
Why are submarines demagnetized? - fgeorgy
http://qi.epfl.ch/en/sondage/show/255/
======
vanderhosen
At my previous job, I worked on software and algorithms for degaussing
systems, it's an interesting field. A fun fact, more modern systems are moving
away from copper wire for deperming and towards high temperature super
conductors. It's quite expensive and difficult to wrap modern ships with tons
of copper wire, so the plan is the ceramic wire will alleviate that. I left
the company, so I'm not sure where that tech left off.

~~~
econophys
I spent a summer working at an accelerator physics lab, where a constant
problem was degaussing large beam steering magnets. We always used a DC
current to drive the magnet through smaller Hysteresis loops, until the
magnetization is roughly zero. Is there a better way?

------
zymhan
4,000 Amps is a ridiculous amount of current. Though when you have a nuclear
power plant nearby...

Also, this line made me shudder:

> "...stringing an electric cable the length of the ship and pulsing 2,000
> amps through the cable."

EDIT: Okay maybe not that ridiculous, apparently the Luxor hotel has higher-
amp lightbulbs
[http://www.wolframalpha.com/input/?i=4000+amps](http://www.wolframalpha.com/input/?i=4000+amps)

~~~
esaym
Well as far as energy, depends on voltage. That's just 4000 watts if the
voltage is just 1 volt.

~~~
mw67
But as far as dangerousness, only 0.1A can kill a human, whereas voltage is
not so important.

~~~
oxplot
Voltage is what drives current so they're directly proportional and their
proportion depends on electrical resistance. Given that human body's
resistance to electricity is more or less constant, that allows calculating a
dangerous voltage just as easily.

~~~
danmaz74
Except, you also need to keep in mind the internal resistance of your power
source. If that one is higher enough than the human body resistance, you could
have a very high voltage which is actually not very dangerous.

~~~
stordoff
I assume this is the reason I was able to safely use a 5000V power source in
school, despite being shocked multiple times (IIRC, the maximum current it
would give, even if you shorted the output terminals, was 3mA. Our teacher at
the time did initially insist on wearing latex gloves, but they were discarded
once we realised the current would just arc through the gloves and leave holes
in them).

You do start to see some odd effects at those voltages though - I recall
holding one of the output plates for ten minutes or so, without realising the
supply was on. Didn't really think much of it, until I realised I could hold
my hand over a piece of tin foil (roughly 1cmx1cm) and it would be attracted
to my hand from 2-3cm away (that persisted for five minutes or so).

------
simonh
A lot of military submarines have titanium hulls because it's non-magnetic.
Interestingly, during the cold war the main source of titanium for the US
military was the USSR.

Edit - It looks like the principal reasons for using Titanium was likely to
reduce weight and dive depth rather than it's magnetic properties. Thanks to
respondents for clarifying that. Much appreciated.

~~~
_djo_
The USSR was the only country to build submarines with titanium hulls, a
practice which ended with the Cold War.

~~~
dogma1138
And only one class the Alfa fast attack submarine the titanium hull was used
more to reduce weight (Half the displacement of a comparable Los Angeles class
US attack submarine) to allow it to be the fastest submarine ever constructed
(double the reported speed of US attack submarines and most likely at least
25% faster than their actual speed) rather than because titanium is non-
magnetic. All Alfa subs were decommissioned in 1990 and sold as scrap.

~~~
noir_lord
Yep, the Alfa was a strange design, Titanium hull, reduced to way below safe
minimum shielding to save the weight, noisy as hell (like insanely noisy).

Basically designed as the submarine equivalent of an interceptor, get
somewhere fast and open fire.

IIRC the top speed of the alfa was above the speed of the NATO torpedoes in
use at the time.

~~~
ethbro
Reported top speed. And it was seen as shocking enough to spur the investment
into new torpedo technology.

[https://en.wikipedia.org/wiki/Alfa-
class_submarine#Impact](https://en.wikipedia.org/wiki/Alfa-
class_submarine#Impact)

Like most fears over high-performance Soviet hardware though, the threat
turned out to be overblown compared to the actual numbers build / operational
state.

~~~
noir_lord
Yep, the Alfa also had a tendency to slowly kill it's crew from radiation
exposure.

