Marine Magnetic Compass adjustment:rules to be followed with regard to the placing of correctors;

Of course. The adjustment of a marine magnetic compass is a precise procedure governed by a clear set of rules. The goal is to counteract the ship’s own magnetic field so that the compass needle aligns with the Earth’s magnetic field as closely as possible on all headings.

The placement of the correctors is fundamental to this process. Here are the key rules and principles to be followed.

The Goal of Corrector Placement

The primary goal is to place correctors in such a way that they create a magnetic field that is equal in strength and opposite in direction to the ship’s deviating field at the compass position. The placement ensures that each corrector targets a specific type of magnetic error.

These errors are mathematically represented by Coefficients A, B, C, D, and E. The correctors are designed to neutralize Coefficients B, C, and D, and the heeling error.

Rules for Placing Each Type of Corrector

The standard binnacle contains the compass bowl and the housings for the correctors.

1. Permanent Magnetism Correctors (Coefficients B & C)

These are small, powerful bar magnets used to correct for the ship’s permanent magnetism.

• What they correct:

  • Fore-and-Aft Magnets correct Coefficient B (deviation that is maximum on North/South headings).
  • Athwartships Magnets correct Coefficient C (deviation that is maximum on East/West headings).

• Placement Rules:

  1. Location: They are placed in dedicated horizontal trays or holes within the binnacle, directly beneath the compass card. The athwartships magnets are typically placed below the fore-and-aft magnets.
  2. Symmetry: Magnets must be placed symmetrically about the center of the binnacle. For example, if two magnets are required, one is placed on each side of the center line at the same distance.
  3. Orientation (The “Red and Blue” Rule):
     • The red end of a magnet is its North-seeking pole.
     • The blue end of a magnet is its South-seeking pole.
     • To Correct Coefficient B: Magnets are placed in the fore-and-aft trays.
       • If deviation is Easterly when the ship heads North (Positive B), place magnets with their red ends aft.
       • If deviation is Westerly when the ship heads North (Negative B), place magnets with their red ends forward.
     • To Correct Coefficient C: Magnets are placed in the athwartships trays.
       • If deviation is Easterly when the ship heads East (Positive C), place magnets with their red ends to port.
       • If deviation is Westerly when the ship heads East (Negative C), place magnets with their red ends to starboard.
  4. Distance: The strength of the correction is adjusted by moving the magnets closer to or further from the compass card. Rule: Place the magnets as far from the compass as possible while still achieving the required correction. This creates a more uniform corrective field.

2. Quadrantal Correctors (Coefficient D)

These are two soft iron spheres (or hollow globes) that correct for deviation caused by induced magnetism in the ship’s horizontal soft iron.

• What they correct: Coefficient D (deviation that is maximum on the inter-cardinal headings: NE, SE, SW, NW).

• Placement Rules:

  1. Location: Placed on brackets on the port and starboard sides of the binnacle, level with the compass card.
  2. Symmetry: The spheres must be placed at an equal distance from the center of the compass. They are almost always placed on the athwartship line.
  3. Distance: The amount of correction is changed by moving the spheres closer to or further away from the compass along their brackets. Moving them closer increases the corrective effect.
  4. No Polarity: Being soft iron, they have no permanent magnetism. They acquire a magnetic field by induction from the Earth’s field, which automatically counteracts the field induced in the ship’s structure.
  5. Interference: The spheres themselves can be affected by the permanent magnets (B and C correctors). This is a key reason for the specific order of adjustment.

3. Flinders Bar (Part of Coefficient B)

This is a soft iron bar placed vertically in a brass case on the binnacle. It corrects for deviation caused by induced magnetism in vertical soft iron on the ship. This effect changes with magnetic latitude, which is why it needs a separate corrector from the permanent magnets.

• What it corrects: The part of Coefficient B that is caused by induced magnetism.

• Placement Rules:

  1. Location: Placed in its housing on the binnacle, usually on the forward side, but sometimes aft. The position (forward or aft) depends on whether the compass is forward or aft of the ship’s main vertical magnetic mass.
  2. Adjustment: The correction is adjusted by changing the length of the soft iron bar inside the case. It is made of several short pieces that can be added or removed.
  3. Latitude Dependence: The Flinders Bar is critical because the deviation it corrects varies with the vertical component of the Earth’s magnetic field (i.e., it changes with magnetic latitude). The permanent magnets for Coefficient B do not. An experienced adjuster sets the Flinders Bar based on the ship’s trading routes and an analysis of deviation across different latitudes.

4. Heeling Error Corrector

This corrects for deviation that occurs when the ship is rolling or has a constant heel. It is caused by a combination of vertical forces from permanent and induced magnetism.

• What it corrects: Heeling error.

• Placement Rules:

  1. Location: A small magnet in a central vertical tube placed directly beneath the pivot of the compass card.
  2. Adjustment: The magnet is moved up or down within the tube.
  3. Orientation:
     • In the Northern Hemisphere, the red end (North pole) of the magnet is typically placed uppermost.
     • In the Southern Hemisphere, the blue end (South pole) of the magnet is typically placed uppermost.
  4. Procedure: The height is adjusted (usually in port by simulating a heel) until the compass shows no deviation when heeled.

General Principles and Order of Adjustment

The placement of correctors must follow a strict order because the correctors can interact with each other.

1. Initial Checks: Ensure the compass is in good working order and properly aligned with the ship’s fore-and-aft line.
2. Remove Coefficient C: Head the ship East or West (magnetic). Place the athwartships magnets to remove all deviation.
3. Remove Coefficient B: Head the ship North or South (magnetic). Place the fore-and-aft magnets to remove all deviation.
4. Remove Coefficient D: Head the ship on an inter-cardinal heading (e.g., NE, SE). Adjust the quadrantal spheres to remove the remaining deviation.
5. Swing for Residuals: Swing the ship through 360 degrees, stopping every 15 or 30 degrees to record the remaining (residual) deviation. This data is used to create the vessel’s deviation card.
6. Set Heeling Error Corrector: This is often done separately in port.
7. Set Flinders Bar: This is a more complex adjustment, often determined by analyzing deviation records over time and across different latitudes.

By following these rules, a certified compass adjuster minimizes the ship’s magnetic interference, ensuring the magnetic compass remains a reliable primary or backup tool for navigation, as required by SOLAS regulations.