Marine Magnetic Compass adjustment:principle of the deflector method and its limitation;

Of course. Here is a detailed explanation of the principle and limitations of the deflector method for marine magnetic compass adjustment.

Introduction

The Deflector Method is a technique for adjusting a ship’s magnetic compass without needing to perform a full “swinging of the ship” (i.e., pointing the ship to various known headings and observing the deviation). It is a quicker, more convenient method often used for preliminary adjustments or when conditions like bad weather, traffic, or being in port make a full swing impractical.

However, its convenience comes at the cost of accuracy and completeness.

Principle of the Deflector Method

The core principle of the deflector method is not to measure deviation directly, but to measure and equalize the magnetic directive force acting on the compass needle on opposite headings.

Let’s break this down:

1. Directive Force: The compass needle aligns itself with the local magnetic field. The strength of the horizontal component of this field, which pulls the needle towards magnetic north, is called the Directive Force.

2. The Problem: On an uncorrected compass, this directive force is the vector sum of two forces:

   • The Earth’s horizontal magnetic field (H).
   • The ship’s deviating magnetic field (caused by its hard and soft iron magnetism).

   As the ship turns, the ship’s deviating field changes relative to the compass, causing the total directive force to become stronger on some headings and weaker on others. This variation is what causes deviation.

3. The Solution (The Deflector’s Role): The deflector is a precision instrument containing magnets. It is designed to be placed on the compass bowl to apply a known, consistent magnetic force.

   • The deflector is used to push, or “deflect,” the compass card by a specific, large angle (typically 90°).
   • The amount of force needed from the deflector to achieve this 90° deflection is a direct measure of the directive force acting on the compass at that moment. A stronger directive force will require a stronger setting on the deflector to push the card 90°, and vice versa.

4. The Adjustment Process: The goal of the adjustment is to make the directive force equal on all cardinal (N, E, S, W) and intercardinal headings. If the directive force is the same on all headings, it implies that the ship’s deviating forces have been successfully cancelled out by the corrector magnets.

The Practical Steps Illustrate the Principle:

• To Correct Coefficient B (Fore-and-Aft Hard Iron):

  1. The ship is headed Magnetic North. The deflector is placed on the compass and adjusted to deflect the card exactly 90°. The deflector’s reading is noted.
  2. The ship is headed Magnetic South. The deflector is used again to deflect the card 90°. The reading will likely be different from the one on North because the ship’s fore-and-aft magnetism is now affecting the compass in the opposite way.
  3. The Adjustment: While on the South heading, the fore-and-aft corrector magnets are moved in or out until the deflector reading required to get a 90° deflection is the same as it was on the North heading (or, more precisely, the mean of the two initial readings).
  4. By doing this, you have ensured the directive force on North and South is now equal, effectively neutralizing the magnetic force that causes Coefficient B deviation.

• To Correct Coefficient C (Athwartships Hard Iron):

  • The exact same principle is applied on East and West headings, but this time the athwartships (port-starboard) corrector magnets are adjusted.

• To Correct Coefficient D (Quadrantal Soft Iron):

  • A similar process is followed on the intercardinal headings (NE, SE, SW, NW), adjusting the soft iron spheres (Kelvin’s Spheres) to equalize the directive force on these four headings.

In summary, the deflector acts as a “force meter.” The adjuster uses it to measure the directive force and then adjusts the correctors until that force is balanced on opposite headings.

Limitations of the Deflector Method

While useful, the deflector method has significant limitations that make it less accurate and less comprehensive than a full compass swing.

1. Cannot Detect or Correct Coefficient A:

   • Coefficient A is a constant deviation on all headings, often caused by a misaligned lubber line or an asymmetrical arrangement of soft iron. Since the deflector method works by comparing opposite headings (e.g., N vs. S), any constant error is present in both measurements and is therefore not detected. This is the most critical limitation.

2. Requires Very Calm Conditions:

   • The method requires precise reading of the deflected compass card. Any rolling or pitching of the vessel will cause the card to oscillate, making it impossible to get a stable and accurate reading. A full swing can tolerate slight motion better, as the observer is simply reading the ship’s average heading.

3. Lower Overall Accuracy:

   • The method is indirect. It relies on the skill of the operator, the precision of the deflector instrument, and the stability of the compass pivot. Small errors in deflecting the card by exactly 90° or reading the deflector scale can lead to significant residual deviations. Pivot friction in the compass can also introduce errors.

4. Instrument-Dependent:

   • The accuracy of the adjustment is entirely dependent on the calibration and condition of the deflector itself. If the magnets in the deflector have weakened over time or have been damaged, the entire adjustment will be flawed.

5. Does Not Address Heeling Error:

   • Heeling error is the deviation caused when the ship rolls or lists. The deflector method is performed when the ship is upright and gives no information about this type of error. Correcting heeling error requires a separate procedure and instrument (the heeling error instrument or clinometer).

6. Complex Adjustment for Coefficient D:

   • While possible, using the deflector to accurately adjust the quadrantal spheres for Coefficient D is particularly tricky and prone to error compared to observing the deviations on a full swing.

Conclusion

The deflector method is a valuable tool for a qualified compass adjuster, especially for making a preliminary adjustment after a ship has been in drydock or for a quick check. It is based on the elegant principle of balancing magnetic forces rather than measuring deviation angles.

However, due to its significant limitations—most notably its inability to correct Coefficient A and its requirement for calm seas—it is not a substitute for a full compass swing and analysis. A proper swing is always required to create an accurate deviation card that the navigator can rely on for safe passage.