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.