Module 9 - Compass
Yachtmaster Offshore Standard: Compass
What you will learn
Run a direction-integrity gate when compass, electronic-heading and ground-track evidence disagree, then preserve a safe option until the conflict is explained.
Yachtmaster compass work is not simply faster CDMVT. It is the command habit of naming what each source measures, tracing shared failures, recording the conditions and refusing to turn unexplained agreement into confidence.
Keep one direction record that another navigator can audit: source, reference, observation time, vessel heading and configuration, conversion, independent corroboration and the action trigger. A bare number such as 112° is incomplete. State whether it is compass, magnetic or true and whether it is heading, a bearing, water track or course over ground.
Heading is where the fore-and-aft line points; water track is the vessel's motion through the water; course over ground is motion over the ground. Tide, leeway, manoeuvring and sensor lag can separate all three. Converting a compass heading to true does not turn it into COG, and a stable COG does not prove the heading source.
Do not vote between the steering compass, a fluxgate display and a plotter. Two displays may share one sensor, power feed, calibration, alignment or data path. A portable speaker, tablet case, charging cable, steel tool or current-carrying conductor can disturb a magnetic compass. Heel can also change the residual deviation pattern. Identify dependencies before calling any agreement independent.
When the discrepancy is material and unexplained, protect sea room and reduce commitment. Hold, slow or move to safer water only as appropriate to the actual vessel and conditions; use known physical directions, depth, radar, position and other genuinely independent evidence. Restrict reliance on the suspect source, record the observation and arrange competent investigation rather than inventing one correction at sea.
Worked example
The steering compass reads 112°C. The current card gives 2°E residual deviation on that heading and the applicable variation is 1°30′W. What true heading does the present record predict?
- 1Write the signed C→T corrections: deviation +2° and variation −1°30′.
- 2Compass to magnetic: 112°C + 2° = 114°M.
- 3Magnetic to true: 114°M − 1°30′ = 112°30′T.
- 4Label 112°30′T as the card-predicted true heading, then compare it with independent evidence. The arithmetic does not prove that the compass, card or variation source is current and reliable.
Sense check: An east deviation makes magnetic greater than compass; a west variation then reduces the true value. The result returning close to 112° is plausible because the two signed corrections nearly cancel, not because they can be skipped.
| Source | What it describes | Failure to test | Stronger check |
|---|---|---|---|
| Steering compass | Installed magnetic compass heading | Heading-specific deviation, heel or nearby magnetic influence | Known direction and current residual-deviation record |
| Fluxgate display | Electronic magnetic heading, possibly corrected by the system | Siting, alignment, calibration, power or shared data | Trace the sensor and compare with an independent physical reference |
| GNSS COG | Movement over the ground | Tide, leeway, manoeuvring, smoothing or low-speed instability | Use only as controlled screening; do not relabel it as heading |
Open or close the direction-integrity gate
After a large alteration near a headland, the steering compass, fluxgate heading and GNSS COG differ while the yacht is heeled in cross-tide.
1. Classify
Name the reference and physical quantity for each value. Do not compare raw compass heading directly with COG.
2. Trace
Identify shared sensors, power, calibration and data paths; record time, heading, heel and equipment configuration.
3. Corroborate
Use a charted transit or other known direction where available, plus independent position, depth, radar or visual evidence appropriate to the situation.
4. Decide
If the conflict remains material, preserve sea room, restrict the suspect source and set a clear trigger to hold, divert or investigate before further commitment.
Sense check: Choosing the display nearest the planned route is confirmation bias. A direction source earns reliance through known reference, traceable corrections and independent corroboration.
Common mistake or limitation
- A true-heading label on a display does not establish how the value was sensed, corrected or shared with other equipment.
- Three matching screens can still be one source when they share a heading sensor, antenna, power supply, calibration or data network.
- A correct CDMVT calculation only states what the recorded inputs predict; it cannot validate a stale deviation card or an interfered compass.
- COG comparison is conditional screening, not a universal compass swing or proof of heading accuracy.
Recap
- Record source, reference, time, heading, configuration and conversion.
- Keep heading, water track and course over ground separate.
- Trace common-source failures instead of voting between displays.
- Preserve sea room and restrict reliance until a material conflict is explained.
Optional quick check
Section 8 of 9
The steering compass and two displays disagree, but both displays use the same fluxgate. What is the strongest interpretation?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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.