The campaigns in the
Bristol Channel were supplemented by many other mine laying operations around the East
Coast of Britain resulting in severe disruption in coastal traffic. The net result of the
campaign was to increase the importance of west coast ports in the reception of North
Atlantic convoys which was exactly what U-boat command had anticipated. The mines were
beginning to take a worrying toll of coastal ships. However, because of the covert way in
which they were laid, and the near impossibility of being able to detect them, many mine
detonations were attributed to torpedo attacks. The result being a large number of
supposed U-boat sightings around the coast even though the number of boats involved at any
one time was very small.Clandestine magnetic mine laying
was clearly a success by the onset of winter 1939. At a senior staff conference to review
the mine war in November, Dönitz and Raeder considered future strategies and determined
the following:
Minefields were to be laid at as many vital point as possible.
The intention being to cause general disruption to shipping traffic.
Narrow access channels were to be given special attention with a
number of minefields to deny them to the enemy for a long period of time.
Experience with the TMB mine highlighted the necessity of laying
the weapon at the appropriate depth. The 25 metre depth limitation was causing problems
for the crews. The Mines Inspectorate were in the process of developing a larger weapon
with a charge of 2000 pounds. This could be laid at greater depths, allowing for more
operational flexibility. This new mine, the TMC (Admiralty designation ‘Type
GS’) was expected to be ready by December 1939.
The Bristol Channel was confirmed as a perfect environment for
mine laying. U-boat command realised that with the virtual closure of the North Sea ports,
Liverpool, Swansea, Barry, Cardiff, Newport, and Bristol would be vital for importing food
and war supplies. It is certain that these ports would have experienced far heavier mine
laying attacks but for the shortage of suitable U-boats.
The TMB and TMC mines would have doubtless remained a formidable
weapon for a long time were it not for the intervention of the Luftwaffe. Senior
German Air Force staff were great admirers of the magnetic mine and pressed for the need
to have an air dropped variant from the outset of the war. Senior naval officers objected
strongly to air dropped mines, fearing that they would be imprecise and run the risk of a
mine falling on land. Naval staff were fully aware that the effect of the weapon depended
solely on the enemy’s ignorance of how it worked. However, the Air Force argued that
the shortage of U-boats meant that mines could never be deployed in the numbers necessary
for mass disruption unless air delivery was adopted. The Air Force argument carried the
day and air dropped variants of the mines were constructed. The magnetic fuse was
comparatively delicate, necessitating the need for a parachute to limit the force of
impact upon the water. This was another unpopular step as it enabled the mine to drift on
the wind, often for a considerable distance, compounding the fear that they would land on
dry land. As a precaution, the parachute mines were given bomb fuses which would explode
the mine a short period after impact if by that time the fuse was not covered by at least
4m (13 feet) of water.
The German Air Force jealously watched the
successful ground mine campaign develop and became eager to develop their own successes.
Air Force units commenced laying parachute mines on 17 November 1939, regardless of the
protests by the navy. Within days the navy’s worst fears had been realised. On 23
November 1939, the first magnetic mine was spotted on the mud flats near Shoeburyness. In
an operation that forms one of the most famous chapters of bomb disposal history,
Lieutenant Commander R.C. Lewis RN, and Chief Petty Officer Baldwin defused the mine.
After a day of careful work, the Admiralty were told that the first magnetic mine had been
recovered. The British knew precisely the value of the information that had been obtained.
On the evening of 23 November, Lieutenant Commander Lewis gave a presentation of his
findings to Winston Churchill and sixty senior naval officers. Within days the scientific
secrets of the mine were revealed and countermeasures were being devised.
The technical experts at HMS Vernon were aware that
the ground mines were magnetically detonated but at last they had an actual example to
work with. The crucial information was the polarity of the detonator and whether they
worked in the horizontal or vertical magnetic field set up by the metal hull of a ship.
The Shoeburyness mine was actuated when subjected to a field of 50 milligauss and it was a
simple matter to reduce the magnetic field of any ship below 50 mg by winding a current
bearing coil around the hull of the ship to give her less North Pole down polarity. This
countermeasure became known as ‘degaussing’. A merchant ship could be
effectively degaussed in three days, although even this short delay often caused problems.
A simpler and faster process known as ‘wiping’ was also devised which would
quickly alter the magnetic signature of a boat for a temporary period (up to six months).
Wiping only took about twelve hours and could be done with a minimum of specialist
equipment in any port.
Once the technical nature of the mine had been laid
bare, its desired effect was much reduced. The Navy’s dire warnings that the Air
Force would give the secret away had come true with a startling speed. The mine war
intensified thereafter. Both sides invested considerable time and resources in producing
more complex fuzz systems and effective countermeasures. U-boat command were aware that
Allied shipping was being fitted with degausssing equipment from April 1940. Towards the
end of 1940, The TMB/C series mines were fitted with magnetic/acoustic fuzzes which caused
great disruption until effective countermeasures were devised. However, the defusing of
the Shoeburyness mine had already robbed the U-boat arm of one of its best assets;
surprise.
A final rather unpleasant postscript to the story
concerns the German Air Force. The incorporation of a bomb fuse into the air dropped
variant enabled the weapon to be used as a high capacity parachute retarded blast bomb.
This method of use proved irresistible to the Air Force, particularly as they were facing
a shortage of large conventional bombs. On the night of 16 September 1940, at least 25
magnetic mines were dropped on London. Seventeen of these weapons failed to explode; due
in part to the fuses being damaged on impact or being hung up on their parachutes and
failing to go into their arming cycle. Those that did explode invariably had a devastating
effect and many British towns were to bear the scars of the massive explosions that these
weapons yielded. Both Swansea and Cardiff suffered dreadful damage on the occasions that
parachute mines were used.
Although U-boat command moved its boats onto other
tasks, the Air Force continued to use mines against trade in the Bristol Channel. Parts of
the Channel were first mined by Heinkel 111s of I/KG 4, based at Soesterberg in Holland,
on the night of 17 July 1940. As an attempt to maintain the pressure on the defences, and
to interrupt vital war production, missions were usually flown on nights when no other
raids were taking place, thereby extending the amount of time an area remained under Red
Alert. Further missions were flown in late October, with the abandonment of the planned
invasion of Britain and a switch to a policy of blockade being resumed by He 111s of KGr
126 flying from Nantes in Brittany. This unit was reorganised as I/KG 28 in December 1940
and continued minelaying until July 1941. All areas considered suitable for U-boat
minelaying were considered appropriate targets for aerial campaigns. With the realisation
that the mines could cause massive damage on dry land, the minelaying Gruppen were often
used to supplement air raids on the west coast ports. Further minelaying took place
between October and December 1941 with the transfer of Ju 88s of III/KG 30 from the
Balkans to Melun in France.