Degaussing and Deperming

22 May
“At 0756 on October 8th, we departed from the Moore yards and proceeded to a deperming pier at San Francisco.  After that operation was completed, we moved out to the degaussing range to have that gear checked and took all day.” (1)


Construction of the Tryon was completed, the crew had assembled, and final preparations were being made for the Tryon to get underway: destination, South Pacific.  Among the last things to be completed before sailing was deperming.

German contact mine in the waters off Austrailia, WW II

“Degaussing” and “deperming” are procedures that came about in response to Germany’s development and use of magnetic mines in the early days of World War II. The Germans were the first to deploy magnetic mines, and they used them with great effectiveness. What we normally think of when we hear the term “mine” in connection with World War II was  the “contact” mine, which floated on or near the surface, had “horns” extending from it’s body, and exploded when a ship made contact with these horns.  The horns contained a glass vial  that was filled with fluid, and when the glass ruptured the fluid activated a battery that detonated the explosive.  A contact mine would blow a hole in the hull of a ship, causing it to sink.

Contact mine technology.

Magnetic mines required no contact.  They settled to the sea floor, and detonation was triggered by the magnetic signature of a passing ship.  A steel-hulled vessel, moving through the magnetic field of the earth, tends to concentrate some of that magnetic field in it’s structure.  As the ship then passes near the magnetic mine the mine’s sensors pick up this magnetic field and detonate the mine’s explosives.  Unlike contact mines, magnetic mines are designed to create a shock wave that hits the middle of the ship, causing it to buckle and break apart.  Part of the appeal of magnetic mines was that they did not explode on impact. They could be dropped from a plane from high altitude, with

This German magnetic mine missed the water – demonstrating that the magnetic mine did not routinely detonate on impact.

or without a parachute, and would sink to the bottom of the sea, where they were difficult to detect, and would there await the next passing ship.  During the war magnetic mines were dropped from bombers as high as 30,000 feet.(2) 

Magnetic mines were highly effective against British shipping, taking out ships faster than the British could replace them.  As Britain was reliant on shipping for food and other necessities, the problem quickly became critical. Methods of neutralizing the effectiveness of magnetic mines had to be found.  Degaussing and deperming proved to be the answer.

By the time the Tryon was built, nearly all warships were undergoing degaussing or deperming. Although the two procedures are similar in purpose, they are a bit different in application.  The pioneering work on degaussing was done by Canadian Charles Goodeve.  Early magnetic mines detected the vertical magnetic field from a ship passing over it, and Goodeve found that by installing a coil of cable around a ship and passing a current through it this magnetic field could be neutralized. These cables could be installed externally on ships already in service.  Degaussing systems required monitoring of the magnetic field with appropriate adjustment of the current to the coil to offset changes in the field.

Shipboard degaussing control panel.

A later development for removing the magnetic signature of a ship (also developed by Goodeve) was called “wiping”or “deperming.”  This involved making permanent changes to the magnetic field by dragging a cable alongside the vessel with a 2,000 amp current flowing through it. This was found to be as effective as degaussing, and initial views that the effects of deperming would be quickly lost by the pounding of the sea and vibrations from the engine proved incorrect; however, it was found necessary to repeat the deperming process periodically.(3)


Troop transport USS McCawley – external degaussing cables are visible around perimeter of ship

By using these methods of neutralizing the magnetic signature of a ship, often in combination, the Allies were able to lessen the effectiveness of the magnetic mine.  The story is told of 60 British ships passing through an area suspected of being mined near the Straits of Gibraltar.  They were ordered to activate their degaussing systems.  All but one successfully made it through the mine field – and that one failed to use it’s degaussing equipment. (3)  Interestingly, magnetic mines were used by the Germans and the Allies, but the Japanese never developed a magnetic fuse for their mines or their topedoes.  While they did use magnetic mines captured from the British late in the war, they did not develop this technology for their own.


Be that as it may, by 1942 Navy ships headed for the Pacific were routinely being depermed in San Francisco.  While Mac Perry doesn’t indicate precisely where this took place, several other resources indicate that deperming in San Francisco was done at Pier 33, and that’s likely where it was done on the Tryon (Pier 33 is now the terminus for the Alcatraz Island Ferry).  Once the deperming and/or degaussing was completed, the Tryon headed to the “degaussing range” to test the effectiveness of the procedures.  An interesting explanation of how this was done was given by Robert M. Knight, who was a naval officer that worked at Pier 33:


How could the ships natural magnetism be determined? This was done by setting up a number of magnetometers (devices for measuring the strength of a magnetic field) in a line about 400 ft. long with the magnetometers 30 ft below the water about 15 feet apart These magnetometers were connected to leads which ran to a “house” on the shore which contained a fluxmeter for each magnetometer beneath the water. Each magnetometer would send its reading to its fluxmeter in the range house which would record its reading on a moving tape. The readings from all of the fluxmeters would be combined into one reading which was called the ship’s “signature”. From this signature we could determine what action to take to reduce the effective magnetism to as low a condition as possible. This could be done by adding turns or decreasing the number of turns in the degaussing cable that went just inside the skin of the ship. The same effect could be accomplished by raising or lowering the current flowing through the turns. An officer from the “degaussing station” would board the ship.

Internally installed degaussing cables can be seen running through channel on the deck of the SS San Emiliano (which was sunk August 6, 1942 by German U-boat).

The ship would run the range. The recommendation would be signaled to the ship and the officer on board would make the necessary adjustments, and then the ship would run the range again to see if further adjustments needed to be made. (4)

The narrative of Knight’s experiences as a “degaussing officer,”first in San Francisco, and later in the South Pacific, is quite interesting.  In it, among other things, he explains how ships’ compasses had to be recalibrated to provide accurate information when the degaussing equipment was in use, as it would alter the magnetic field influencing the compass’ pointer.

It is not clear whether the Tryon was equipped with degaussing equipment.  There is no evidence of external cabling in photos – it is possible that degaussing coils were installed inside the exterior hull wall.



One Response to “Degaussing and Deperming”


  1. February 4, 1943 | A Sentimental Journey - February 4, 2013

    […] is a good explanation from a web site for the USS Tryon that explains deperming and degaussing, why it was done, and how the procedure took […]

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: