Marine Magnetic Compass adjustment:"wiping" and degaussing;

Of course. This is an excellent and specific question that gets into the more advanced and remedial aspects of marine magnetic compass adjustment. Let’s break down “wiping” and “degaussing” in this context.

The Core Problem: The Ship’s Own Magnetism

A magnetic compass points to Magnetic North. However, a ship is a large mass of steel, which has its own magnetic field. This field deflects the compass needle, causing an error called Deviation.

The ship’s magnetism is composed of two types:

1. Permanent Magnetism (P): “Hard iron” magnetism that is “baked into” the ship’s structure during construction (from hammering, welding, and sitting in one orientation on the building ways). It’s relatively stable.
2. Induced Magnetism (I): “Soft iron” magnetism that is induced by the Earth’s magnetic field. It changes depending on the ship’s heading and location on Earth.

Standard compass adjustment uses a combination of small permanent magnets and soft iron correctors (Flinders bar and Quadrantal spheres) placed in the compass binnacle to counteract these effects.

However, sometimes the ship’s permanent magnetism becomes too strong, unstable, or localized for the standard correctors to handle. This is where “wiping” and “degaussing” come in.

1. Degaussing: The “Whole Ship” Solution

Degaussing (or “deperming”) is the process of reducing or neutralizing the entire vessel’s permanent magnetic field.

Why is it done?

There are two primary reasons for degaussing a ship:

1. Military Purpose (Primary): To reduce the ship’s magnetic signature, making it “invisible” to magnetic mines and torpedoes or harder for magnetic anomaly detectors (MAD) to find. This is a critical aspect of naval stealth. Modern warships have built-in degaussing coils (DG coils) that are energized to actively manage the ship’s signature.
2. Navigational Purpose (Secondary): For commercial vessels, a full degaussing is a remedial action taken when the ship’s magnetic field has become excessively strong or unstable. This can happen after:
   • A lightning strike.
   • Major structural repairs involving extensive welding.
   • Being laid up for a very long time on a single heading.
   • Carrying highly magnetic cargoes (e.g., iron ore).

A ship with an unstable or powerful magnetic field is a nightmare for a compass adjuster because the deviation will be large and will change unpredictably, making accurate correction impossible. Degaussing essentially “resets” the ship’s magnetic state to a more neutral and stable baseline.

How is it done?

The process involves subjecting the ship to a powerful, alternating, and slowly decreasing magnetic field. This scrambles the magnetic alignment of the molecules in the ship’s steel, leaving very little residual magnetism.

• Degaussing Ranges: The ship is sailed through a facility with massive underwater electrical coils that generate the required magnetic field.
• Wrapping: For temporary degaussing, heavy electrical cables are wrapped horizontally and vertically around the ship’s hull, and specific patterns of current are passed through them.

Crucially for navigation, after a ship is degaussed, its magnetic compass must be completely re-adjusted (“swung”) because its entire magnetic signature has been fundamentally altered.

2. “Wiping”: The “Surgical” Solution

“Wiping” is a term of art used by compass adjusters for localized degaussing. It’s a precise technique used to neutralize unwanted permanent magnetism in a specific piece of steel in the immediate vicinity of the compass.

Think of it as the difference between a sledgehammer (degaussing the whole ship) and a scalpel (wiping a single beam).

Why is it done?

Sometimes, the source of a large and troublesome deviation isn’t the entire ship, but one specific structural member. For example:

• A steel beam directly under the compass.
• A new piece of equipment installed on the bridge wing.
• A bulkhead near the binnacle that was heavily welded during a repair.

This localized “hotspot” can create a magnetic field so strong that the standard corrector magnets in the binnacle are not powerful enough to counteract it. Attempting to correct it with magnets might work on one heading but throw the compass way off on others.

How is it done?

The compass adjuster uses a portable degaussing coil or a powerful electromagnet.

1. Identification: First, the adjuster uses a magnetometer (or a simple handheld compass) to pinpoint the exact location of the offending magnetism.
2. The “Wipe”: They energize the coil with an alternating current (AC).
3. They physically “wipe” or pass the coil over the surface of the magnetized steel.
4. While continuing the wiping motion, they slowly and smoothly decrease the current in the coil down to zero.

This process, just like large-scale degaussing, randomizes the magnetic domains in that specific piece of steel, effectively “wiping” away its permanent magnetism.

After a successful wipe, the compass must be swung and adjusted again. The goal of wiping isn’t to fix the deviation itself, but to remove an anomaly that was preventing a proper, stable adjustment.

Summary and Relationship

In practice, a compass adjuster will always try a standard adjustment first. If they find the deviation is too large, erratic, or “non-linear” to be corrected by the binnacle correctors, they will investigate further. If they find a localized source, they will perform a wipe. If the entire ship seems magnetically “noisy,” they might recommend a full degaussing at a proper facility before they attempt the adjustment again.