Shipboard magnetic compass adjustment.
Of course. Here is a comprehensive guide to shipboard magnetic compass adjustment.
Shipboard Magnetic Compass Adjustment: A Complete Overview
The magnetic compass is a fundamental and mandatory piece of navigational equipment on any vessel. However, a ship is a large mass of steel and electrical systems, all of which create their own magnetic fields. These fields interfere with the Earth’s magnetic field, causing the compass to point away from Magnetic North.
Compass adjustment, also known as compass swinging, is the process of counteracting these unwanted magnetic forces to make the compass as accurate as possible.
1. The Core Problem: Why Adjustment is Necessary
To understand the adjustment, you must first understand the different “Norths” and the errors involved.
- True North (T): The direction of the geographic North Pole. All charts are oriented to True North.
- Magnetic North (M): The direction the Earth’s magnetic field lines point. The location of the Magnetic North Pole slowly wanders.
- Compass North (C): The direction a ship’s magnetic compass needle points.
The two main errors that separate these are:
- Variation (or Declination): The angle between True North and Magnetic North. This is caused by the Earth itself and varies depending on your geographic location. Variation is found on nautical charts and is not correctable by the adjuster.
- Deviation: The angle between Magnetic North and Compass North. This is caused by the ship’s own magnetic field. Deviation is the error that is corrected during compass adjustment.
The goal of compass adjustment is to reduce deviation to a minimum (ideally less than 2-3 degrees on any heading).
The relationship can be remembered with the mnemonic: True Virgins Make Dull Company At Weddings True bearing Variation (E+/W-) Magnetic bearing Deviation (E+/W-) Compass bearing
To convert from Compass to True, you work up the list, applying the corrections. A simple mnemonic for this is CADET: Compass ADd East to get True. (This means East errors are added, West errors are subtracted).
2. Sources of Deviation on a Ship
The ship’s magnetic field is a combination of two types of magnetism:
A. Permanent Magnetism (Hard Iron) This is “baked in” to the ship’s steel structure during construction, primarily due to hammering, welding, and riveting while the ship was sitting in a constant orientation in the Earth’s magnetic field. It does not change as the ship changes heading.
B. Induced Magnetism (Soft Iron) This is temporary magnetism induced in the ship’s soft iron components by the Earth’s magnetic field. Unlike permanent magnetism, it changes as the ship changes its heading.
- Vertical Soft Iron: Masts, funnels, deck cranes.
- Horizontal Soft Iron: Beams, decks, and other horizontal structures.
3. The Tools of Adjustment: The Binnacle and its Correctors
The compass sits in a housing called a binnacle, which contains the correctors used to counteract deviation.