points, lost a large part of its surface fleet, and for a time, was rendered virtually incapacitated, while the conquest of France allowed the Kriegsmarine to establish new bases in western France, at Brest, Lorient, La Rochelle, and Saint-Nazaire, which greatly reduced (by 450 miles) the distance to the Atlantic, allowing the U-boats to range further west and spend more time at sea. The campaigns also eased the difficult war-time economy of Germany, as more agricultural and industrial resources became available. Germany’s position would later improve further with more conquests, as well as with forming Axis treaties, in Eastern Europe, rendering the British blockade (temporarily) ineffective.
But for Britain, these campaigns were disastrous: in Norway, the Dunkirk evacuation, and clashes in the English Channel and the North Sea, the Royal Navy lost 23 destroyers sunk, and dozens more damaged; there loomed the possibility that the Germans might seize the French fleet and use it to invade Britain; and more Royal Navy ships had to return for the defense of the homeland, thus reducing security for the merchant convoys in the Atlantic. To preclude the possibility that the French ships would fall to the Germans, in July 1940, the British Navy attacked the French fleet at Mers-el-Kebir, French Algeria, while the French squadron at Alexandria, Egypt, was forced to be interned by the British fleet there. These naval actions infuriated the new, nominally sovereign Vichy government in France, which had declared its neutrality in the war, and also because it had assured Britain that the French Navy would not fall into German hands.
In July 1940, Hitler launched the Luftwaffe over the English Channel and British skies, starting the air war known as the Battle of Britain. The air attacks peaked in August 1940, when the Luftwaffe turned its attention from attacking British military and industrial infrastructures to bombing civilian targets in London and other cities, which would continue with some intensity until early 1941. By then, the threat of a German cross-channel invasion (Operation Sea Lion) had diminished, and ended completely in May 1941 when Hitler was fully engaged in the forthcoming invasion of the Soviet Union, set for June 1941.
Meanwhile, in the second half of 1940, hostilities in the Atlantic again escalated following the end of the campaigns in Scandinavia and Western Europe. Launching from their new bases in western France, in June 1940, U-boats in increasing numbers prowled the Atlantic, immediately coming upon and attacking and sinking many merchant ships. In a radical change from World War I U-boats that operated singly as lone ambushers of isolated ships, in September 1940, Admiral Karl Donitz, head of the U-boat arm of the Kriegsmarine, devised Rudeltaktik (“pack tactics”), where a squadron of U-boats would simultaneously attack a convoy of ships. This strategy, soon called “wolf pack” by the British, consisted of several U-boats spaced out in a single long line across the anticipated path of an incoming convoy. One U-boat, upon sighting the convoy, would maintain contact with it, while the other U-boats were alerted by radio and be brought forward. Together, the U-boats would attack at night, generally with impunity against the lightly escorted convoys, inflicting heavy losses in men and ships. Convoy protection was provided by corvettes, which were too slow to chase away a U-boat. ASDIC also proved unreliable in the turbulent conditions that the battles generated and in inclement weather, and underwater detection was further defeated by U-boats that stayed at the surface at night.
The German effort also was strengthened when in August 1940, the Italian Navy (Regia Marina) sent a fleet of submarines to operate in the Atlantic from a naval base in Bordeaux, France. Over-all, the Italian contribution was small, with only a few dozen submarines taking part, and accounted for 3% of the total number of merchant ships sunk in the Battle of the Atlantic. From June to October 1940, in what German U-boat crews celebrated as “The Happy Time” (German: Die Glückliche Zeit), German U-boats sunk 274 Allied ships (totaling 1.5 million tons) for the loss of only 6 U-boats. This stunning success brought instant fame to many U-boat commanders and their crews, who were welcomed as heroes on their return to Germany.
In November 1940, Britain introduced some counter-measures: convoys were diverted away from the regular trade routes to further north near Iceland and shipping codes were changed. More measures were adopted in early 1941: the merchant convoys and British reconnaissance aircraft were equipped with radar to detect surfaced U-boats; the British Western Approaches Command (tasked with safeguarding the Atlantic trade) was moved to Liverpool, allowing better strategic control; and the convoys were given naval escort protection all along the length of the Atlantic. In the latter, the convoys at their assembly point in St. John’s, Newfoundland, Canada were escorted by Royal Canadian Navy ships to a designated point off Iceland, where the British Royal Navy then would take over escort protection for the rest of the way to Britain. Furthermore, the British Navy introduced a new convoy system: a few large convoys (rather than many smaller convoys) were organized, as British experience thus far showed that their less frequency meant that they were exposed to less time to attack, and they required fewer escorts measured on a prorated basis against smaller convoys.
The end of the U-boat’s “Happy Time” in November 1940 coincided with the German Navy’s surface ships rampaging through the Atlantic Ocean. In early November 1940, the German heavy cruiser Admiral Scheer came upon an Allied merchant convoy, sinking five ships and damaging many others. In January-March 1941, in a series of actions, two German battleships, the Scharnhorst and Gneisenau, sunk or captured 22 merchant ships. And in February 1941, the cruiser Admiral Hipper ambushed a 19-ship unescorted convoy, sinking 13 ships.
By then, British battleships were tasked to protect merchant ships, and in a number of incidents, they warded off German surface raiders from attacking the convoys. This measure paid off materially when two German ships, the new battleship Bismarck and the cruiser Prinz Eugen, were sighted off Iceland by a British naval squadron, and in the ensuing clash, the Bismarck was damaged, although it sank the British battle cruiser HMS Hood. While attempting to escape to France, the Bismarck was intercepted and sunk. The increasing British Navy presence in the Atlantic and Hitler’s displeasure with the loss of the Bismarck compelled the Fuhrer to suspend surface fleet operations in the Atlantic. The German Navy’s surface vessels finally ceased to have any impact in the Atlantic when in February 1942, in the “Channel Dash”, the Scharnhorst, Gneisenau, and Prinz Eugen boldly crossed the heavily protected English Channel from their base in western France to Norway. The transfer was prompted by reports of an imminent British invasion of Norway, as well as the need for greater German naval presence in the Norwegian Arctic to stop the Allied convoys supplying the beleaguered Soviet Union.
In the second half of 1941, Admiral Donitz focused U-boat operations along the “mid-Atlantic air gap”, which accounted for 70% of Allied merchant ship losses during this period. Improved Allied aircraft technology, which allowed greater air range, was yet unable to provide cover for the full distance of the vast Atlantic Ocean. Allied shipping losses in the Atlantic war was eased somewhat when many U-boats were withdrawn to other sectors, first in June 1941 to the Arctic to help stop the flow of Allied supplies to the Soviet Union, and in October 1941, in the Mediterranean Sea to cut British supply lines in the North African campaign.