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Discussion Starter #1 (Edited by Moderator)
  • 1 Introduction
  • 2 Early Degaussing Systems
  • 3 Operating the Degaussing gear
  • 4 Later Developments
  • 5 Recollections of Degaussing gear by SN members
  • 6 External References
  • 7 Images
  • 8 Contributors

A steel-hulled ship acquires a permanent magnetic field during the course of its constuction. This magnetic field either adds to, or subtracts from, the Earth's magnetic field. Magnetic mines were built with sensors to trigger when they detected the distortion in the Earth's magnetic field caused by a passing ship. Magnetic mines had originally been built during WW1 but research on them had been discontinued - except in Germany. The mines could be laid by ships or aeroplanes.

The UK was, and to a large extent still is, dependent on food supplies from abroad to feed the population. The high rate of loss of ships due to mines at the outset of WW2 threatened the country's survival. Efforts were made to remove mines with minesweepers but this was not sufficient to control the problem. An answer had to be found by reducing the magnetism of ships so that they were not detected by magnetic mines.

The term Degaussing was coined for this process being named after Carl Freidrich Gauss - a German genius and pioneer of mathematics and magnetism who died in 1855, and in whose honour the cgs unit of magnetic induction had been named.

Early Degaussing Systems[edit]

Pioneering work on degaussing was done by Charles F Goodeve, a Canadian who also contributed to the development of the Hedgehog anti-submarine weapon. The aim of degaussing was to, as far as possible, cancel out the ship's magnetic field - or at least reduce it to the extent that it would not trigger magnetic mines.

Early magnetic mines detected the vertical magnetic field from a ship passing over it, so the first approach was to install a coil of cable around a ship and pass an electic current through it so that it neutralised this magnetic field. The coil was installed externally on ships already in service - this technique was known as Coiling.

The photograph below shows the degaussing coils installed on the ill-fated tanker San Emiliano

Image 1: Degaussing-1.jpg

The two photographs below shows the degaussing coils installed on Sheaf Royal and were provided by SN Member Captkenn. The coils are in the box-shaped trunking that goes diagonally down to the deck.

"Here is a picture of degaussing equipment fitted to the Sheaf Royal. It consisted of 4/6 massive cables boxed in brackets, running on the deck under the rails all round the ship and then you can see it in a trunk going from the deck into the forecastle. It was a pain painting it!"

Image 2: Degaussing-2-Sheaf-Royal.jpg

Image 3: Degaussing-3-Sheaf-Royal.jpg

The size of many ships meant that it was not a simple matter to reduce the magnetic field. The current flowing through the coils needed to be adjusted, and measurements taken to ensure that the optimal adjustment had been made. To do this the government built a number of "Degaussing Inspection Stations". There was one at Harwich, though this was considered a bit too close to the enemy; the most safely located site was based at an old fort on the Clyde opposite Greenock and Gourock.

Calibrating the degaussing gear was a tedious business with the ship having to sail up and down over a marked course with underwater sensors measuring the magnetic field each time. The currents through the coils were adjusted until the best result was obtained. An account of this process is given at External Reference #1.

Operating the Degaussing gear[edit]

Stan Mayes notes that he is unsure where the control gear for the degaussing equipment was on San Emiliano but thought it most likely to be in the engine room as the cables went in that direction.

Protection from magnetic mines would only be required in coastal waters so it is presumed that the degaussing gear was turned on as the depth of water decreased to the depth where they would be able to operate.

Looking at the illustration below, the equipment was turned on and off using knife switches and presumably there was a fair sized spark when they were used.

Image 4: Degaussing-control-panel.jpg

Later Developments[edit]

Ships built with Coiling after the succesful trials incorporated the cables within the structure of the ship instead of having it exposed on the outside. A big disadvantage of the Coiling approach was the cost - especially with more and more ships needing to be protected.

Goodeve next invented a technique called Wiping. This involved making permanent changes to the magnetic field of a ship by dragging a cable alongside it with a 2,000 amp current flowing through it. This was found to be as effective as Coiling, and initial views that the magnetism would be quickly lost by the pounding of the sea and vibrations from the engine proved incorrect. However it was found that the magnetic field of the ship was affected over time as it travelled through the earth's magnetic field. This could be reduced by frequent changes of direction, but it was also necessary to repeat the Wiping progress at intervals.

Increasing sophistication in the construction of magnetic fuses required increasing complexity in the design of degaussing coils used as the improved fuses were able to detect small differences between the magnetic field of different parts of a large ship as it passed overhead.

External Reference #2 provides further information about the need for various types of coil needed to protect against the increasingly sophisticated detection system used in magnetic mines since the end of WW2.

