Module 11 - Tides & Tidal Streams
Advanced Scenario Check: Tides
What you will learn
Manage a compound tidal conflict in which the predicted height, a delayed gauge value, observed depth, cross-stream and shared electronic sources no longer support the planned harbour approach.
This fictional scenario rehearses evidence control and early fallback decisions. It cannot provide the live under-keel clearance, course, speed or go/no-go decision for a real vessel and does not demonstrate practical Yachtmaster competence.
A yacht is approaching a fictional harbour after a machinery delay. The plan uses a direct digital secondary-port prediction of 2.4 m at 0815. A harbour gauge panel copied into both navigation apps shows 2.1 m, but its datum is not displayed and the timestamp is twelve minutes old. The echo-sounder trend is 0.4 m shallower than the passage plan expected, and the cross-stream measured from track error is stronger than the atlas estimate.
Do not average the 2.4 m prediction and the 2.1 m display. The first is a modelled value; the second is not comparable until station, datum, timestamp, variable and quality are verified. The shallower depth trend and stronger cross-stream are operational observations that challenge the plan even though they do not by themselves diagnose one cause.
Keep the entrance gate closed and preserve the briefed holding or diversion option. Assign one person to the con, one to helm and lookout as the vessel requires, and one to rebuild the navigation picture. Confirm the electronic data paths, obtain current authoritative harbour or gauge information if available, recheck tide, depth, stream, squat and clearance assumptions, and compare the result with a route that retains sea room.
Reopen only when the revised calculation, applicable observation and independent navigation evidence agree with the minimum clearance, time window and fallback. If the discrepancy remains at the last safe point, hold, divert or abandon the approach. Harbour permission or pressure to berth does not replace the missing tidal and position evidence.
Worked example
Classify the four principal inputs before deciding whether the harbour gate can reopen.
- 12.4 m direct digital tide: a prediction tied to a source, station, time and datum; do not add paper secondary-port corrections again.
- 22.1 m app gauge panel: unresolved until station, datum, timestamp, variable and quality are confirmed; two apps sharing it remain one source.
- 3Depth 0.4 m shallower than expected: an observed discrepancy after instrument offset and vessel response are checked, not proof of one cause.
- 4Stronger cross-stream from track error: an operational estimate that should be corroborated and used to revise timing and course-control margin.
- 5Decision: keep the gate closed until the revised depth, stream, position and fallback are coherent, or use the holding/diversion plan at the decision limit.
Sense check: Averaging incomparable numbers would create a tidy answer without establishing which datum, time or physical location it represents.
| Conflict | What it does not prove | Safe response |
|---|---|---|
| Prediction 2.4 m; app panel 2.1 m | Which value shares the applicable datum and time | Verify both source paths before comparison |
| Two apps show the same gauge panel | Independent corroboration | Count the shared feed as one source |
| Depth is 0.4 m shallower than planned | A single cause or a safe remaining clearance | Protect sea room and rebuild tide, position, offset, squat and seabed assumptions |
| Cross-stream is stronger than estimated | That the planned arrival window still holds | Revise course-control, timing and gate margin |
Use closed-loop roles
The crew ask whether the lower gauge number should simply be used as the conservative height while continuing towards the entrance.
1. Con
State that the gate is closed, name the holding or diversion command and preserve the last safe point.
2. Navigator
Verify source dependencies, station, datum, time and variable; rebuild depth, stream and clearance with current inputs.
3. Helm and lookout
Maintain the named safe command and report traffic, depth trend and environmental changes without steering towards an assumed solution.
4. Skipper
Reopen only against explicit clearance, timing, evidence and fallback criteria; otherwise hold, divert or abandon.
Sense check: Using the lower value may be conservative for one calculation, but it does not resolve the stronger stream, position, datum or stale-data conflicts that control the whole approach.
Common mistake or limitation
- Two screens do not provide two observations when both repeat one prediction or gauge feed.
- A lower number is not automatically usable merely because it looks conservative; its datum, time and location still matter.
- Observed depth disagreement should trigger a whole-picture check, not an unsupported assumption that weather alone explains it.
- This written scenario cannot prescribe the live approach, clearance allowance or manoeuvre for a real yacht.
Recap
- Do not average incomparable prediction and observation data.
- Trace shared sources and verify datum, time, variable, location and quality.
- Use depth and stream discrepancies to close the gate and rebuild the plan.
- Preserve holding, diversion or abandonment before the last safe point.
Optional quick check
Section 13 of 13
A 2.4 m prediction conflicts with a stale 2.1 m gauge panel of unknown datum, shallower depth and stronger cross-stream. What should open the gate?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
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