Module 9 - Compass
Deviation
What you will learn
Use the current heading-specific deviation card and recognise when vessel changes or local magnetic items make it unreliable.
Deviation is the difference between magnetic direction and what a particular compass indicates in the vessel. It can change with heading and after equipment, wiring or structural changes.
Engines, steelwork, speakers, electric currents, tools, phones and other magnetic items can affect a compass. Because the vessel's orientation to the Earth's field changes, both the amount and direction of deviation can vary around the compass card.
A compass swing compares reliable known directions with compass readings on a range of headings, adjustments are made where appropriate, and the residual error is recorded on a deviation card. Use the entry for the intended compass heading and interpolate only as the card and procedure allow.
Treat the card as configuration-specific evidence, not a permanent property. Moving a speaker, adding electrical equipment, lightning, repairs or a material change from expected bearings warrants investigation and, where needed, a fresh check by a competent person. Keep portable magnetic items away during normal steering and observations.
Respond to a changed compass environment
After a cockpit loudspeaker is replaced, a known transit bears 4° differently on the steering compass while the hand-bearing compass agrees with the charted magnetic bearing.
1. Remove easy causes
Move portable magnetic items and check whether new wiring or the speaker sits near the steering compass.
2. Do not edit one card cell
A change on one heading can imply a new deviation pattern around the card.
3. Arrange a proper check
Use independent navigation meanwhile and have the compass and card checked by a competent person before relying on the old corrections.
Sense check: Variation cannot explain a difference that appeared after a change aboard while place and time remained essentially the same.
Common mistake or limitation
- Deviation is not one fixed boat value; it is taken for the compass heading concerned.
- A hand-bearing compass can also suffer local deviation if used beside a speaker, phone, steelwork or current-carrying cable.
Recap
- Use the card for this compass, configuration and heading.
- Keep magnetic items away and compare unexpected readings with independent evidence.
- Recheck after relevant changes rather than preserving a stale table.
Optional quick check
Section 3 of 5
Why can one deviation value not be used on every heading?
Continue studying Compass
This public lesson is part of Revision Module 9. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.