Module 9 - Compass
Using a Hand-Bearing Compass
What you will learn
Take repeatable magnetic bearings clear of local interference and combine position lines without claiming the vessel must lie inside a cocked hat.
A hand-bearing compass lets the observer sight a target directly from a useful position on deck. A clean observation is normally treated as magnetic, then converted and plotted on the chart with its time and target.
Choose conspicuous charted objects with useful crossing angles, identify each before reading, and stand clear of speakers, phones, steel rigging, wiring and the steering compass. Hold the instrument as designed, let the card settle, read without parallax and repeat doubtful observations.
Record target, magnetic bearing and time. Convert to true if plotting on the chart, then draw the reciprocal from the object towards the vessel. Bearings taken while the vessel moves should be obtained quickly or advanced to a common time when the required accuracy justifies it.
Three lines that do not meet form a cocked hat, but the vessel is not guaranteed to be inside it. Random observation errors may surround the position; shared errors—wrong variation, compass interference, misidentified object or plotting bias—can displace every line in the same direction. Use depth, position, additional lines and safe-side judgement rather than treating the triangle centre as truth.
Diagnose a displaced cocked hat
Three neatly plotted bearings form a small triangle over shallow water, while depth and GPS place the yacht consistently south of it.
1. Do not average blindly
A small triangle can still be displaced by one shared systematic error.
2. Check common inputs
Verify target identities, variation sign, magnetic interference, plotting reciprocals and observation times.
3. Navigate on the safe side
Use depth and additional independent evidence while keeping clear of the charted danger.
Sense check: Precision is how tightly lines agree; accuracy is whether they agree with the vessel's real position. A small cocked hat proves only the first.
Common mistake or limitation
- A portable compass is not immune to deviation; moving beside a loudspeaker can corrupt the reading immediately.
- Three intersecting position lines do not automatically place the vessel inside their triangle when errors share a cause.
Recap
- Take identified, timed, repeatable bearings away from interference.
- Convert magnetic to true and plot reciprocals carefully.
- Test a cocked hat against independent evidence and the safest plausible position.
Optional quick check
Section 5 of 5
Why might a small cocked hat still miss the vessel's true position?
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.