A magnetic mine damages HMS Belfast

At 10.58 a.m. on 21 November 1939 the modern British light cruiser HMS Belfast struck a magnetic mine near the Isle of May, at the mouth of the Firth of Forth in eastern Scotland. The mine did not pierce her plating, but the explosion beneath the hull broke the ship’s keel and wrecked her machinery. The cruiser, which had been commissioned less than four months earlier, returned to operations only in November 1942. It was the most spectacular success of the German mining campaign of autumn 1939, which reached its peak in November. A few days earlier, on 13 November, the destroyer HMS Blanche, the first Royal Navy destroyer lost in this war, had gone down on mines in the mouth of the Thames, and on 20 November aircraft had joined in mining British waters.

Popular chronologies link these three events with the dates given above, but are wrong on two points. The mines on which Blanche sank were laid not by a submarine but by German destroyers. The mine that damaged Belfast was laid not by U-52 but by U-21, and two and a half weeks earlier. What is correct is that the cruiser was out of service until the autumn of 1942.

The mine war off the British coast

From the first days of September German submarines had been secretly mining the British coast. The most dangerous weapon was a new one: the ground mine with a magnetic firing mechanism. It lay on the bottom of a shallow channel and exploded when a steel ship passed over it, disturbing the earth’s magnetic field. The shock wave of an explosion under the hull broke ships’ backs. British minesweepers, designed to sweep moored mines, were helpless against it, and the Admiralty still did not know exactly how the German firing mechanism worked.

In the autumn the Germans extended the campaign. Submarines laid mines not only off the east coast of England but also in the Scottish firths, and from mid-October destroyers joined in, approaching right up to the coast at night. The leadership of the Kriegsmarine, headed by Admiral Erich Raeder, saw mines as a way of inflicting heavy losses on Britain with small forces before larger numbers of U-boats entered service. In the same weeks the Germans were also step by step intensifying the torpedo war against merchant ships.

13 November: destroyers in the Thames Estuary and the loss of Blanche

During the night of 12–13 November 1939 three German destroyers, Z18 Hans Lüdemann, Z19 Hermann Künne and Z20 Karl Galster, covered by Z21 Wilhelm Heidkamp, approached the Thames Estuary in darkness and laid in the channels leading to London, according to British tallies, 288 mines with magnetic firing mechanisms. Three more destroyers were to have taken part in the operation, but two turned back with engine failures and the third escorted them home. The German ships returned to base unobserved.

On the morning of 13 November British ships sailing from Grimsby to Portsmouth ran into the new minefield: the minelayer HMS Adventure, escorted by the destroyers HMS Blanche and HMS Basilisk. At 5.18 a.m., near the Tongue lightship, Adventure struck a mine and was badly damaged. According to British figures more than twenty of her men were killed or missing and several dozen were wounded. With the help of tugs the ship limped into Sheerness, and her repairs lasted until September 1940.

Blanche, commanded by Lieutenant Commander R. M. Aubrey, stayed with the damaged ship and took one of the salvage tugs in tow. At 8.25 a.m., close to the same spot, she herself struck a mine. The explosion knocked out her power, the ship began to list, and before noon she capsized and sank. One sailor was killed, another was missing and more than a dozen were wounded; the rest of the crew were picked up by tugs and a trawler. Blanche was a B-class destroyer built in 1930, which during the Spanish Civil War had carried out patrol duties off the Spanish coast. She was the first British destroyer sunk in the Second World War.

The minefield laid that night proved one of the most effective of the whole campaign. According to British tallies, in the following weeks thirteen merchant ships totalling nearly 49,000 GRT were sunk on it in addition to Blanche, among them ships of neutral Italy and the Netherlands. On 13 November itself the freighter Matra, from a convoy coming from Halifax, struck a mine and sixteen men were killed, and on 27 November the Dutch liner Spaarndam followed. The Admiralty diverted shipping to other channels, but there were not many of them: by mid-November only one of the three deep-water channels into the Port of London remained open.

20 November: mines from the air

Chronologies say that on 20 November the Luftwaffe and U-boats began to mine the Thames Estuary. In fact the only new thing that day was mining from the air. Submarines had been laying mines off the east coast of England since 4 September and went on doing so in those days too: on 17 November U-19 laid mines on the banks off the Lincolnshire coast, and during the night of 21–22 November U-20 did the same near the Newarp lightship off Great Yarmouth.

During the night of 20–21 November 1939 nine Heinkel He 59 seaplanes flew out over the Thames Estuary carrying magnetic mines, which descended into the water on parachutes. According to an American study of aerial minelaying, five of them turned back because their crews could not find the target, while four dropped seven mines. Over the next two nights aircraft dropped several dozen more. Some sources date the start of aerial minelaying to 18 November; most, including the British daily tallies, point to the night of 20–21 November. The aircraft belonged to the coastal air arm (Küstenflieger), units that were formally part of the Luftwaffe but operationally under the navy. The more modern Heinkel He 115 seaplanes were also used for minelaying.

An aircraft could lay a mine where no ship could go: in shallow channels right by the coast and in river mouths. But aerial minelaying had a weakness. A mine dropped at night from low altitude could land on a sandbank that the ebb tide would uncover. That is exactly what happened two nights later off Shoeburyness, where the British found an intact German mine lying in the mud. Its disarming opened the way to an effective defence.

