How to Correct and Adjust Ship’s Compass While Swinging

Accurate navigation begins with a well-calibrated compass, which ensures a ship stays on course. One critical step in achieving this is compensating for magnetic deviations caused by the environment or structural interference. This blog post outlines the sequence of placing correctors (Bi, D, E, HE, Bp, Cp), precautions to take, and key steps during swinging to ensure accurate compass adjustments.

Whether you’re a mariner preparing to calibrate the compass or just looking to understand this intricate process, we’ve got you covered.

Understanding Magnetic Deviation and Correctors

Magnetic deviation occurs when the ship’s structure and onboard magnetic sources interfere with the compass’s accurate readings. To combat this, correctors, such as flinder bars, quadrantal spheres, and magnets, are strategically placed around the compass. Here’s a breakdown of each corrector and its role:

Key Correctors

  1. Soft Iron Correctors
      • Flinder Bar: Controls vertical magnetic fields.
      • Quadrantal Spheres: Balances horizontal deviations caused by soft iron.
  1. Permanent Magnet Correctors
      • Heeling Error Magnet: Corrects magnetic forces influencing compass readings when the ship tilts.
      • Horizontal Magnets: Compensates for constant deviations (Bp and Cp).

By properly configuring these correctors, you can ensure that the compass provides unerring directional guidance.

Sequence of Placing Correctors

Corrector placement requires precision to counteract specific deviations. Below is the sequence to follow when compensating for various error components.

1. Bipolar Index Error (Bi)

      • If -Bi, place the flinder bar forward of the compass.
      • If +Bi, position the flinder bar aft of the compass.
        • Note: If the superstructure is located abaft the compass, the flinder bar is placed on the forward part. Conversely, if the superstructure is forward of the compass, position the flinder bar on the after part.

2. Quadrantal Deviation (D)

      • If +D, place the quadrantal spheres athwartship (across the ship).
      • If -D, position the quadrantal spheres fore and aft (F&A).

3. Diagonal Quadrantal Spheres

      • Situations requiring complex balancing may call for placing the quadrantal spheres diagonally.

4. Vertical Force Instrument (VFI)

      • Make adjustments to the compass when the vessel is upright to account for vertical magnetic forces.

5. Horizontal Magnetic Deviations (Bp)

      • If +Bp, place the permanent red (forward pole) toward the forward side of the compass.
      • If -Bp, position the permanent red pole aft.

6. Circular Magnetic Deviations (Cp)

      • If +Cp, set the permanent red pole starboard.
      • If -Cp, place the permanent red pole portside.

Recap of Correctors Placement

  • Soft Iron Correctors
    • Flinder bar (forward or aft).
    • Quadrantal spheres (athwartship, fore-aft, or diagonally).
  • Permanent Magnet Correctors
    • Heeling error (HE) magnet and horizontal magnets (Bp and Cp) correctly positioned for deviations.

Precautions Before Swinging

Prior to swinging the compass, a series of precautions should be observed to ensure accuracy and safety. These practical steps reduce the likelihood of errors affecting calibration.

Pre-Swinging Preparations

  • Vessel’s Position:
    • Ensure the vessel is upright to avoid introducing tilt-related errors.
  • Cargo Gear Security:
    • Secure all movable cargo gear to prevent interference with the ship’s magnetic field during adjustments.
  • Distance from Movable Steel:
    • Arrange for movable steel components to be well outside the magnetic field’s radius.
  • Correct Initial Adjustments:
    • Correct the flinder bar and quadrantal spheres for initial soft-iron interference.
  • Eliminate Bubbles:
    • Make sure no bubbles block the compass fluid, as this could impede its accuracy.
  • Azimuth and Gimbals:
    • Calibrate azimuth readings correctly and confirm the free movement of the gimbal arrangement for seamless swinging.

Precautions During Swinging

When calibrating the compass while swinging, follow these important steps to prevent errors and ensure a stable process.

1. Maintain Safe Distance

      • Ensure no ship is within 3 cables of the swinging vessel. This prevents external interference from impacting the readings.

2. Steady Headings

      • Keep the ship on a steady heading for at least 2 minutes at each cardinal and intercardinal point. This allows consistent, reliable data collection.

3. Swing Duration

      • The entire swinging session should last no less than 45 minutes to ensure comprehensive calibration.

4. Deviation Recordkeeping

      • Throughout the swinging process, record all observations in the deviation book. These records serve as a reference for future adjustments and checks.

Actionable Insights for Accurate Compass Calibration

Ensuring accurate compass readings is essential for the safe and efficient navigation of any vessel. By properly placing correctors and following the recommended precautions before and during swinging the compass, you not only eliminate magnetic deviations but also strengthen the reliability of your navigation systems.

Regular checks and proper log maintenance ensure that these procedures are not just effective but also repeatable, offering long-term benefits for mariners.

Do you have questions about other essential compass corrections or advanced navigation techniques? Feel free to reach out or explore our in-depth guides for further insights.