Ferry van Eeuwen
During WWII all sorts of mines were developed and used, sometimes complete minefields were created. A particularly deadly variety was the magnetic mine which was engineered by the German naval research into torpedo and mine fuses. They successfully developed a murderous magnetic proximity fuse. The magnetic mine was based on the principle that when the residual magnetism of a ship distorted the local geomagnetic field of the sensor, it activated the mine's magnetic needle of the trigger.
One of the countermeasures was the installation of wiping stations which degaussed or demagnetised the ship's natural magnetic field. For years after WWII Dutch ships had either to be degaussed by a shore based wiping stations or they carried special degaussing equipment on board the ship. All Dutch ships in the post-war period had degaussing equipment installed. Also Shell tankers like the Kermia on which I sailed for two years were provided with a degaussing unit.
Each ship has a unique magnetic signature which is the total result of the materials used to build the ship. As already stated above the materials used such as steel, iron, rivets, welded seams and the copper screw all submerged in an electrolyte, which sea water is, would help to build such a signature. This was proven by studies of magnetic ship signatures utilizing detector coils placed on the bottom of harbours which indicated that the intensity of the stray magnetic field varied from ship to ship, even within the same class. Furthermore there is the induced magnetization which is the result of the interaction of the ship with the geomagnetic field. A ship moving north will have a north pole induced in its bow and a south pole induced in its stern, and if heading south, the opposite results. The extent of the induced magnetization depends on the strength of the local magnetic field, the heading of the ship, but also the age of the ship, its composition and its speed. The objective of completely degaussing the ship was to eliminate or drastically reduce its induced, residual or stray magnetism. A disturbing factor was that the return of the magnetic signature after degaussing had a tendency to increase unpredictably. Natural processes and even minor modifications to the ship were also responsible for this process.The magnetic mine
Mines in general are a cheap means of warfare, both offensive and defensive. Magnetic bottom mines could be built simply, inexpensively and in very large quantities by unskilled labour. The going cost for a magnetic mine was around US$10,000. They are hard to locate and can cause huge damage to ships, their cargo and their personnel. Magnetic mines laid by Germany in 1939 virtually brought infested ports to a standstill until sweeping techniques could be developed. The mines can be laid by special minelayers or U-boats and dropped by airplanes by parachutes. The magnetic mine once grounded on the seafloor or riverbed armed itself and sat and patiently waited for a disturbance in the earth's magnetic field which would then detonate the deadly charge.
The German minelayer Brummer, 1940. Here seen in action off the Norwegian coast during a World War II mine laying operation. Note the mine launching doors in the ship's transom, and the snow on her gun blast shield.
Shallow water magnetic mines were developed by the British and first used in 1918 off the Belgian coast as anti-submarine devices. The original magnetic mines built by the British used the vertical component of the ship's magnetic field to trigger the mine when a given field density was reached. Later the British developed a superior derivative of their 1919 mines which worked off the horizontal component of the ship's magnetic field. This apparently insignificant change made it possible to design a mine fuse which responded to the rate of change of field strength rather than absolute field strength. This made defence against magnetic mines by degaussing and magnetic sweeping procedures far less effective.
German magnetic mines where built in several varieties, each suited for a special purpose. The magnetic ground mines for up to 50 meters water depth were indicated with RM and those for up to 700 meters laid by U-boats were referred to as SM. A mine could be detonated from its ground position or in deeper water once triggered left its sea bottom anchors and by means of compressed air it would propel upwards. Some were built to detonate on impact with the ship's hull, others were detonated by a pressure switch set for maximum effect. The gas and foam explosion would lift the ship's hull in a certain position. After the pressure was lost when breaking the surface for a short time a large and strong cavity was created underneath the ship causing another opposite bending moment. The result usually was that the ship would break in two pieces.
An example of a hemispherical ground magnetic mine laid by surface craft. The charge was approximately 1,764 lbs. (800 kg). It was known as the Schildkröte (turtle mine).
A later improvement came with the introduction of double-tap fusing. In this case the mine would be activated by increasing magnetic field strength but only detonated by decreasing field strength. Thus the mine would explode as the ship passed, under the screws rather than under the bows. The combination of horizontal-component and double-tap fusing was an order of magnitude more damaging and was also much more difficult to simulate when sweeping. It made the whole generation of Second World War built minesweepers obsolete.
Degaussing or Wiping Station
The word 'degaussing' was invented after the scientist Karl Friedrich Gauss who researched the principles of magnetism and it was adopted to cover all methods of protecting ships against magnetic mines by reducing the ship's magnetic signature. In Britain ten wiping stations were active during the war which degaussed in total about 10.000 ships. In Holland a wiping station was located in a harbour in Rotterdam, the Waalhaven until the 1970's.
