During the war Great Britain got most of its food, war materials and other supplies from North America. Throughout the war Germany tried to destroy the ships that brought goods to the island. German U-boats or submarines were the biggest threat to the British navy. At first, these U-boats were very successful in tracking down British cargo ships and sinking them. They often attacked in groups called wolf packs.
To survive such attacks British ships travelled in groups called convoys. They were protected by destroyers and also supported by planes. As time went on America gave British ships more and more protection.
In 1943 the Allies began to escort the convoys with small aircraft carriers. The planes that could take off from the carriers and long-range bombers from land began to attack and destroy more and more German submarines. The Allies began winning the sea war also because they started using an underwater radar system called sonar to locate submarines. By the middle of 1943 the Allies were sinking U-boats faster than the Germans could replace them.
Officers on board a British destroyer on the lookout for German U-boats