When navigating at sea, understanding compass deviation is just as important as knowing about magnetic variation. Both affect how you interpret compass readings and steer an accurate course.

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⛵ 1. What Is Compass Deviation?

Compass deviation is the error introduced to your vessel’s compass by magnetic fields generated onboard. These fields come from the engine, electrical wiring, metal structures, and other ferrous objects within the boat.

Unlike magnetic variation, caused by natural, regional differences in the Earth’s magnetic field, deviation is unique to each vessel and can change depending on your heading.

On modern fiberglass yachts, deviation is usually small, often within ±2°, but it can vary when the boat turns. To know your boat’s deviation for any heading, consult your vessel’s deviation card — a chart listing deviation values corresponding to different compass headings.

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🌊 2. Applying Deviation and Variation: The Correct Order

When converting between different types of bearings — True, Magnetic, and Compass — it is crucial to apply variation and deviation in the correct sequence. A popular mnemonic to remember this process is:

"True Virgins Make Dull Company"

• T = True Course (from the chart, relative to True North)

• V = Apply Variation (correction for geographic magnetic variation)

• M = Magnetic Course (adjusted for variation)

• D = Apply Deviation (correction for vessel-specific magnetic effects)

• C = Compass Course (what you actually steer by the compass)

Using this method ensures you correctly translate a True Course into the Compass Course needed at the helm.

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🛠 3. Adding or Subtracting Corrections

Whether you add or subtract variation and deviation depends on their directions (East or West). The mnemonic CADET helps remember this:

• Correct

• Apply

• Deviation

• East

• True

In brief, for East variation or deviation, subtract the value, and for West, add it. Always double-check your deviation card and local variation charts for accurate values.

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🏆 4. Why It Matters

Clear communication is vital. When discussing bearings or courses, always specify if the figure is a True bearing, Magnetic bearing, or Compass bearing, as these can differ significantly. Accurate application of deviation and variation corrections keeps your navigation precise and your vessel safely on course.