Module 11 - Tides & Tidal Streams
Tidal Anomalies
What you will learn
Recognise when a generic smooth-tide assumption fails and replace it with the port-specific curve, notes and a conservative decision window.
Double high waters, prolonged stands, rapid changes and bores are local behaviours. Their operational meaning comes from the current local publication, not a memorised national pattern.
The simple six-hour rise-and-fall model does not describe every port. Basin and coastal response can produce double high or low waters, long stands, asymmetric rise and fall, bores and stream behaviour that does not reverse at local HW or LW.
An anomaly changes the passage decision. A stand can extend one part of an entry window, while a rapid fall after it can close depth unexpectedly. A stream turn can precede or follow the local height event. The source note and local curve therefore outrank a generic rule of twelfths.
Read the almanac or sailing-direction note, the named reference port and the port-specific curve. Build a latest safe time and an earlier abort gate. Do not infer a precise cause or timing from a regional label alone.
| Observed or published feature | Planning risk | Required response |
|---|---|---|
| Prolonged stand | Window looks wider than the later fall allows | Use the full local curve |
| Double high water | Wrong peak selected | Identify both events and required one |
| Rapid fall | Depth margin closes quickly | Set an earlier leave/abort time |
| Stream offset | Wrong atlas hour or gate | Use named reference and local note |
Plan the feature, not the nickname
A harbour note shows a stand near high water followed by a faster fall, and the yacht needs a falling-tide departure.
1. Mark the complete curve
Identify the stand, later fall and the required depth threshold.
2. Calculate the limiting time
Find when predicted depth reaches the vessel's requirement, including margins.
3. Move the decision earlier
Set a leave/abort point that preserves manoeuvring time before the rapid fall.
Sense check: A stand near high water does not prove the entrance will remain usable through the following rapid change.
Common mistake or limitation
- Rule of twelfths is not a substitute for a supplied local curve or anomaly note.
- A named regional example does not make every nearby port share the same double-water timing or cause.
Recap
- Treat anomalies as local published behaviour.
- Use the complete curve and named reference.
- Turn the feature into a conservative operational gate.
Optional quick check
Section 7 of 11
A port has a prolonged stand followed by a rapid fall. What should control the departure window?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
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