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Minesweeping, Mines, and Malta: The Deadly Seabed

 

In February of 1941, the Germans dropped mines over the grand harbour. These mines did not float. Instead, they sunk to the bottom waiting to be triggered by a passing ship. These weapons were called ‘influence mines’ and there were several types. 

 

Acoustic mines

One type was capable of hearing: the acoustic mine. They appeared in October of 1940. It was also noted that ships travelling at higher speeds (therefore making more noise) were usually the ones hit by mines. Their potential sensitivity was so acute that enemy planes switched off their engines just before they dropped their sound-sensitive mines.

2 minutes after entering the sea, water pressure would start a hydrostatic clock, arming the sensitive mine 17 minutes later. Using a (carbon-button) microphone/transformer, the resonance created by the sound of a ship’s engine would force it to make electric contact with its circuitry, transferring an electric charge into a series of relays. This way, it allowed itself to be triggered solely by the specific frequency and duration of a passing ship.

 

To sweep these mines, the allies used a sound-acoustic device located inside or outside the hull just beneath the bow. There were several variants: the ‘towed box’, ‘oscillator’ and ‘kango hammer’. The latter being a large device that could be lowered into and raised from below the bow. Using a jack-hammer like mechanism, it emitted an extremely loud acoustic signature into the sea in front of the sweeper, detonating any ground mines sensitive to such sound. In Malta, this was used on the minesweeping/drifter Ploughboy as well as on the drifters ‘Girl Margaret’ and ‘Justified’.

 

With such technological developments and improved techniques Malta’s sweepers could better defend the island from such underwater threats. Nevertheless, the Germans had many other types, such as mines with both magnetic-acoustic capabilities combined, not to mention influence mines fitted with pressure-acoustic triggers in 1944.

Magnetic Mines

Cutting of the mooring did not work for magnetic mines. Firstly, these highly engineered weapons rested directly on shallow seabeds waiting for a ship to pass by close enough to be electromagnetically significant. Using gimbals and mu-metal, a passing ship would interfere with the magnetic field enough to effect a magnet mounted on a swing. When the distorted magnetic field pushes it downwards (not upwards, ie: not attracted to the ship), it touched a contact point which would complete a circuit detonating the mine.

Later mines were even more sensitive. They could detonate with any change in the magnetic field rather than the intensity of one pole. Their trigger was a special setup consisting of a spindle placed precisely between two small magnets in a ‘neutral position’. Therefore, the ship’s magnetic polarity would then be irrelevant to the mine. Both unipolar and ‘M3’ bipolar magnetic units were in use by 1939). 

 

To sweep them, two wooden boats (of a specialised class called ‘MMS’ or ‘motor minesweeper’ class) would each tug two cables with an electrode on both ends. One set of cables would have a short negative and a longer positive. At predetermined and highly synchronised intervals (5 seconds, every 30 seconds), the lines on both minesweepers would pulsate, emitting a strong magnetic signature onto the seabed which would detonate any magnetic mine within the large area. This was known as the double L sweep and it could create safe corridors for ships and convoys faster than ever before.

 

Sometimes, mines would damage the cables, causing extensive damage and halting the minesweeping operation entirely. Ernest Goodall recounts one such situation when the chief engineer called him down to the engine room to check if there was a problem with the ammeter:

There certainly was.  It was reading over 4,000 amps instead of the normal 3,000 amps.  It was then obvious that the cable had been stripped of its insulation causing a short circuit. I reported to the Skipper that we had damaged the cable with our last mine so we couldn’t continue sweeping.  He said that it was out of the question to pack up as there was a convoy due in a few hours time.  I told him that I hadn’t got the jointing equipment – very large brass lugs and sweating gear like they use at the shore maintenance base – but I was willing to have a go at doing a joint by hand, to which he agreed. The cable was pulled aboard until we got to the damaged section, and there was about two hours of daylight left in which I thought I could just about manage.   I cut the cable completely through and stripped back the insulation about four feet along each piece. I then proceeded to do a married joint of about 300 strands of wire. 

 

This ingenious method was pioneered by Charles Goodeve. Through sheer alacrity and passion for naval technology, he went ahead with his plans despite administrative red tape and all sorts of set backs. On the same day he proved his concept with model boats in a lake, he was told to terminate his experiments for the ‘LL’ sweep since they “will prove self-cancelling, and cannot work”. After a successful test using model boats, Goodeve organised a sea Trial on boxing day in 1939. ‘LL sweep’ equipment was in full production by January, and the MMS class of sweepers by Autumn the following year. So adamant was Goodeve to put his will into reality irrelevant of the difficulties present that young recruits working with Charles on ‘stone frigate’ HMS Vernon suffering to get promoted or asked to perform the impossible were instructed to ‘do a Goodeve’.

Furthermore, to reduce their ship’s magnetic signature, wire coils or ‘girdles’ were passed alongside hulls to create a strong magnetic field. This would disorient and diffuse the ship’s magnetic pole between North and South, greatly reducing its magnetic signature overall. The process was referred to as ‘de-magnetization’ or ‘degaussing’ and the royal navy immediately set on equipping all its ships with it. This soon proved to be too expensive and a faster method was devised. Heavily charged coils could be passed from underneath the hull of ships in a special station, which enabled it to be ‘wiped’. This was not a permanent degaussing method and had to be done twice a year to maintain its full efficiency. Interestingly, submarines had batteries powerful enough to be able to degauss themselves during operations.

All the while until such equipment reached the far corners of the Empire, in Malta’s harbour the drifter Ploughboy would drag (skid) a magnet underneath and fire her lewis gun hoping it wouldn’t blow itself up in the process. The first drifter to be equipped with ‘LL’ magnetic equipment was the Trusty star in April of 1941. Meanwhile, mines were also directly responsible for civilian deaths. The same month, 28 dockyard workers were killed by a mine inside the harbour. Earlier, in February, mines were responsible for sinking the all important ferry between the two main islands. During the war the air, sea, and seabed were all deadly.