This is the post war wiping station located in the Waalhaven harbour in Rotterdam. It was a heritage of the German occupiers and was dismantled in the 1970's. The height of the vertical structure was 42 meters. It could therefore only handle ships up to a certain maximum tonnage.
On board degaussing equipment
Degaussing equipment was also installed on the Dutch Shell tanker Kermia on which I served for two years during the second half of the 1950's. Thick cables forming a loop, aligned with the main deck, were fastened around the length of the vessel. The wire was energized with an electric current that neutralized or greatly reduced the ship's magnetism signature. On the Kermia the electric current was provided by a three phase motor connected to a direct current generator which fed the loop. Such systems saved without doubt countless numbers of ships during the war and also in the post war period from destruction.
Still in the 1960/1970's it was normal procedure to switch on the degaussing equipment after leaving a Dutch harbour. When sailing to German ports ships followed a swept channel marked by buoys. The Dutch and German navies guaranteed that a ship sailing in this lane was safe from mines. The degaussing was switched on for the stray mines, just in case. When sailing the Atlantic through the English Channel the degaussing equipment was operational to about Ouessant, France.
Needless to say that the degaussing equipment did not protect a ship against a variety of other mines such as acoustic mines, contact mines and the later developed pressure mine. The latter worked on the principle that when a ship passed overhead a small under-pressure - suction effect - was created by the ship's hull which triggered the mine's charge. The problem was that no countermeasures could or can be taken against this type of mine.
The drawing shows the principle of the pressure mine. Me= Pressure membrane Ba= Battery H= Protective dome Ue= Bypass Bl= Rubber bubble Ko= Contact switch Z= to detonator By the looks of it this is a simpler device than the magnetic mine, but even more deadly.
Damage by magnetic mines
Even in inland waterways magnetic mines can cause huge damage like in the case of the Dutch "Pilot Boat 19" on 11 May, 1940. This pilot vessel was to carry the gold supplies of the ‘Nederlandsche Bank’ (Dutch Central Bank) to England. On 10 May, 1940, she loaded her precious cargo at the Boompjes quay opposite the Bank. The next day the ship steamed up the river ‘Nieuwe Waterweg’ heading for Hook of Holland and the North Sea. Unfortunately, the vessel struck a German magnetic mine close to Vlaardingen, about halfway between Rotterdam and Hook of Holland. The vessel sank, taking literally all that sailed on her to the bottom of the river. The Dutch Pilots’ Corporation Rotterdam-Rijnmond still has a plaque in the office with the names of the twelve pilots and crewmembers who lost their lives in this tragic accident. The unfortunate ship was salvaged during the war, together with part of the precious cargo of gold. However, 111 bars of gold were missing. Finally, in post-war years 56 bars were retrieved. This must have been the first magnetic mine incident in Holland after the start of German occupation.
The menace presented by a magnetic mine and the devastating effect it could have on ships is also illustrated by another even more tragic incident which happened shortly after the sking of the "Pilot Boat 19" and which took place on 30 May, 1940 off Willemstad. The inland waterway ship 'Rhenus 127' hugely overloaded with about 1200 (!) prisoners of war triggered a magnetic mine which exploded amidships. The ship broke in two pieces and sank. Almost 200 Belgian soldiers went down with the ship. A special honour field at the Willemstad's War Cemetery keeps the memory of this bizarre incident alive. Last year 30 May, 2005 a large remembrance was held in Willemstad.
A picture of the ill fated "Rhenus 127" could not be found, but this is probably a very similar ship, the "Rhenus 150" built in 1943 by the yard Schimag in Neckarsulm, Germany. The ship is still going strong as the "Adios" and now owned P. Schouwenaar in Rotterdam. The ship's length is 55 metres, width 7,31 metres. Tonnage: 576. The 1200 prisoners were probably stowed like cattle in the ship's holds and then could hardly breath at all. A nice illustration of German misbehaviour and lack of respect of prisoners of war.
The river Rhine which runs through six countries got its name after the river god "Rhenus Bicornis", the "Two Horned Rhine". The river god was supposed to bring fertility and wealth to the surrounding areas. The Roman relief shown in the picture was found during excavations along the borders of the river Rhine in 1911 and stems from the 2nd Century AD.
I have no further particulars of the degaussing equipment on Dutch ships like the make, cabling used and current sent through the loop. Any extra information would be helpful.