Marine Magnetic Compass adjustment:Magnetic effects of electrical currents;

Of course. This is an excellent and critical topic in marine navigation. The magnetic effects of a ship’s own electrical currents are a significant source of compass deviation, and understanding how to manage them is crucial for safe navigation.

Here is a detailed breakdown of the topic.

Marine Magnetic Compass Adjustment: Magnetic Effects of Electrical Currents

A standard magnetic compass works by aligning itself with the Earth’s magnetic field. The process of “adjusting” or “swinging” the compass is designed to compensate for the magnetic fields created by the ship itself, which deflect the compass needle from magnetic north. This deflection is called deviation.

While much of this deviation comes from the ship’s permanent and induced magnetism in its steel hull (the “hard iron” and “soft iron”), a significant and often variable component comes from electrical currents.

1. The Fundamental Principle: Electromagnetism

The core principle was discovered by Hans Christian Ørsted in 1820: Any wire carrying an electrical current generates a magnetic field.

• Right-Hand Grip Rule: If you point the thumb of your right hand in the direction of the conventional current flow (positive to negative), your fingers will curl in the direction of the magnetic field lines that encircle the wire.
• The Problem: This magnetic field generated by the ship’s wiring will interact with the compass needle, just like the steel in the hull, causing deviation.

2. DC vs. AC Currents: The Critical Distinction

The type of electrical current is fundamentally important to its effect on the compass.

• Direct Current (DC): This is the primary culprit. DC flows in one constant direction. As a result, it creates a stable, uni-directional magnetic field. This field acts exactly like a permanent magnet (hard iron) as long as the current is flowing. If a high-amperage DC cable runs near the compass, it can cause a very large and steady deviation.

• Alternating Current (AC): This is far less of a problem. AC rapidly reverses its direction of flow (typically 50 or 60 times per second, i.e., 50/60 Hz). This causes the magnetic field it generates to also rapidly reverse its polarity. To the relatively slow-to-react magnetic compass needle, these rapid fluctuations average out to a net magnetic effect of almost zero. Therefore, AC circuits are not considered a significant source of compass deviation.

3. Common Sources of DC-Induced Deviation on a Vessel

Any piece of equipment that uses a significant DC current can affect the compass. On modern vessels, these include:

• Generators and Alternators: While they produce AC for the ship’s main supply, their excitation fields use DC.
• Large DC Motors: Anchor windlasses, mooring winches, and some types of cargo cranes. These draw enormous currents when in operation.
• Navigation and Communication Equipment: Radars, Echo Sounders, GPS units, VHF/HF Radios, and satellite communication systems all run on DC power (often converted from the ship’s AC supply).
• Steering Gear Systems: Many electro-hydraulic steering systems use DC control circuits or DC motors.
• Cathodic Protection Systems: Impressed Current Cathodic Protection (ICCP) systems pass a DC current through the hull to prevent corrosion. This can create a significant magnetic field.
• Degaussing Coils (Naval Vessels): These are specifically designed to manipulate the ship’s magnetic signature and will have a massive, though controlled, effect on the compass.
• Lighting Circuits: Particularly older DC lighting systems or single-wire/earth-return systems.

4. Characteristics of Electrically-Induced Deviation

The deviation caused by electrical currents is particularly dangerous because it is variable.

• It only appears when the equipment is switched on.
• Its magnitude depends on the amount of current being drawn (e.g., the deviation from a winch motor is much greater under heavy load than when running free).
• This creates a situation where the deviation card, created during a compass swing under one set of conditions, may be dangerously inaccurate under another.

5. The Adjustment Process: How to Compensate for Electrical Effects

A professional compass adjuster must account for these electrical effects during the swing.

Step 1: Preparation - Simulating Normal Conditions Before the swing begins, the ship’s electrical equipment should be set to its normal seagoing condition. This typically means:

• The normal running generator(s) are online.
• The steering gear is running.
• Radars, echo sounders, and primary navigation systems are switched on (even if in standby mode).
• The ICCP system is operating normally.
• All other heavy DC consumers (winches, deck cranes) are switched off.

Step 2: Swinging the Compass and Identifying Coefficient E The compass is adjusted for the permanent (Coefficients B and C) and induced (Coefficient D) magnetism as usual. The deviation caused by electrical currents often manifests as part of what is known as Coefficient E.

• Coefficient E is a component of deviation caused by the asymmetrical arrangement of soft iron, particularly horizontal iron that is not aligned fore-and-aft or athwartships.
• However, the magnetic fields from electrical wiring can interact with the ship’s soft iron, creating a similar effect that gets bundled into Coefficient E.
• It is corrected by slewing the quadrantal correctors (Kelvin’s Spheres) slightly away from their pure athwartships alignment.

Step 3: Post-Adjustment Checks and Creating Multiple Deviation Cards After the main adjustment is complete, the adjuster and the ship’s officers must test for the effect of variable equipment.

1. With the ship steady on a cardinal heading (e.g., North or East), note the compass reading.
2. Switch on a major piece of DC equipment (e.g., turn the radar from standby to transmit, or briefly run a deck winch).
3. Note any change in the compass reading. The difference is the deviation caused by that specific piece of equipment.
4. Repeat for other major systems.

If the effects are significant (more than 1-2 degrees), it is best practice to create separate deviation cards for different, predictable operating conditions. For example:

• Card 1: Normal Condition at Sea (as adjusted).
• Card 2: Cargo Operations (with deck lights on, specific winches powered up).
• Card 3: Maneuvering (if bow thrusters or other equipment cause a noticeable effect).

6. Mitigation and Best Practices

• Proper Installation: All wiring for equipment near the binnacle must use twisted-pair (two-wire) supply and return cables. This causes the magnetic field from the supply wire to be almost perfectly cancelled by the field from the return wire. Never use a single-wire system with a hull return near the compass.
• Distance: Keep all high-current DC cables and equipment as far away from the magnetic compass as possible.
• Awareness: The Officer of the Watch must be aware that starting or stopping electrical equipment can affect the compass. It is good seamanship to observe the compass when major machinery is used.
• Regular Checks: The compass deviation should be checked regularly against the gyrocompass or by celestial observation to ensure the deviation card remains accurate.

Summary