~~~
dogma1138
well thats what you get for putting a fast nuclear reactor with minimal
shielding in a relatively tiny sub.

------
mannykannot
During WW1, the British Royal Navy became concerned that U-boats could enter
its harbor at Scapa Flow. In addition to installing nets, mines and
hydrophones, large loops of wire were installed on the channels that could be
opened for navigation, encircling the entire channel, in order to detect
vessels by induction.

There were no successful attacks in WW1, but perhaps they were less careful in
WW2 (or submarine degaussing rendered the technique ineffective), as a U-boat
torpedoed and sank the battleship HMS Royal Oak in the harbor, with great loss
of life.

Update: It seems that Gunther Prien made his attack before the defenses were
fully restored.

------
snake_plissken
Not limited to subs, the same principle was also used on ships via degaussing
coils:
[https://en.wikipedia.org/wiki/Degaussing](https://en.wikipedia.org/wiki/Degaussing)

------
twoodfin
Related, interesting HN discussion a month ago:

[https://news.ycombinator.com/item?id=10810390](https://news.ycombinator.com/item?id=10810390)

Submarines can alter their magnetic fields, but gravitational fields are a
good bit more tricky, and can apparently also be exploited for detection.

~~~
VLM
The mass of a bucket of water, and an equally full bucket with a sub in it, is
equal. Otherwise the sub would sink like an anchor or float like a boat.

~~~
ISL
True. The gravity _gradients_ from the sub may be different, though.

------
esaym
The MAD detector
[https://en.wikipedia.org/wiki/Magnetic_anomaly_detector](https://en.wikipedia.org/wiki/Magnetic_anomaly_detector)

~~~
crikli
Yup. My dad was a RAD/MAD operator on a P3 during the Vietnam War. One of my
indelible memories from childhood is him explaining how it worked and a story
about seeing a Soviet sub on the surface. Of course as a kid having watched
Das Boot I'm expecting there to be guys scrambling to the conning tower, dive
dive dive, depth charges, etc. He's like "nah, the skipper just circled the
thing and we waved at each other." Seeing this story on HN makes me wish I
could hear the story again but he passed of cancer a few years back.

~~~
paulmd
Vietnam was a proxy war and neither the US or Russia wanted it to escalate
into World War III. US policy was to stringently avoid anything that might
lead to escalation.

------
daredevildave
Tom Scott talked a bit about magnetic warships and the magnetic mines in one
of his "Things you might not know" videos:
[https://www.youtube.com/watch?v=LG-X23aVud0&list=PL96C35uN7x...](https://www.youtube.com/watch?v=LG-X23aVud0&list=PL96C35uN7xGI9HGKHsArwxiOejecVyNem&index=25)

------
krschultz
The US Navy has a few pictures of the setup they use for submarines. They use
the term 'deperming' rather than 'demagnatizing'.

[http://www.navy.mil/view_image.asp?id=38120](http://www.navy.mil/view_image.asp?id=38120)

------
smegel
What a strange title for an article. It's like back-to-front thinking.

Surely someone uninformed on the topic would want to read something along the
lines of "anti-submarine warfare" or even "submarine detection", during the
course of which would probably learn about magnetic anomaly detection and
attempts to defeat it.

I mean, who and how would someone know (or want to know) why submarines are
demagnetized without already knowing something about ASW?

~~~
metasean
The full title is, "Monday January 25, 2016 Poll Why are submarines
demagnetized?" and the article starts off with the results of Monday's poll.
So the article we're ready today is a response to the results of the poll, but
the site administrators kept the poll question as part of the title.

------
cmdkeen
Interestingly the new P8 Poseidon doesn't have Magnetic Anomaly Detection
(MAD) equipment on the US variant, though the Indian one has had it fitted.
You need to fly low for it to work which is less efficient for the jet powered
P8 compared to the older P3. So if you're generally flying higher and up
against submarines which have been demagnetised there's less need to fit MAD.

Surface warships are also degaussed to help avoid magnetically triggered
mines.

~~~
mannykannot
> Surface warships are also degaussed to help avoid magnetically triggered
> mines.

