Sink the Bismarck!

In 1939, Nazi Germany's largest and most powerful battleship, Bismarck, is launched in a ceremony at Hamburg with Adolf Hitler attending. The launching of the hull is seen as the beginning of a new era of German sea power.
Two years later, in 1941, British convoys are being ravaged by U-boats and surface raider attacks that cut off supplies essential for Britain's abilities to continue the war. In May, British intelligence discovers the Bismarck and the heavy cruiser Prinz Eugen are about to break out of the Baltic and into the North Atlantic to attack convoys.
Meanwhile, a spy in Norway spots the Bismarck and its escort Prinz Eugen; he attempts to alert the British by telegraph but he is discovered by the Germans and killed. The spy is only able to message that one of the ships is Prince Eugen but is shot before he can message that the second ship was the Bismarck.
The man assigned to co-ordinate the hunt is the Admiralty's chief of operations, Captain Jonathan Shepard (Kenneth More), who has been distraught over the death of his wife in an air raid and the sinking of his ship by German ships commanded by Admiral GÃ¼nther LÃ¼tjens (Karel Å tÄpÃ¡nek). Upon receiving his new post, Shepard discovers LÃ¼tjens is the fleet commander on the Bismarck. Shepard's experience of conflict with Nazi Germany's Kriegsmarine and his understanding of LÃ¼tjens allow him to predict the Bismarck's movements. Shepard acts coldly to his staff but comes increasingly to rely on the coolness and skill of his assistant, WRNS Second Officer Anne Davis (Dana Wynter).
LÃ¼tjens is also bitter. After the First World War, he considered that he had received no recognition for his efforts in the war. LÃ¼tjens promises the captain of the Bismarck, Ernst Lindemann (Carl MÃ¶hner), that this time, he and Germany will be remembered in greatness.
Next morning near Iceland Bismarck and Prinz Eugen encounter HMSÂ Hood and HMS Prince of Wales. The four warships engage in a heavy battle. During the battle a shell from Bismarck hits the Hood slightly damaging her. Bismarckâ fires again and both sides watch as three shells hit the water near the Hood, but the fourth hits it just below its mast; suddenly the ship's deck bursts up and explodes in a massive fireball, even blowing one of the turrets off and sending it flying into the ocean. Both sides are shocked and horrified at the devastation as the Hood's sinking remains are enveloped by smoke. Now Prince of Wales is alone and gets fired at by the two German ships and is severely damaged. The ship makes smoke and retreats. The Bismarck's escape is shadowed by smaller British ships. Meanwhile, Shepard, obsessed with Bismarck, must endure the likely death of his son as an air-gunner on a Fairey Swordfish torpedo bomber from HMSÂ Ark Royal, one of the British ships deployed to the hunt. He gambles that LÃ¼tjens is returning to friendly waters where U-boats and air cover will make it impossible to attack.
Shepard commits large forces stripped from convoy escort and uses Catalina flying boats to search for the battleship. His hunch proves correct, and Bismarck is located, apparently steaming towards the German-occupied French coast. British forces have a narrow window to destroy or slow their prey before German support and their own diminishing fuel supplies prevent further attack. Swordfish aircraft from HMS Ark Royal have two chances. The first fails: they misidentify HMSÂ Sheffield as Bismarck; also the new magnetic torpedo detonators are faulty and most explode as they hit the water. Switching to conventional contact detonators, the second attack is successful, with damage jamming Bismarck's rudder.
Unable to repair the rudder, the German battleship steams in circles. During the night Bismarck is attacked by two British destroyers. They fire torpedoes at Bismarck, and one torpedo hits the battleship, but Bismarck returns fire, sinking the destroyer HMS Solent.[Note 2] The main force of British ships (including battleships HMSÂ Rodney and HMSÂ King George V) find Bismarck the next day and rain gunfire on her. LÃ¼tjens in his final moments insists to Lindemann that German forces will arrive to save them, but he dies when a shell destroys Bismarck's bridge. After that the remaining officers declare Abandon Ship! In the King George V Admiral Tovey orders the newly joined cruiser HMSÂ Dorsetshire to finish Bismarck off with torpedoes. The cruiser fires a salvo of six torpedoes at the already sinking and severely damaged vessel. Four torpedoes strike the hull, causing the ship to list faster than the men can get out. The Captain in K.G.V bows his head as the Bismarck rolls over and sinks beneath the waves. The Admiral orders Dorsetshire to pick up the remaining survivors, and finally says tersely: "Well gentlemen, let's go home"
After the sinking of the Bismarck, and having been told that his son has been rescued, Shepard asks Davis out to dinner, believing it to be nine o'clock at night, only to realise it is nine in the morning. Davis suggests breakfast, and they walk off together.

Question: What are being ravaged by U-boats?
Answer: British convoys.