<P> In the South Atlantic, British forces were stretched by the cruise of Admiral Graf Spee, which sank nine merchant ships of 50,000 GRT in the South Atlantic and Indian Oceans during the first three months of war . The British and French formed a series of hunting groups including three battlecruisers, three aircraft carriers, and 15 cruisers to seek the raider and her sister Deutschland, which was operating in the North Atlantic . These hunting groups had no success until Admiral Graf Spee was caught off the mouth of the River Plate between Argentina and Uruguay by an inferior British force . After suffering damage in the subsequent action, she took shelter in neutral Montevideo harbour and was scuttled on December 17, 1939 . </P> <P> After this initial burst of activity, the Atlantic campaign quieted down . Admiral Karl Dönitz, commander of the U-boat fleet, had planned a maximum submarine effort for the first month of the war, with almost all the available U-boats out on patrol in September . That level of deployment could not be sustained; the boats needed to return to harbour to refuel, re-arm, re-stock supplies, and refit . The harsh winter of 1939--40, which froze over many of the Baltic ports, seriously hampered the German offensive by trapping several new U-boats in the ice . Hitler's plans to invade Norway and Denmark in the spring of 1940 led to the withdrawal of the fleet's surface warships and most of the ocean - going U-boats for fleet operations in Operation Weserübung . </P> <P> The resulting Norwegian campaign revealed serious flaws in the magnetic influence pistol (firing mechanism) of the U-boats' principal weapon, the torpedo . Although the narrow fjords gave U-boats little room for manoeuvre, the concentration of British warships, troopships and supply ships provided countless opportunities for the U-boats to attack . Time and again, U-boat captains tracked British targets and fired, only to watch the ships sail on unharmed as the torpedoes exploded prematurely (due to the influence pistol), or hit and failed to explode (because of a faulty contact pistol), or ran beneath the target without exploding (due to the influence feature or depth control not working correctly). Not a single British warship was sunk by a U-boat in more than 20 attacks . As the news spread through the U-boat fleet, it began to undermine morale . The director in charge of torpedo development continued to claim it was the crews' fault . In early 1941 the problems were determined to be due to differences in the earth's magnetic fields at high latitudes and a slow leakage of high - pressure air from the submarine into the torpedo's depth regulation gear . These problems were solved by about March 1941, making the torpedo a formidable weapon . </P> <P> Early in the war, Dönitz submitted a memorandum to Grand Admiral Erich Raeder, the German navy's Commander - in - Chief, in which he estimated effective submarine warfare could bring Britain to its knees because of the country's dependence on overseas commerce . He advocated a system known as the Rudeltaktik (the so - called "wolf pack"), in which U-boats would spread out in a long line across the projected course of a convoy . Upon sighting a target, they would come together to attack en masse and overwhelm any escorting warships . While escorts chased individual submarines, the rest of the "pack" would be able to attack the merchant ships with impunity . Dönitz calculated 300 of the latest Atlantic Boats (the Type VII), would create enough havoc among Allied shipping that Britain would be knocked out of the war . </P>

How many allied ships were sunk by the end of ww2