External Reference #3 is a bit confused but suggests that Wiping was conducted in Falmouth Dock.

The following thread on Ships Nostalgia is about degaussing on ships after the end of WW2: SN Degaussing thread

Recollections of Degaussing gear by SN members[edit]

SN member Steve Hodges: "..... the Patience, which as I recall had full electrical degaussing gear for countering magnetic mines, paid for by HM Government. I was told that she was the largest vessel ever thus fitted. Is my memory playing tricks or can anyone confirm?I seem to remember some special switchgear on the flat below the ECR in a caged off section, with massive cables leading off to port and starboard. These went in a complete circuit of the main deck, the cost must have been enormous."

SN member Blythespirit: "One of the shell tankers I sailed on had degaussing gear on and when we had orders to scrap it the degaussing gear got cut up for scrap, the wiring was armoured and an absolute nightmare to retrieve the copper for the scrap man! But we managed it in the end and a very welcome bonus for us engineers!"

SN member Raybnz: During my time with Shaw Savill the ships I sailed on had deguasing gear which if I remember rightly was checked on a regular basis. The apparatus was situated in a mesh cabinet."

SN member John F: "I joined the British Glory in 1959, built in 1957, & she had degaussing gear, although it was never explained to me how it worked. I think that most of BP's vessels built about that time, certainly the larger ones, had it fitted."

SN member Henneganol: "One of the ships on which I sailed, and the British Valour springs to mind, after drydocking on the Tyne called in at Portland and made several runs over the degaussing range in order to test our system. I also think that there may have been another degaussing range in Gibraltar!"

SN member Vital Sparks: "Maintenance of the degaussing gear was paid for by the MOD and switchgear was in locked cabinet not normally accessible to ship's staff. I saw the gear removed from British Holly after vessel deemed too old for military service and MOD stopped funding. Cables were 2 inches is diameter, copper stranded, but surprisingly flexible once cut free. Short lengths passing through each frame were left in place."

SN member Sarky Cut: "Degaussing was fitted to many BP Tankers. Remember that the government owned a majority share in the company and could in fact dictate what was fitted. A demurrage was paid on the gear as it was heavy and therefore took away a few tons of capacity that could mount up over the period of the ships life.

Four multicore cables were fitted and they were connected so that there was in effect one conductor around the entire hull giving a huge coil. The power was DC via a plate rectifiers fed with low voltage from a transformer. The current was adjusted by altering the value of the ballast resistance.

Most naval bases had a range with in easy reach. There was a range in Gibralter and most of the new DG equipted ships built in the UK with coated tanks would load fresh water in Southampton and proceed to Gibralter,discharge the FW into tanks ashore ballast with seawater and run the range. Reading would be taken of the magnetic signature by the range officer and passed to the ship for the adjustments to be made to the coil for optimum effect. The maintenance would consist of a visual inspection and checking the connections in the various boxes around the ship. Followed by a "megger" test. There were also compass correction coils fitted to the Magnetic Compass Binnacle that had to be opened ,checked and tested for resistance and insulation value. There was a range outside of Simonstown as well.

Many of the systems were useless after a few years as the armouring deterioted in the weather or became damaged. The cables were often underwater the moment the ships left port as the swell broke over them continously. The insulation values were low at the best of times but in an emergency situation they may have improved with the current being applied.

RN ships were much more sophisticated with heeling coils and all other sorts of additions. There was a special department in MOD with men in suits based at Fort Rowner Gosport that would visit merchantmen with their little box that contained a compass. This would be walked around the ship with the system on and end up by the binnacle to set the coils for compass correction. As the vessel passed into the Southern Hemisphere the coils set up in the North became somewhat useless.

External References[edit]
  1. World War 2 Living Memorial: Reminiscences of a WREN working at a degaussing station on the Clyde: The Navy at Work
  2. Federation of American Scientists: Degaussing Military Analysis Network
  3. People's War: Degaussing ships in Falmouth Docks People's War

  1. Photo of San Emiliano by SN member Stan Mayes
  2. Photo of Sheaf Royal by SN member Captkenn
  3. Photo of Sheaf Royal by SN member Captkenn
  4. Illustration of control gear from Britain's Merchant Navy Ed.Sir Archibald Hurd
  1. Initial entry completed by Benjidog
  2. Information provided by Stan Mayes and many SN members in threads

1 Posts
I see the contributor Vital Sparks was on British Holly when the degaussing gear was removed. We were in Piraeus in Greece at the time in 1975. I was one of three engineer cadets and it was my very first trip to sea. I was involved in helping remove it from along the edge of the main deck. I have a foggy memory that certain sections behind bulkheads had already been removed at some time in the past, no doubt for scrap to fund the officers bar.
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