I just learned during a company training course that submarines and ships need to be degaussed (remove residual magnetism) in order to decrease probability of being detected by mines etc.

While it's clear that a huge mass of magnetized material could trigger a bomb detecting a varying magnetic field I don't understand why the mine itself couldn't contain magnets (like a compass) . It would then still detect a ship/submarine passing by because a magnet is attracted to steel, or it could just sense the changing earth's magnetic field because the flux lines are concentrated through the ship...

I admit not having done too much research Maybe someone already has the answer?

http://www.onr.navy.mil/en/Media-Ce...ure-Superconducting-Degaussing-for-Ships.aspx

 

There may be a number of potential problems here. The first would be that if there is going to be more than one of these mines in a given region, all emitting search fields, how would they avoid setting each other off?

Another issue may be difficulty in emitting enough field to reach a passing ship. This could require the field coils (or whatever was used) to be large. Mines operating by sending out a magnetic field might also be more easily be detected at a distance, making them easier to search out and destroy.

Detecting distortion of the Earth's field may be another matter. I would not be surprised if this method is used in some way.

 

I was thinking more of a passive magnetized device, electronic compass etc. Not something that emits an LF magnetic field...

Anyway, we are only guessing how it's actually done and what are the problems involved, since you won't find any bomb schematics on the net.
I'm only interested from a technological standpoint of course.

I still wonder how much kA/m they need to degauss that amount of metal.

 

No idea how they do it nowadays but old last century vessel had their own degaussing system installed to reduce what was called the "magnetic signature".

Bulky to say the least.

In some shipyards they had special equipment to apply degaussing to vessels drydocking there.

There were mines able to actually count nn times before exploding in the idea that damage to a vessel in the middle of a convoy was much more effective than to the first detected one.

 

Mines operating by sending out a magnetic field might also be more easily be detected at a distance, making them easier to search out and destroy. .

I'd go along with this.

 

Detecting distortion of the Earth's field may be another matter. I would not be surprised if this method is used in some way.

Correct.

If the ship has any charge, the signature it creates will be greater.

MAD (Magnetic Anomaly Detector)

 

Correct.

If the ship has any charge, the signature it creates will be greater.

MAD (Magnetic Anomaly Detector)

That was interesting, thanks.

This is what I meant more or less. An MAD could be used for a mine IMO. Unless the MAD itself is also detectable.

Probably the degaussing of the submarines/ships is not meant to make it NOT detectable but LESS detectable, that's all...

 

Spent some time on a coastal mine sweeper--wooden hull

 

The germans during WWII deployed a number of shallow water mines in harbors that used a compass type of trigger. When a steel ship came close, the steel hull would deflect the compass needle possibly triggering the mine. A magnetic target would presumably deflect the compass by a larger amount. This is a good reason for de-gaussing any ship hull.

The other (main) reason is when a ship turns thru 360 degrees, you don't want the ship's compass to be influenced by the hull's possible magnetism. As you know (or maybe you don't) the compass card (part with the numbers) remains stationary as the ship turns thru 360 degrees. The part (of the compass) that moves is the lubber line.

Cheers, DPW [ Everything has limitations...and I hate limitations.]

 

Why not make the hull out of titanium or aluminum?

 

Why not make the hull out of titanium or aluminum?

titanium = cost
aluminum+dissimilar metals+saltwater = http://www.bloomberg.com/news/2011-...ive-corrosion-on-austal-s-combat-ship-1-.html

 

Why not make the hull out of titanium or aluminum?

Aluminum would more than likely get eaten by seawater thanks to galvanic corrosion and titanium is just plain too expensive.

 

Aluminum is also a tricky material for a battle repair after deformation from pressure or blast as it tends to crack and fracture instead of bending and ripping. http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=217&pageid=2144417145

and with a normal steel hulled ship a quick plate repair batch is possible using tools available in almost any port.

USS Cole: http://www.cargolaw.com/2000nightmare_cole.html#disaster

USS Stark: http://www.navybook.com/nohigherhonor/pic-stark.shtml


I would hate to think about what could have happened with an Aluminum hull.
HMS Sheffield:
http://www.g2mil.com/aluminum.htm

 

Why not make the hull out of titanium or aluminum?

It's a subject of debate, but it's rumored the Russsians made their hulls out of titanium at one point. The story I heard in sub school is that they could go down to 10,000ft but only once or twice, afterwards they had to derate the dive depth because of stress cracks.

 

Us Brits tried making ships out of cardboard.

 

Us Brits tried making ships out of cardboard.

based on the past tense I assume it didn't work out?

 

If I remember correctly it did work quite well - in some cases it was stronger than steel.

I forget why they stopped, I'll find out for you.