Marine Magnetic Compass adjustment:Errors of magnetic compass;

Of course. This is a fundamental and critical topic in marine navigation. Here is a comprehensive breakdown of the errors of a marine magnetic compass and the process of its adjustment.

Understanding the Marine Magnetic Compass

A magnetic compass is a simple, robust, and non-power-dependent instrument essential for safe navigation. It works by aligning its magnetized needles with the Earth’s magnetic lines of force to point towards Magnetic North.

However, the reading you see on the compass (Compass Heading, C) is almost never the actual direction you want to steer (True Heading, T). This is due to several predictable and correctable errors.

The primary errors can be broken down into two major categories:

1. Variation (V)
2. Deviation (D)

1. Variation (V)

Variation is the error caused by the Earth itself. It is the angular difference between True North (the geographic North Pole) and Magnetic North (the point where the Earth’s magnetic field lines are vertical).

• Cause: The Earth’s magnetic poles are not located at its geographic poles, and their positions wander over time.
• Characteristics:
  • It is a geographical error. Its value depends entirely on your location on the Earth.
  • It is the same for all vessels in the same location.
  • It changes slowly over time (annual change is noted on charts).
• How to Find It: The value of Variation for a specific area is printed on the compass rose of a nautical chart. For example, it might say: VAR 9°30'W (2020) annual decrease 8'.
• Naming Convention:
  • If Magnetic North is to the east of True North, the variation is named Easterly (+).
  • If Magnetic North is to the west of True North, the variation is named Westerly (-).

Correction for Variation: Variation cannot be “adjusted” on the compass itself. It is an external factor that must be accounted for in all course calculations.

2. Deviation (D)

Deviation is the error caused by the ship’s own magnetic field interfering with the compass.

• Cause: The ship is constructed of steel and has electrical equipment, engines, and other metallic objects that create their own magnetic fields. This local magnetic environment deflects the compass from pointing to Magnetic North.
• Characteristics:
  • It is unique to each ship.
  • Its value changes as the ship’s heading changes. As the vessel turns, its metallic structure interacts differently with the Earth’s magnetic field, causing the deviation to change.
  • It can be influenced by cargo (especially metal ores), the position of cranes, or large metal objects brought on board.
• Sources of Ship’s Magnetism:
  • Permanent Magnetism: “Hard iron” in the ship’s structure acquires a permanent magnetic signature during construction (e.g., from hammering, welding) based on the shipyard’s location and orientation.
  • Induced Magnetism: “Soft iron” in the ship temporarily becomes magnetized by the Earth’s magnetic field. The strength and polarity of this induced magnetism change depending on the ship’s heading and its latitude.
• Naming Convention:
  • If the compass needle is deflected to the east of Magnetic North, the deviation is named Easterly (+).
  • If the compass needle is deflected to the west of Magnetic North, the deviation is named Westerly (-).

Correction for Deviation: Deviation is the error that is physically adjusted or compensated for by a professional compass adjuster. The goal is to reduce it to a practical minimum.

The Compass Adjustment Process (“Swinging the Compass”)

The process of correcting for deviation is known as compass adjustment or swinging the ship. It involves placing small magnets and soft iron correctors near the compass to counteract the ship’s magnetic fields.

The Binnacle and its Correctors: The compass sits in a housing called a binnacle, which is equipped with correctors:

1. Fore-and-Aft Magnets (P): Correct for permanent magnetism acting on the ship’s fore-and-aft axis.
2. Athwartships Magnets (Q): Correct for permanent magnetism acting across the ship (port-starboard).
3. Quadrantal Spheres (Kelvin’s Balls): Two soft iron spheres on either side of the compass that correct for induced magnetism in the ship’s horizontal soft iron.
4. Flinders Bar: A vertical soft iron bar placed in a tube on the binnacle that corrects for induced magnetism in the ship’s vertical soft iron.
5. Heeling Magnet: A vertical magnet placed directly beneath the compass pivot to correct for changes in magnetism when the ship heels or rolls.

The “Swinging” Procedure:

1. The ship is taken to a location with a known magnetic direction and minimal interference.
2. The ship is slowly turned (“swung”) through 360 degrees, stopping at various headings (e.g., every 15 or 30 degrees).
3. At each heading, the ship’s magnetic heading is determined (using a gyrocompass, GPS, or by aligning with known landmarks called “ranges”).
4. This known magnetic heading is compared to what the ship’s compass is reading. The difference is the deviation on that heading.
5. The compass adjuster uses this information to strategically place the corrector magnets and soft iron to reduce the deviation on the cardinal (N, E, S, W) and intercardinal (NE, SE, SW, NW) headings as much as possible.

The Deviation Card: It is impossible to remove all deviation. After adjustment, the small remaining error, called residual deviation, is recorded on a Deviation Card. This card is mounted near the compass and shows the deviation for different ship headings, allowing the navigator to apply the correct value for any course being steered.