I believe the large pipe seen in this photograph running along the hull of the
Queen Elizabeth, as fitted out as a WW2 troop transport, contained wires for
degaussing.

[http://www.ssmaritime.com/RMS-Queen-Elizabeth-War-
Trooper-2....](http://www.ssmaritime.com/RMS-Queen-Elizabeth-War-
Trooper-2.jpg)

------
mef
See also
[https://en.wikipedia.org/wiki/Deperming](https://en.wikipedia.org/wiki/Deperming)

~~~
kcorbitt
Also known as "degaussing," which brings me back to my school days when I
would sometimes degauss the school's CRT monitors when I got bored just for
fun. Looks like this, if you didn't live through those times:
[https://www.youtube.com/watch?v=Sd9nOtX-
CJg](https://www.youtube.com/watch?v=Sd9nOtX-CJg)

~~~
nitrogen
There was a lot more ripple if the refresh rate was set higher but the
degausser still ran at 60Hz.

------
gregwtmtno
I love that simple and direct writing style.

------
djyaz1200
Maybe if one were to have EXTREMELY accurate baseline magnetic field data for
the whole world subs could still be detected?

[http://www.gfz-potsdam.de/en/media-communication/press-
relea...](http://www.gfz-potsdam.de/en/media-communication/press-
releases/details/article/satellitentrio-zur-erforschung-des-erdmagnetfeldes/)

~~~
VLM
Look up the noise spectrum data. You'd need a rather dense sensor field to
detect a signal outside of the noise levels.

------
brandmeyer
[https://www.google.com/maps/place/Naval+Submarine+Base+Kings...](https://www.google.com/maps/place/Naval+Submarine+Base+Kings+Bay/@30.7739173,-81.4917521,479a,20y,48.24t/data=!3m1!1e3!4m2!3m1!1s0x88e502b7217f9073:0x3f42c66eb3317248!6m1!1e1)

Here's one of the degaussing facilities on the East coast.

~~~
ethbro
[https://www.google.com/maps/search/electric+boat+groton+ct/@...](https://www.google.com/maps/search/electric+boat+groton+ct/@41.3444238,-72.0856987,1122m/data=!3m1!1e3)

While we're mapping, there's also where they're built: Electric Boat in
Groton, CT.

~~~
brandmeyer
EB doesn't have an externally visible degaussing facility.

------
jammaloo
Some more information on the advances in sea mines, and how they were
countered:

[https://medium.com/war-is-boring/how-britain-beat-
germanys-w...](https://medium.com/war-is-boring/how-britain-beat-germanys-
wwii-magnetic-sea-mines-bfec5558704c)

------
amgin3
The obvious answer is so that nobody catches a submarine while magnet fishing.

~~~
preinheimer
thanks, I haven't laughed out loud while reading HN in a long while.

------
joering2
You learn something every day, but thanks to this site I learnt 100 things
today :)

Here is the list:
[http://qi.epfl.ch/en/sondage/list/](http://qi.epfl.ch/en/sondage/list/)

------
chasinglight
Article suggests -- or outright says -- that relatively crude magnometer tech
is still used by militaries and 'coalition' expeditionary search vehicles
today.

And thus why the MH370 airliner remains missing? Only a thought.

~~~
exar0815
The same people who shot Kennedy hide the plane. You will be disintegrated in
3 minutes.

Or you could use the grey matter between your ears. Modern airliner hulls and
structure are made mostly out of aluminium, which is -suprise- antimagnetic.

And finding a Submarine or anything else is hard. Like really hard. You need
to know where to look. Because of that, NATO would have to had stopped Red
Fleet at the GIUK-Barrier or it would have gotten ugly. Or, look at the
problems people had to find ships like Titanic or Bismarck, where they had
much more precise locations to start searching for.

------
buro9
> It took them several months to come up with the answer

Maxwell's equations provides the answer:
[https://en.wikipedia.org/wiki/Maxwell%27s_equations](https://en.wikipedia.org/wiki/Maxwell%27s_equations)

But then, I personally think Maxwell to be under-appreciated by the mass
population whereas at least Faraday people can name even though they may not
remember what he achieved.

I bet they were also inspired by Faraday's experiments across the Thames.

------
rehevkor5
TLDR: WWII