21 November: Belfast in the Firth of Forth

HMS Belfast was one of the newest and most powerful cruisers in the Royal Navy. She belonged to the last group of the Town class, armed with twelve 6-inch (152 mm) guns in four triple turrets. She was built by Harland and Wolff in Belfast, and was launched in March 1938 by Anne Chamberlain, wife of the Prime Minister, Neville Chamberlain. The ship was commissioned on 5 August 1939 under the command of Captain G. A. Scott. In the first weeks of the war she served on blockade duty in the north with the 18th Cruiser Squadron from Scapa Flow. On 9 October, north-west of the Faroe Islands, she intercepted the German liner Cap Norte, which, disguised as a Swedish ship, was trying to slip home from Brazil with German reservists on board. During the night of 13–14 October she was lying in Scapa Flow when U-47 sank the battleship Royal Oak in the anchorage.

On 10 November Belfast was transferred to the 2nd Cruiser Squadron, which was to form a strike force for operations in the North Sea at Rosyth, far up the Firth of Forth. The firth was no quiet place: on 16 October German bombers had attacked warships lying near the Forth railway bridge. On 21 November Belfast put to sea with the cruiser Southampton and the destroyer Afridi for gunnery exercises, the new squadron’s first cruise together. At 10.58 a.m., near the Isle of May, a mine exploded beneath her hull.

The mine had been laid on 4 November by U-21 under Kapitänleutnant Fritz Frauenheim. This small Type II coastal boat had left Kiel on 22 October, during the night of 4–5 November laid nine magnetic mines in the firth, and returned to base on 8 November. The British knew nothing about it: sweeps designed for moored mines could not detect mines lying on the bottom. Two merchant ships were later sunk on the same field.

The explosion, although it tore only a small hole in the bottom beneath one of the boiler rooms, shook the whole ship. The hull buckled upwards, the keel cracked, the decks were distorted, and the engines and boilers were torn from their mountings. According to British figures more than twenty sailors were wounded and many others suffered lighter injuries; one of the wounded died on 30 November. The tug Krooman, which had been towing gunnery targets for the squadron, cast them off and took the cruiser in tow, and other tugs soon joined her. Belfast was hauled back to Rosyth. Destroyers and escort vessels searched the firth for the submarine that had supposedly torpedoed the cruiser, but U-21 had already been in Kiel for two weeks.

A long repair

The first survey looked grim. The damage to the hull and machinery was so extensive that writing the ship off was considered. It was decided, however, to rebuild her. After temporary repairs at Rosyth the cruiser was paid off on 4 January 1940, and in July 1940 she was moved to the naval dockyard at Devonport. There, over more than two years, her hull was straightened and strengthened, her side armour lengthened and thickened, bulges were added to improve stability, her anti-aircraft armament was reinforced and new fire-control radars were installed. She returned to service in November 1942, in effect a new and more powerful ship.

The mine that put a cruiser worth more than two million pounds out of the war for three years had been laid by a submarine displacing some 280 tons. That disproportion showed better than anything what the magnetic mine meant for the Royal Navy in the autumn of 1939. For the Admiralty, headed by Winston Churchill, and for the First Sea Lord, Admiral Dudley Pound, it was another blow after the sinking of Royal Oak: two of the fleet’s most important anchorages had turned out to be dangerous, one because of a submarine, the other because of mines.

The height of the mine crisis

November 1939 was the worst month of the mining campaign. Mines sank 27 merchant ships of some 121,000 GRT off the British coast, more than U-boat torpedoes sank in the Atlantic. Among the victims were neutral ships, such as the Dutch liner Simon Bolivar, sunk off Harwich on 18 November with heavy loss of life among her passengers. The British government responded by announcing that German exports carried in neutral ships would be stopped, which provoked protests from the Netherlands, Belgium and the Scandinavian countries. The press and Parliament demanded explanations, and the Admiralty worked hard on countermeasures at the mine school, HMS Vernon, in Portsmouth.

Significance

The damage to Belfast was the most painful loss that German mines inflicted on the Royal Navy in the first months of the war. A few days later, however, the British recovered an intact mine at Shoeburyness, learned how its firing mechanism worked and began to degauss the hulls of warships and merchant ships by wrapping them in cables carrying an electric current. On 4 December the battleship Nelson was still damaged by a mine in Loch Ewe, but in the winter of 1939–1940 the mine crisis gradually died down, partly because the Germans ran short of magnetic mines. The mine war went on until the end of the war, with ever newer acoustic and pressure firing mechanisms, and the east coast convoys sailed along narrow channels swept every day.

After returning to service Belfast covered the Arctic convoys to the Soviet Union. On 26 December 1943 she took part in the Battle of the North Cape, in which the German battleship Scharnhorst was sunk, and in June 1944 she bombarded German positions in Normandy. After the war she served in the Far East and fought in the Korean War. Since 1971 she has been moored as a museum ship on the Thames in London, in the care of the Imperial War Museum, not far from the waters where German destroyers and aircraft laid mines in November 1939. She is a reminder that one of the first lessons of the Battle of the Atlantic was that the war for the sea lanes was also fought in the shallow waters right off the British coast.