By their nature these ‘influence mines’ were restricted to use in shallow water, the type TMB in 15 fathoms and the TMC no deeper than 20 fathoms. Smaller mines could be laid through the U-boat’s torpedo tubes; three of them together were about the length of a torpedo. Laid by U-boats or parachuted from low-flying aircraft into harbours, estuaries and coastal sea routes, the German mines caused consternation. The battleship HMS Nelson and the cruiser HMS Belfast were seriously damaged by mines, three destroyers were sunk and so were 129 merchant ships. The Thames Estuary became so littered with German mines that there was talk of closing down the Port of London.
It seemed at first as if it was a problem easily solved, since the Royal Navy had used magnetic mines in 1917. In the interwar years Admiralty scientists had experimented with the magnetic properties of ships’ hulls, but the emphasis was upon counter-measures to the magnetic torpedo. This meant a powerful magnetism that would prematurely explode a torpedo as it approached. But scientists and technicians were few in number and their work was overruled by officials at the Admiralty who thought that mines, like submarines, were weapons for inferior naval powers. Dusting off their old research, the navy tried it out. But they found that magnetic sweeps that countered British magnetic mines exploded German ones. The sweeping devices were often destroyed and sometimes mine-sweepers were badly damaged.
The night of 22/23 November 19391 was dark and moonless. Between 9 and 10 pm a Heinkel He 111, following the Thames Estuary, flew very low over the tip of Southend pier. It was a good landmark: probably Luftwaffe crews were briefed to use the pier as a navigating fix. A machine-gun team at the pier’s far end opened fire and saw two parachutes fall from the plane. Startled by the unexpected gunfire, the Germans had dropped two mines into the shallow tidal water. Its load lightened, the Heinkel sped away.
The report that men had jumped out of the aircraft was discounted. Before midnight, Churchill was told that there was probably going to be a chance to examine the new weapon. By 1.30 am on that same night two experts briefed by Churchill, and the first sea lord, were on their way by car to Shoeburyness, where the mines were exposed by the outgoing tide. By 4 am – with rain falling heavily – the investigating team was out on the exposed mud-flats. Using a powerful portable signal lamp, they were looking at a black aluminium cylinder, seven feet in length and about two feet in diameter. Before anything much could be done, beyond securing the mines, the incoming tide had swallowed them out of reach until the following afternoon.2
Next day steel-nerved technicians from HMS Vernon (the RN’s mine school) defused the weapon. By a stroke of fortune, a mechanical device to keep the mine safe until it had settled on the seabed had jammed. It was actuated by the technicians rolling the mine over but by that time the mechanism had been rendered safe. Stripped of its detonators and priming charges the mine was taken for examination to a ‘non-magnetic laboratory’. In a matter of hours the new weapon was understood: it operated on a vertical magnetic field and required about 50 milligauss to fire it. The threat remained.
When, on Saturday morning, Rear-Admiral W. F. Wake Walker told government scientist Frederick Brundett about the capture of the mine and that he would need twelve engineers by Monday morning, Brundett drove to the south coast to seek out individually men with special engineering skills and sign them up on the spot. One whom he considered essential was already being paid £2,000 a year. ‘As it happened the Director of Scientific Research at the time was only getting about £1,700 a year, and I was subsequently told by the Treasury that I couldn’t do it. I pointed out that I’d already done it and that he was in fact already working for us.’3
Some anti-mine experiments were performed by sailors who towed toy ships backwards and forwards over cables in ‘canoe lake’, a children’s boating pool in Southsea, near to HMS Vernon. Various counter-measures were devised and put into operation immediately. These included ‘de-gaussing’, which neutralized the vertical magnetism of a ship. The mines could be swept by floating electrical cables (through which a current pulsed) behind a de-gaussed ship. Large coils were also installed in low-flying aircraft as a quick way to neutralize minefields.
Such measures were not enough to solve the problem completely. Channels swept by aircraft were narrow and unmarked. The German aviators were bold: one minelaying seaplane landed in Harwich harbour, carefully placed its mines, and then took off again. It was the minelaying operations of low-flying aircraft that led to the construction of new radar stations with apparatus designed for low-level detection. When the Battle of Britain began, these stations were to play a vitally important part in detecting low-flying formations that would otherwise have got in under the radar screen.
The de-gaussing that all British warships were given at this time probably saved some of them from attack by magnetic torpedoes, notably during the Norwegian campaign. At the time these failures were considered to be due to faults in German torpedoes, and no one can be sure of what exactly happened inside the warheads.
The Germans hit upon the idea of sowing mixed minefields with both magnetic and moored mines. These required tricky sweeping techniques. They reversed polarity to catch ships that had been ‘over-de-gaussed’. Delayed-action fuses kept the mines inactive for a period; thus sweeping would be without result.
Then came acoustic mines, which had to be swept with noisy ‘hammer boxes’. There were double fuses that would work only when two triggers were activated, such as noise and polarity. But by the summer of 1940 the magnetic mine had ceased to be a real danger.
On 7 May 1940 a new threat arose. A modified version of the BM 1000, intended as a sea mine in the Clyde, overshot and landed on the Clydeside hills near Dumbarton. These ingenious dual-purpose ‘bomb mines’ were fitted with Rheinmetall inertia fuses when used against land targets. Falling into water at least 24 feet deep they functioned as a magnetic mine; on soft mud or in shallow water they would self-destruct. They would also self-destruct (by means of a hydrostatic valve) when the water pressure lessened, as it would when they were lifted to the surface. It was an example of German thoroughness that, despite all the foregoing precautions, the BM 1000 also incorporated one of the most cunning booby traps ever built. A set of photo-electric cells connected to a detonator would explode the bomb if light got inside it. This was a way of killing any bomb team who got to see the workings. By amazing luck, the bomb found at Dumbarton had suffered a circuit failure.4
A few months later this sort of bomb-mine was extensively used during the night bombing of London. They parachuted down and caused widespread destruction without forming a crater. Londoners found it easy to recognize the results and called them ‘land mines’.
The menace of the magnetic mine had been overcome by the scientists, and this was the important fact. Before the war the admirals and generals in Whitehall had showed little interest in science and technology, but success with the magnetic mine changed that attitude. According to Dr C. F. Goodeve, who was a physicist and RNVR officer: ‘it was the first technical battle in which we won a decisive victory over the enemy; but more important still, it was one which brought science fully into the war in the very early days’.5
In Germany the Nazis had broken with the nineteenth- and early twentieth-century tradition of political encouragement and social respect for science and its practitioners, and there was little or no collaboration between the scientists and the military until the end of 1943, when German scientists were invited to help with the Battle of the Atlantic. Even then they were simply asked to identify Allied radio and radar transmissions.
But Nazi distrust of science and top-level obstruction of research did not change the way in which German industry employed men who knew how to apply science and engineering to design and production. The magnetic mine provides a good example of the excellence of German design. The only reason for the mine’s failure to cripple British shipping was that Germany went to war with only 1,500 of them in stock. After the first sequence of minelaying operations, the Germans had to wait until March 1940 for more to be manufactured.6 It was this respite that saved Britain’s defences from being overwhelmed, and provided an interval during which the menace could be countered.