What is the difference between variation and deviation?

Variation is the angular difference between True North and Magnetic North at a given location; it is caused by the Earth's geomagnetic field and changes with position and slowly with time (secular variation). Deviation is the error of the ship's compass caused by the vessel's own magnetic field; it changes with the ship's heading and is unique to each compass and each vessel. Both are tabulated in degrees East or West: variation on Admiralty charts; deviation on the compass deviation card posted near the binnacle.

What are the five deviation coefficients?

Coefficient A (constant deviation, independent of heading — caused by misalignment of compass card or fluxgate sensor); B (sine component — caused by fore-and-aft permanent magnetism, corrected by fore-and-aft permanent magnets); C (cosine component — caused by athwartships permanent magnetism, corrected by athwartships permanent magnets); D (quadrantal deviation — caused by symmetrical soft iron, corrected by soft-iron spheres); E (octantal deviation — caused by asymmetric induced magnetism, rarely significant on modern steel ships). Heeling error is a separate term caused by vertical soft iron.

What is a Flinders bar and when is it used?

A Flinders bar is a vertical soft-iron bar mounted forward or aft of the compass binnacle to correct for induced vertical magnetism in the ship's structure. It is particularly important on vessels whose bow frequently points toward the equator during construction (inducing a vertical magnetic component in the bow structure). The length and position of the Flinders bar is determined by the compass adjuster during the deviation swing and adjusted as the ship ages and the permanent magnetism changes.

How often must the deviation table be updated?

SOLAS V/19 requires ships to carry a magnetic compass capable of being corrected, and Reg V/2.9 defines 'deviation table' as a record to be maintained. A full swing (on at least 8 headings — the four cardinals and four intercardinals) is required after major structural alterations, removal of large ferromagnetic masses, and at least every two years. Additionally, residual deviations should be checked whenever the vessel moves to a significantly different magnetic latitude (geomagnetic latitude change of 15° or more).

Magnetic compass adjustment

SOLAS Regulation V/19 requires every ship to carry a properly adjusted magnetic compass. Adjustment (also called compass compensation) reduces the deviation caused by the ship's own magnetism using correcting magnets, soft-iron spheres, and the Flinders bar. A deviation table showing residual errors on all headings must be posted near each compass. Understanding the theory of deviation is required for OOW and Master certificates.

Sources of ship's magnetism

· Permanent (hard iron) magnetism: acquired during construction when the ship is struck by hammers and welded in a fixed orientation within the Earth's field. Produces Coefficients B and C. Changes slowly over years as the ship vibrates and ages.
· Induced (soft iron) magnetism: steel components that are magnetised by the Earth's field in proportion to the field strength; changes continuously as the ship changes heading and latitude. Produces Coefficients D and E.
· Heeling error: vertical component of soft-iron magnetism; causes deviation when the ship rolls; significant in high magnetic latitudes. Corrected by a vertical permanent magnet in the binnacle.
· Sub-permanent magnetism: semi-permanent remanence of large steel masses near the compass; decays over time but can be significant after major repairs.

Binnacle correctors

· Fore-and-aft permanent magnets (red/blue): placed in fore-and-aft trays in the binnacle to correct Coefficient B (deviation maximised on East/West headings).
· Athwartships permanent magnets: placed in port/starboard trays to correct Coefficient C (deviation maximised on North/South headings).
· Soft-iron spheres: two iron spheres mounted on brackets either side of the compass; their induced magnetism opposes the ship's induced quadrantal deviation (Coefficient D). Position determined by trial during the swing.
· Flinders bar: vertical unmagnetised soft-iron bar, forward or aft of the binnacle; corrects induced magnetism from vertical structural members; particularly important for vessels built in northerly latitudes with bow pointing north.
· Heeling error magnet: vertical magnet below the compass rose; adjusted with the ship upright but in high-latitude waters where heeling error is greatest.

Compass swing procedure

· Swing conducted in an area free of local magnetic anomalies (steel structures, harbours, submarine cables); in calm weather.
· Steady the ship on each of 8 or more headings (N, NE, E, SE, S, SW, W, NW); each heading held for at least 3 minutes.
· Compass heading compared to true heading (from GPS COG or by bearing of a distant transit); difference = compass error = variation ± deviation.
· Apply Coefficient B correction (athwartships magnets) on East and West headings; Coefficient C correction (fore-and-aft magnets) on North and South headings.
· Apply Coefficient D correction (sphere position) on NE/SW and NW/SE headings after correcting B and C.
· Residual deviation on each heading entered into the deviation table; maximum acceptable residual per IMO Res A.382(X) guidance is 3° on any heading.

Deviation table and daily use

· Deviation table posted at the standard compass and on the bridge; tabulates compass heading vs deviation (E or W) for every 10° or 15° of compass heading.
· OOW converts: Compass → Magnetic (apply deviation) → True (apply variation). Or in reverse for steering: True → Magnetic → Compass.
· Daily check: compare gyrocompass and magnetic compass readings; discrepancy exceeding 3° from the tabulated deviation indicates the table may be out of date or a corrector has shifted.
· Log the gyro-compass error and magnetic compass error in the official log book at noon each day (SOLAS V/28 requirement).

Sources: SOLAS Chapter V, Regulation 19 — Carriage requirements for shipborne navigational systems; Admiralty Manual of Navigation Volume 2 (NP220) — Magnetism and the ship's compass; IMO Resolution A.382(X) — Magnetic compasses — carriage and performance standards. See also deviation, variation, passage planning.

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