Module 11 - Tides & Tidal Streams
Yachtmaster Offshore Standard: Tides
What you will learn
Command a tidal decision gate by separating source-controlled predictions from observations and assumptions, showing the arithmetic and preserving a workable fallback before the margin disappears.
Yachtmaster tidal work is not a longer calculation. It is a defendable decision built from the correct port, date, time basis, datum, method, uncertainty and a clear point at which the plan will be held, revised or abandoned.
All worked values in these advanced sections are fictional, non-navigational teaching data. Use the current publication, applicable observations and vessel-specific margins for a real decision.
Start by controlling the source. Record the named standard or secondary port, date, edition or service, time basis, vertical datum and the specific curve, correction or stream method in use. A direct digital prediction for a secondary port is not corrected again with a paper secondary-port table. Two apps repeating the same dataset are one prediction path, not independent confirmation.
Separate three evidence classes. A fact is directly established for this calculation, such as the printed HW time, the charted sounding or a depth trend observed on a checked instrument. An estimate is a modelled result, such as predicted tidal height or an interpolated stream rate. An assumption is a condition the plan depends on, such as boat speed, gauge comparability or an unverified weather allowance. Label uncertainty instead of hiding it inside a precise-looking answer.
Turn the result into a gate with an arrival window, minimum depth or clearance, an operational allowance, a last safe decision point and a fallback. Recalculate when actual progress, depth, stream, pressure, wind or sea state differs materially from the plan. A favourable permission, desired ETA or best-case calculation does not reopen a gate whose evidence or margin has failed.
Observed data also needs source control. Before comparing a tide gauge or stream station with the plan, check that it is the intended station, that the timestamp and variable are current, and that its datum, quality and location are applicable. An ordinary navigation buoy or a display labelled 'tide' is not automatically an authoritative live observation.
Worked example
A fictional secondary-port calculation gives corrected HW 0550. The headland stream becomes foul at HW +0400, and the passage plan requires the yacht to be clear 30 minutes earlier. At 0750, revised underway time is 85 minutes and the selected operational allowance is 20 minutes. Is the gate open?
- 1Foul-stream start: 0550 + 0400 = 0950.
- 2Clear-by time: 0950 - 0030 = 0920.
- 3Revised time needed from 0750: 85 + 20 = 105 minutes.
- 4Revised clear time: 0750 + 1 hour 45 minutes = 0935.
- 50935 is 15 minutes after the 0920 clear-by time, so the planned gate is closed. Hold, divert or rebuild the route rather than spending the allowance as if it were spare time.
Sense check: Even the 85-minute best case reaches 0915, leaving only five minutes before the clear-by time and none of the chosen 20-minute allowance. A precise best case is not the same as a defendable margin.
| Evidence | Classification | Command use |
|---|---|---|
| Current tide-table HW/LW and stated time basis | Source-controlled input | Use in the calculation and record the source |
| Predicted intermediate height or stream rate | Estimate | Apply only the publication's method and add operating margin |
| Unverified gauge value from a shared app | Assumption or unresolved evidence | Check station, datum, time, variable and data path before comparison |
| Actual progress removes the planned allowance | Gate failure | Hold, divert or rebuild the plan while the fallback remains open |
Brief the tidal gate before departure
The route still looks possible on the plot, but a slower speed trend and stronger observed cross-stream are consuming the original allowance.
1. Prediction
State the port, source, time basis, datum, HW/LW data, stream reference and calculation method.
2. Observation
Record actual time, progress, depth trend, sea state and any checked gauge or stream evidence without treating shared displays as independent.
3. Gate
Name the clear-by time, minimum clearance, chosen allowance and the latest point at which the fallback remains controllable.
4. Command
Keep the gate closed when a material input or margin fails; hold, divert or replan before the situation becomes forced.
Sense check: The gate reopens only after the revised source, arithmetic, observations and contingency form one coherent and controllable plan.
Common mistake or limitation
- A tidal prediction is not an observation of the water now present at the yacht.
- Adding more decimal places does not reduce uncertainty in the source, method, boat speed or weather effect.
- A direct secondary-port digital prediction must not receive the paper correction a second time.
- A best-case arrival exactly at the clear-by time has no allowance for the uncertainty the plan already identified.
Recap
- Control port, date, source, time basis, datum and method.
- Separate facts, estimates and assumptions.
- Convert the arithmetic into a timed gate with an operating allowance.
- Keep a fallback controllable before evidence or margin fails.
Optional quick check
Section 12 of 13
The best-case clear time is 0915, the planned clear-by time is 0920 and the passage requires a 20-minute allowance. What is the defensible call?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
This public lesson is part of Revision Module 11. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.