Module 10 - Chartwork
Yachtmaster Offshore Standard: Chartwork
What you will learn
Replan from the last accepted fix when progress, cross-track error or a timed gate no longer supports the intended passage.
Yachtmaster chartwork is a command decision, not a longer pencil exercise. The plot must expose current evidence, remaining uncertainty, the monitoring limit and a fallback that is still usable before the route becomes forced.
The RYA Yachtmaster Offshore examination includes navigation and safety, while the theory scope includes position fixing, course shaping and plotting, tidal knowledge, publications and electronic position equipment. At this level, a correct historic construction is not enough: the skipper must decide whether the current plan still works and issue a safe, understandable command.
Use the last position that the navigation team has accepted, with its time and uncertainty, as the start of the revised problem. Do not continue measuring from the departure point or chase an old track line after the evidence has changed. Recalculate distance and time remaining, compare actual progress and cross-track error with the briefed limits, then protect a holding, diversion or sea-room option before making a new route commitment.
Keep evidence classes visible. A timed visual or radar fix is an observation with identification and plotting uncertainty. DR and EP are estimates built from course, distance, leeway and stream assumptions. A chartplotter route is a planned geometry using chart, position and user-entered data. Agreement between displays that share those inputs is repetition, not independent confirmation.
All values in this lesson are fictional revision data, not a route or instruction for use aboard. A real plan requires current charts and publications, vessel-specific performance, applicable weather and tide, a suitable lookout and the skipper's live assessment.
Worked example
A fictional 18.0 M leg began at 1200. At the planned 6 kn it would take 3 hours and arrive at 1500, ahead of a 1515 clear-by gate. At 1330 an accepted fix leaves 10.2 M, current SOG is 5.0 kn, cross-track error is 1.2 M and the briefed action limit is 0.5 M. What does the revised plot say?
- 1Original duration: 18.0 M ÷ 6 kn = 3 hours, so the original ETA was 1500.
- 2Observed progress by 1330: 18.0 M − 10.2 M = 7.8 M in 1.5 hours, an average SOG of 5.2 kn rather than the planned 6 kn.
- 3Time remaining at the current 5.0 kn SOG: 10.2 M ÷ 5.0 kn = 2.04 hours, about 2 hours 02 minutes.
- 4Revised ETA: 1330 + about 2 hours 02 minutes = 1532, which is 17 minutes after the 1515 clear-by gate.
- 5The 1.2 M cross-track error also exceeds the 0.5 M action limit. Close the original gate and select the briefed hold or diversion, or build a new route from the accepted fix with current hazards, tide and margins.
Sense check: The revised ETA fails the timed gate before any extra allowance is added, and the cross-track limit has already been exceeded. Cutting a corner or steering back towards the historic line would hide those two independent warning signals rather than restore the plan.
| Evidence | Meaning | Command consequence |
|---|---|---|
| 10.2 M remaining at 1330; SOG 5.0 kn | 2.04 hours remaining; revised ETA about 1532 | The 1515 clear-by gate is closed |
| Cross-track error 1.2 M | The briefed 0.5 M action limit has been exceeded | Do not chase the old line or cut an unverified corner |
| Accepted fix with stated uncertainty | Current start point for replanning | Rebuild from this evidence, not from departure |
| Holding or diversion remains available | The situation is still controllable | Protect that option before committing to a new route |
Turn a revised plot into a command brief
The crew can still see the original route on both plotters and asks whether to steer harder towards it to recover the schedule.
1. State the evidence
Name the accepted fix and time, 10.2 M remaining, 5.0 kn current SOG, 1.2 M cross-track error and the source of the 1515 gate.
2. State the failed limits
Report the 1532 revised ETA against the 1515 gate and the 1.2 M error against the 0.5 M action limit.
3. Protect the fallback
Use the briefed hold or diversion while there is sea room, or construct a fresh route from the accepted fix with current tide, hazards and clearance margins.
4. Close the loop
Set the next fix, depth, track and time checks and name the discrepancy that will trigger another command decision.
Sense check: A useful brief lets the helm and navigator say what is being done now, why the original plan is closed and what evidence will support the next decision.
Common mistake or limitation
- A route displayed on two screens is not two independent plans when both share the same position, chart and route data.
- Steering back towards an old track line can create a new danger; first establish the vessel's current position, safe water and revised objective.
- A schedule does not justify cutting a corner or spending an allowance whose assumptions have already failed.
- A precise ETA does not remove uncertainty in position, speed, tide, chart data or vessel response.
Recap
- Replan from the last accepted fix and current evidence.
- Compare revised ETA and cross-track error with explicit action limits.
- Close a failed gate before selecting a new route.
- Preserve a hold, diversion or sea-room option and state the next check.
Optional quick check
Section 12 of 13
At 1330, 10.2 M remain at 5.0 kn, the gate closes at 1515, and cross-track error is 1.2 M against a 0.5 M limit. What is the defensible first action?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
Continue studying Chartwork
This public lesson is part of Revision Module 10. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.