Michael McFadyen's Scuba Diving - Magnetic fields and Degaussing
As most people would be aware, the principle of a normal car battery is that a positive electrode and a negative electrode are placed in an electrolyte. This causes ions to travel from the negative to the positive and an electric current is generated. Likewise, people may recall from high school science experiments placing two dissimilar metals in an acid (for example) and then connecting a voltmeter to the metals and discovering that this creates electricity.
Likewise, when a ship is constructed of two different metals (eg a hull of steel and a bronze or iron prop), then it creates its own electrical current when in saltwater which acts as an electrolyte. This causes, amongst other things, a huge magnetic field to be created around the ship. Over a period of time, this can cause parts of the ship to receive a positive attraction (like a magnet) and thus cause metal items in the ocean to be attracted to the ship. In the normal cause of events this is not of much consequence in itself. However, in wartime it can have a tragic outcome. Another problem caused by this is that the negative electrode gradually loses ions and over a period of time (which can be very quick if the metals are widely apart in nature), corrode away. An obvious problem of this is that the prop or hull could corrode very quickly. In vessels with dissimilar metals (eg an aluminium runabout with an outboard motor with a steel prop), a piece of metal that is further down the electrode scale (???) is placed near the prop so that it can corrode away. This saves the hull and prop from damage.
One of the most favoured of war weapons is the floating mine. These were, of course, constructed of iron or steel and placed in a position likely to be used by warships and other important vessels. As you may have now figured out from the above, a ship passing close to a mine tethered to the bottom would normally miss it and keep on its way. However, a ship that has built up a magnetic field could attract a mine from some distance (I have no idea what distance this may be) and the mine would then touch the ship's hull and explode. Result, severe damage or loss of the vessel.
There are many ways to try to combat this problem. One is to build a vessel totally of one metal (or even from a non-metal) but this is easier said than done. Even so called identical metals are likely to be a little different from each other and still cause the problem, albeit over a longer period of time. Things like rivets, portholes, prop, rudder, driveshafts, inlet and outlet pipes for engines etc all need to be taken into consideration. The short answer is that it is impossible to provide a totally inert vessel and that ship will over time become magnetic.
Another method is to degauss (to neutralise the magnetic field of) the ship. This can be done by placing the ship in a degaussing station (there is a Royal Australian Navy station in Sydney Harbour between Nielsen Park and Shark Island) and run an electric current through the coils on the sea bed and demagnetise the ship. This is generally done before and after a vessel heads to open sea. An alternative is to build into the vessel degaussing. This can be constructed as part of the vessel or, in the case of a vessel built for non-military purposes and converted (like the Rio de Janeiro Maru and Amagisan Maru in Chuuk Lagoon, have a series of coils run around the hull and an electric current passed through to set up reverse polarity, thus neutralising the magnetic field.
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