1.2 Compass Deviation, Deviation Cards & Swinging the Compass

1.2 Compass Deviation, Deviation Cards & Swinging the Compass

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What Causes Vessel Deviation

Every vessel carries an internal magnetic field that distorts the magnetic environment around the steering compass. This localized distortion pulls the compass needle away from Magnetic North toward an erroneous heading known as Compass North. The angular difference between Magnetic North and Compass North is defined as Compass Deviation.

Deviation is named by its direction of pull:

• Easterly Deviation (E / +): The needle is deflected east (right) of Magnetic North.
• Westerly Deviation (W / -): The needle is deflected west (left) of Magnetic North.

Uncorrected 5° deviation causes roughly 1 nautical mile of lateral drift for every 12 miles sailed.

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Hard Iron vs. Soft Iron Magnetism

Vessel magnetism originates from two primary physical sources, plus electromagnetic interference:

1. Hard Iron Magnetism (Permanent)

Hard iron consists of ferrous metals retaining permanent magnetism. During construction, steel hull plating, the engine block, and bulkheads absorb Earth's magnetic field as they are welded and worked, creating a permanent magnetic field fixed to the hull.

• Behavior: Constant field strength and fixed polarity regardless of heading.
• Correction: Compensated by inserting permanent magnetic correctors (B and C magnets) into binnacle adjustment slots.

2. Soft Iron Magnetism (Induced)

Soft iron includes ferrous components with high permeability and low retentivity, such as mild steel rigging, deck winches, anchor chains, and brackets. Soft iron does not retain permanent magnetism; instead, it is temporarily magnetized by Earth's field.

• Behavior: As the vessel turns, soft iron components alter their orientation relative to Earth's flux lines, causing induced field strength and polarity to shift every 90 degrees of turn (quadrantal deviation).
• Correction: Compensated by mounting soft-iron spheres (Kelvin's spheres) on opposite sides of the binnacle.

3. Electromagnetic Interference

Direct current flowing through shipboard wiring produces localized magnetic fields. Common culprits include autopilot drives, VHF microphones with internal magnets, mobile phones, searchlights, and wiper motors.

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Why Deviation Changes with Ship's Heading

Deviation varies continuously as the vessel turns:

1. Heading North or South, the longitudinal hard-iron field aligns parallel to Earth's magnetic meridian, producing minimal lateral needle deflection.
2. Heading East or West, the longitudinal field swings perpendicular to the needle, exerting maximum lateral pull and producing peak deflection.
3. Plotted across 360°, deviation forms a characteristic S-curve alternating between Easterly and Westerly values.

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The Deviation Card

Residual errors after physical compensation are tabulated in a Deviation Card mounted at the helm:

For intermediate headings, interpolate linearly (e.g., heading 045°C between 030°C [+3°E] and 060°C [+4°E] yields +3.5°E, applied as 3°E or 4°E).

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The Full Conversion Chain: CADET & Reverse Rules

Converting between compass readings and chart tracks involves five linked components:

• Compass (C) ⟷ Deviation (D) ⟷ Magnetic (M) ⟷ Variation (V) ⟷ True (T)

1. Converting Compass to True (CADET Rule)

CADET: Compass to Add Direction East True

• Moving from Compass toward True: Add East errors (+), Subtract West errors (-).

2. Converting True to Compass (Chart to Steering)

• Moving from True toward Compass: Subtract East errors (-), Add West errors (+).

Mnemonic: "True Virgins Make Dull Companions: At Each Wedding" (True → Variation → Magnetic → Deviation → Compass: Add West, Subtract East).

3. Compass Best / Compass Least Rule

• Error West, Compass Best: If error is West, Compass reading is larger than Magnetic.
• Error East, Compass Least: If error is East, Compass reading is smaller than Magnetic.

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Worked Navigation Conversion Scenarios

Scenario A: Chart Course to Compass Course to Steer

A navigator requires a heading of 199° True. Variation is 7° West. Using the deviation table above:

1. True to Magnetic: Add West variation:
   • Magnetic Heading = 199°T + 7° = 206°M.
2. Find the Compass Heading: The table is indexed by compass heading. Its 210°C row gives 4°W deviation and the required 206°M heading.
3. Check Magnetic to Compass: Add West deviation:
   • Compass Heading = 206°M + 4° = 210°C. The skipper orders 210°C. For an intermediate value, interpolate between the magnetic-heading entries to find the corresponding compass heading.

Scenario B: Observed Compass Bearing to Chart True Bearing

A vessel is steering 090°C. The crew sights a lighthouse bearing 088°C on the steering compass. Variation is 4° West.

1. From the table, deviation on heading 090°C is 3° East.
2. Compass to Magnetic: CADET dictates adding East:
   • Magnetic Bearing = 088°C + 3°E = 091°M
3. Magnetic to True: CADET dictates subtracting West:
   • True Bearing = 091°M - 4°W = 087°T The navigator plots 087°T from the lighthouse onto the chart.

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Swinging the Compass on Charted Transits

Swinging the compass determines residual deviation across headings:

1. Identify two charted visual landmarks in line forming a transit (leading line).
2. Measure True transit bearing directly from chart (e.g., 184°T).
3. Convert to Magnetic transit bearing using current variation: 184°T + 3°W = 187°M.
4. Motor across transit line on eight principal compass headings (000°, 045°, 090°, etc.).
5. As marks align, record observed compass bearing of transit.
6. Calculate deviation using: Deviation = Magnetic Bearing - Compass Bearing.
   • If transit reads 183°C: Compass is least, so deviation is 4° East.
   • If transit reads 191°C: Compass is best, so deviation is 4° West.