Module 6 - Position, Course & Speed
Yachtmaster Offshore Standard: Position
What you will learn
Turn position, course and speed evidence into a labelled movement picture, then act before off-track progress consumes the planned margin.
At Yachtmaster level, a position or speed is useful only with its time, source, reference and physical meaning. The skipper must distinguish where the bow points, how the yacht moves through the water and how it moves over the ground before giving a command.
The RYA theory scope includes position fixing, course shaping and plotting and electronic position-finding equipment; the Yachtmaster Offshore examination assesses navigation and skippering. A defensible update therefore says more than ‘we are doing five knots east’. It identifies the accepted position and time, intended ground track, heading reference, water-track or ground-track evidence, speed frame, uncertainty, monitoring limit and next action.
Heading is where the fore-and-aft line points. Course is a selected direction. Water track describes movement through the water after effects such as leeway. Ground track and COG describe movement over the ground; SOG is the corresponding rate. A log commonly supplies speed through water, while GNSS derives movement over the ground. None of those pairs can be substituted silently in an ETA or DR/EP construction.
A COG and SOG sustained over a stated interval form a ground-movement vector. If COG differs from the intended track, not all of the travelled distance advances the route: one component is along track and another is cross track. A plot can show those components directly; the trigonometry below makes the same geometry explicit for revision.
Treat the calculation as a monitored estimate rather than an automatic helm order. Verify the position and time behind the vector, check whether the motion was steady, compare depth, radar, visual and other suitable evidence, and protect a hold, diversion or sea-room option before a danger-side action limit is exceeded.
All numbers in this lesson are fictional revision data. They do not prescribe a live course, speed, passing side or clearance, and written work cannot demonstrate practical Yachtmaster command or instrument competence.
Worked example
The intended ground track is 090°T. Over a steady 30-minute review interval, checked GNSS history shows COG 108°T and SOG 5.0 kn. The danger-side cross-track action limit is 0.50 M. What movement has actually been made good?
- 1Ground distance in 30 minutes: 5.0 kn × 30 ÷ 60 = 2.50 M.
- 2The track differs from the intended 090°T by 18°. Along-track progress = 2.50 × cos 18° = about 2.38 M.
- 3Cross-track displacement = 2.50 × sin 18° = about 0.77 M to the south, the starboard side of this eastbound track.
- 4The 0.77 M displacement exceeds the 0.50 M danger-side action limit by about 0.27 M. Matching the planned 5 kn speed did not preserve the planned track.
- 5Close the original track gate, establish an accepted current position and protect the briefed safe-water option before constructing or commanding a revised route.
Sense check: An 18° error should leave most of the 2.50 M as eastward progress but create a material sideways displacement. Treating all 2.50 M as along-track distance would hide the part that has moved towards danger.
| Quantity | Meaning in this example | Command boundary |
|---|---|---|
| Heading | Direction of the yacht's fore-and-aft line | Do not substitute it for COG or ground track |
| COG 108°T / SOG 5.0 kn | Recent ground-movement vector over the checked interval | It describes movement; it is not a steering order |
| 2.38 M along track | Progress projected onto the intended 090°T route | Use this frame for remaining ground-route distance |
| 0.77 M cross track | Danger-side displacement from the intended line | The 0.50 M action limit has failed |
Give a reconstructable movement update
The helm hears ‘COG 108, speed five’ and asks whether to steer eighteen degrees back towards the route.
1. Name the state
State the accepted position and time, intended 090°T ground track, 108°T COG, 5.0 kn SOG, review interval and the source and quality of each value.
2. State the geometry
Report about 2.38 M along-track progress and 0.77 M danger-side cross-track displacement against the 0.50 M action limit.
3. Protect before correcting
Do not issue an eighteen-degree arithmetic correction. Preserve safe water and re-establish the route from the accepted position with current tide, hazards, traffic and vessel response.
4. Close the loop
Name the next fix, track, depth and time checks and the discrepancy that will trigger another hold, diversion or replan.
Sense check: A useful command separates evidence from action: COG describes recent ground movement; it is not automatically the course alteration required next.
Common mistake or limitation
- SOG equal to planned speed does not prove the vessel is making the planned along-track progress.
- COG is recent movement over the ground, not an automatic heading or course correction to give the helm.
- Subtracting heading from COG and calling the result ‘the tide’ ignores leeway, manoeuvring, sensor reference, lag and changing conditions.
- A precise vector result remains conditional on the accepted position, time interval, sensor integrity and whether motion was sufficiently steady.
Recap
- Label position, time, source, direction reference and speed frame.
- Separate heading, water movement and ground movement.
- Project a ground vector into along-track progress and cross-track displacement.
- Act on a failed limit from current accepted evidence; do not steer a raw arithmetic correction.
Optional quick check
Section 9 of 10
COG and SOG show 2.50 M travelled, but only 2.38 M is along the planned track and 0.77 M is towards danger against a 0.50 M limit. What is the defensible first response?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Position, Course & Speed
This public lesson is part of Revision Module 6. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.