gap in mid-Atlantic air cover. Cross-Atlantic trade in war supplies and food was extensive and critical for Britain's survival. The continuous action surrounding British shipping became known as the Battle of the Atlantic, as the British developed technical defences such as ASDIC and radar, and the German U-boats responded by hunting in what were called "wolfpacks", in which multiple submarines would stay close together, making sinking a specific target easier for them. Britain's vulnerable shipping situation existed until 1942, when the tides changed as the U.S. merchant marine and Navy entered the war, drastically increasing the tonnage of supplies sent across the Atlantic. The combination of increased tonnage and increased naval protection of shipping convoys made making a significant dent in British shipping much more difficult for U-boats. Once the United States entered the war, U-boats ranged from the Atlantic coast of the United States and Canada to the Gulf of Mexico, and from the Arctic to the west and southern African coasts and even as far east as Penang. The U.S. military engaged in various tactics against German incursions in the Americas; these included military surveillance of foreign nations in Latin America, particularly in the Caribbean, to deter any local governments from supplying German U-boats. Because speed and range were severely limited underwater while running on battery power, U-boats were required to spend most of their time surfaced, running on diesel engines, diving only when attacked or for rare daytime torpedo strikes. The more ship-like hull design reflects the fact that these were primarily surface vessels that could submerge when necessary. This contrasts with the cylindrical profile of modern nuclear submarines, which are more hydrodynamic under water (where they spend the majority of their time), but less stable on the surface. While U-boats were faster on the surface than submerged, the opposite is generally true of modern submarines. The most common U-boat attack during the early years of the war was conducted on the surface and at night. This period, before the Allied forces developed truly effective antisubmarine warfare tactics, which included convoys, was referred to by German submariners as "die glückliche Zeit" or the First Happy Time. Torpedoes The U-boats' main weapon was the torpedo, though mines and deck guns (while surfaced) were also used. By the end of the war, almost 3,000 Allied ships (175 warships; 2,825 merchant ships) had been sunk by U-boat torpedoes. Early German