Terrestrial and ship's magnetism :Sub-permanent magnetism and its effects at sea;

Of course. This is a fundamental topic in marine navigation. Let’s break down sub-permanent magnetism and its practical effects at sea in a clear, structured way.

Part 1: The Foundation - Terrestrial and Ship’s Magnetism

To understand sub-permanent magnetism, we first need to understand the context.

1. Terrestrial Magnetism: The Earth acts like a giant magnet, with magnetic poles roughly near the geographic poles. It creates a magnetic field that a ship’s compass needle tries to align with. This is our reference direction: Magnetic North.

2. Ship’s Magnetism: A steel ship itself becomes a magnet. This is unavoidable. The ship’s own magnetic field interferes with the Earth’s field, deflecting the compass needle away from Magnetic North. The angle between Magnetic North and where the compass needle actually points is called Deviation.

Ship’s magnetism is traditionally divided into three types based on the type of iron in the ship’s structure:

• Permanent Magnetism (Hard Iron): Magnetism “hammered in” during the ship’s construction. It’s relatively strong and stable, acting like a permanent bar magnet fixed within the ship.
• Induced Magnetism (Soft Iron): Temporary magnetism induced in the ship’s soft iron components by the Earth’s magnetic field. This magnetism changes instantly as the ship changes its heading or latitude.
• Sub-permanent Magnetism (Intermediate Iron): This is the “in-between” and often most troublesome category.

Part 2: A Deep Dive into Sub-permanent Magnetism

What is Sub-permanent Magnetism?

Sub-permanent magnetism is a semi-permanent form of magnetism acquired by the ship’s “intermediate” or “semi-hard” steel. This type of steel is not soft enough to change its magnetism instantly (like soft iron) but not hard enough to retain it forever (like hard iron).

Think of it as a magnetic memory. It’s slow to acquire and slow to lose, changing over periods of weeks, months, or after specific significant events.

How is Sub-permanent Magnetism Acquired or Altered?

Sub-permanent magnetism is induced when the ship’s steel is subjected to physical stress or energy while sitting in a magnetic field (the Earth’s). Key causes include:

1. Construction and Major Repairs: The intense vibration, hammering, and welding during construction or a major refit “locks in” a magnetic signature based on the ship’s orientation on the building slip or in dry dock. This is a primary source.
2. Heavy Weather and Pounding: The constant vibration and stress on the hull from navigating in rough seas can gradually alter the existing sub-permanent magnetism or induce a new signature.
3. Laying Up for Long Periods: If a vessel is moored on the same heading for several months, the Earth’s magnetic field will slowly magnetize the intermediate iron, creating a new sub-permanent field.
4. Long Voyages on a Single Heading: A vessel sailing across an ocean for weeks on a constant course (e.g., a long east-west transit) will experience a similar effect to being laid up. The constant orientation in the Earth’s field, combined with engine vibration, will cause a slow, steady change in its magnetic signature.
5. Exceptional Events:
   • Lightning Strike: A massive electrical discharge can drastically and unpredictably change the ship’s entire magnetic profile.
   • Degaussing: Being subjected to a strong, external magnetic field for degaussing (a process to reduce the ship’s magnetic signature for military purposes) will significantly alter its sub-permanent magnetism.
   • Carrying Magnetic Cargoes: Loading or discharging large quantities of magnetic material like iron ore can have a lasting effect.

Part 3: The Effects of Sub-permanent Magnetism at Sea (The Mariner’s Problem)

The primary effect of sub-permanent magnetism is that it makes the ship’s magnetic compass unreliable over time.

1. The Deviation Card Becomes Inaccurate

A ship has a Deviation Card or curve which tells the navigator what the deviation is for any given heading. This card is created during a procedure called “swinging the compass,” where the ship is turned through 360 degrees and the deviation is recorded.

• The Problem: The compass correctors (small magnets and iron spheres near the compass) are adjusted during the swing to counteract the permanent and induced magnetism present at that time. However, sub-permanent magnetism changes after the swing.
• The Result: As the sub-permanent magnetism slowly changes, the deviation of the compass also changes. The values on the deviation card, which were correct on the day of the compass swing, are now wrong.

2. Practical Scenarios and Dangers

• Scenario A: After a Long Pacific Crossing

  • A ship is swung and has its deviation card certified in Los Angeles.
  • It then sails for 3 weeks on a heading of 270° to Tokyo.
  • During this voyage, engine vibration and the constant orientation in the Earth’s field cause the sub-permanent magnetism to change.
  • When the ship arrives in Japan and starts maneuvering on different headings, the deviation it experiences does not match the deviation card from LA. If the navigator relies on the old card without checking, their steered courses will be in error, potentially by several degrees. This can lead to a dangerous navigational position.

• Scenario B: After Winter Lay-up

  • A vessel is laid up in a port for 4 months, facing North.
  • It slowly acquires a new sub-permanent magnetic signature aligned with that heading.
  • When it resumes service, its deviation on southerly headings will be significantly different and larger than before. The old deviation card is now useless and dangerous.

3. The Need for Constant Vigilance

Because of sub-permanent magnetism, a navigator can never blindly trust the deviation card. It is only a snapshot in time. This is why procedural checks are critical.

Part 4: Mitigation and Management

Mariners cannot eliminate sub-permanent magnetism, but they can and must manage its effects.

1. Regular Compass Checks: This is the most important day-to-day task. The ship’s officer must frequently check the actual deviation and compare it to the value on the deviation card. This is done by:

   • Taking an Azimuth: Comparing the compass bearing of a celestial body (like the Sun or Polaris) with its true calculated bearing.
   • Using Transits: Observing the compass bearing when two fixed shore objects (like lighthouses) are perfectly aligned.
   • Comparing with Gyrocompass: Regularly comparing the magnetic compass heading with the gyrocompass heading (after applying gyro error and variation). A changing difference between the two indicates a change in deviation.

2. Compass Logbook: All these checks are meticulously recorded in a Compass Logbook. This log provides a running record of the compass’s performance and helps the navigator detect gradual changes caused by sub-permanent magnetism.

3. Compass Adjustment (Swinging the Compass): If the recorded deviations consistently differ from the deviation card by an unacceptable amount (e.g., more than 2-3 degrees), or after any of the major events listed above (dry-docking, lightning strike, etc.), the ship’s compass must be professionally re-adjusted. This creates a new, accurate deviation card for the ship’s current magnetic state.

Conclusion

Sub-permanent magnetism is the slow, creeping change in a ship’s magnetic field caused by stress, vibration, and time. Its primary effect at sea is to make the compass deviation unstable and the deviation card unreliable over time. For the mariner, it serves as a crucial reminder that the magnetic compass is a dynamic instrument that requires constant observation, regular checks, and periodic re-adjustment to ensure the safety of navigation.