Fatal attraction
You’d be forgiven for guessing that ‘degaussing’ and ‘deperming’ are treatments used by Hollywood hair stylists. But the truth is way more exotic – the terms relate to magnetic fields and electricity – and more specifically, to techniques used to hide nav
Degaussing and deperming are odd terms – but nowhere near as odd as the processes they refer to.
By definition, large, steel-hulled vessels project a magnetic field around them – a field typically acquired during the construction process where there is lots of cutting, hammering and welding.
As they move through the water their magnetic fields interact with a much larger magnetic field – the earth’s. This interaction gives every vessel a distinctive ‘magnetic signature’ – easily spotted by other ships, submarines or aircraft equipped with ‘magnetic anomaly detection’ (MAD) equipment.
Logically, if it were possible to minimise a vessel’s magnetic signature, detection by the enemy would be much more difficult – and much better for clandestine operations.
Degaussing and deperming are techniques used to neutralise a vessel’s magnetic signatures. Both use plenty of electricity to do this – and the process usually involves highamperage cables running within or outside the entire vessel.
The need for degaussing and deperming began at the beginning of WWII when Germany’s engineers developed a sophisticated magnetic trigger for sea mines. Rather than requiring actual contact with a ship’s hull, the mine detonated when it sensed a nearby ship’s magnetic field.
Luckily for the Allies, one of these new mines was poorly deployed and ended up on a sand bank in the Thames estuary. After being disarmed and examined by a British team, it quickly became apparent that minimising a ship’s magnetic field was a desperately-needed counter-measure.
Degaussing was born. And in case you’re curious about the origin of the name, ‘gauss’ is the scientific unit of measurement for a magnetic field – named, ironically, after the German mathematician and physicist Carl Friedrich Gauss.
Early degaussing systems saw cables wrapped outside a vessel’s hull – bow to stern, on both sides. The electrical current passed through the cables neutralised the ship’s magnetic field – but only temporarily. Ships had to undergo the process a few times a year at strategicallylocated degaussing ‘stations’ – a bit like sailors needing a regular shot of penicillin.
ABOVE A conventional ‘contact’ mine with horns.
Such stations were built all over the world during WWII – including the two in Auckland and Wellington. Many of the steel ships in the flotilla used to rescue soldiers from the beaches of Dunkirk were ‘cleansed’ in a marathon four-day effort by degaussing stations.
Later, degaussing systems were built into the vessels themselves – and could be activated whenever the ship was sailing through waters likely to contain magnetic mines. The legendary liner Queen Mary became one of the most well-documented examples of the on-board degaussing system during WWII – when she was converted into a troop carrier.
Submarines, though, posed a different problem. They couldn’t use large-diameter cables running inside, along the length of the hull. That would require cutting holes in bulkheads, and that could compromise the vessel’s structural integrity. Today,