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As an island nation with an overseas empire, the United Kingdom was highly dependent on sea-going trade. Britain required more than a million tons of imported food and materiel per week in order to be able to survive and fight on against Nazi Germany. In essence, the Battle of the Atlantic was the struggle to maintain or cut off the shipping that enabled Britain to survive.
The first actions of the naval campaign started the day that war was declared. Royal Navy vessels dredged up and cut German transatlantic communication cables, forcing the Germans to communicate to their interests in the Americas by less secure means for the rest of the war.
The U-boat fleet, which was to dominate so much of the battle of the Atlantic, was very small at the beginning of the war and much of the early action by German forces involved miningNaval mines are anti-ship or anti-submarine weapons which, like landmines, are static weapons deposited and left to wait until they are triggered by the approach of an enemy ship (cf depth charge). They can be quite cost-effective. Since World War II, min convoyThis article is about the general concept, particularly its use by the military. Convoy is also the name of a trucker's song by C. McCall and of a movie by Sam Peckinpah and Kris Kristofferson inspired by the song, as well as the Japanese name for the Tra routes and ports around Britain.
Initially, contact mines were employed, which meant that a ship had to physically strike one of the mines in order to detonate it. Contact mines are usually suspended on the end of a cable just below the surface of the water and laid by ship or submarine. By the beginning of World War II most nations had also developed mines that could be dropped from aircraft, making it practicable to lay them in enemy harbours (although they simply floated on the surface). The use of dredging and nets was effective against this type of mine, but nonetheless this process was time-consuming and involved the closing of harbors while it was completed.
Into this arena came a new mine threat. Most contact mines leave holes in ship's hulls, but some ships surviving mine blasts were limping back to port with buckled plates, popped rivets, and broken backs. This appeared to be due to a new type of mine that was detonating at a distance from the ships, and doing the damage with the shockwave of the explosion. Often ships that had successfully run the gauntlet of the Atlantic crossing were destroyed entering freshly mineswept harbors on Britain's coast. More shipping was now being lost than could be replaced, and ChurchillChurchill" redirects here. For other meanings, see Churchill (disambiguation). The Right Honourable Sir Winston Leonard Spencer-Churchill KG, OM, CH, FRS ( November 30, 1874 January 24, 1965) was a British politician, best known as Prime Minister of the U ordered that the recovery, intact, of one of these new mines was to be given highest priority.
Then the British experienced a stroke of luck in November 1939. A German mine was dropped from an aircraft laying mines onto mud flats in the Thames estuary, well above the waterline. As if this was not sufficiently good fortune, the land happened to belong to the army, and a base, including men and workshops, was close at hand. Experts were quickly dispatched from London to investigate the mine. They had some idea by this time that the mines used magnetic sensors, so they had everyone remove all metal, including their buttons, and made new tools out of non-magnetic brass. They then safed the mine and rushed it to labs at Portsmouth, where scientists discovered a new type of arming mechanism inside.
The arming mechanism had a sensitivity level that could be set, and the units on the scale were milligauss. Gauss is a measurement for the strength of a magnetic fieldIn physics, a magnetic field is an entity produced by moving electric charges ( electric currents) that exerts a force on other moving charges. The quantum-mechanical spin of a particle produces magnetic fields and is acted on by them as though it were a, and so they knew why it went off before coming into contact with the ship. Using the detector from the mine, they were able to study the effect of a ship passing over it. A ship or any large ferrous object passing through the earth's magnetic field will concentrate the field at that point. The detector from the mine measured this effect, and was designed to go off at the mid-point of the ship passing overhead.
From this crucial data, methods were developed to clear the mines. Early methods included the use of large electromagnets dragged behind ships, or on the undersides of low-flying aircraft (a number of older bombers like the Vickers WellingtonThe Vickers Wellington was a twin-engine, medium bomber designed in the mid-1930s at Brooklands in Weybridge, Surrey, by Vickers-Armstrongs' Chief Designer, R. It was widely used in the first two years of World War II, before being replaced as a bomber by were used for this purpose). However both of these methods had the disadvantage of "sweeping" only a small strip at a time. A better solution was found in the form of electrical cables dragged behind ships, passing a large current through the seawater. This induced a huge magnetic field and swept the entire area between the two ships. The older methods continued to be used in smaller areas; the Suez Canal continued to be swept by aircraft, for instance.
While these methods were useful for clearing mines from local ports, they were of little or no use for enemy controlled areas. These were typically visited by warships only, and the majority of the fleet then underwent a massive de-gaussing process, where their magnetic fields were reduced to such a degree that it was no longer "noticed" by the mines. This started in late 1939, and by 1940 British warships were largely immune for the few months at a time until they once again built up a field. Many of the boats that sailed to Dunkirk were de-gaussed in a marathon four-day effort by hard-pressed de-gaussing stations.
The Germans had also developed a pressure-activated mine and planned to deploy it as well, but they saved it for later use when it became clear the British had beaten the magnetic system.