Module 11 - Tides & Tidal Streams
Using Tide Tables
What you will learn
Extract the correct bracketing high and low waters after checking publication, port, date, time basis, datum and whether a digital value is already a direct prediction.
Tide-table work starts with source control. A plausible time from the wrong port, date or time zone is more dangerous than an obvious arithmetic error.
ADMIRALTY Tide Tables cover about 500 standard and 5,700 secondary ports and provide daily HW/LW times and heights, secondary-port methods, harmonic constants where known and exceptional tidal information. A compact almanac or authorised digital service may present a selected subset or direct prediction in a different layout.
Confirm the publication, edition or service, port and date. Then read the displayed time basis. UK paper examples commonly use UT, while a digital service can display a selected zone or local time. Add one hour for BST only when the source is explicitly UT and BST applies.
For an intermediate height, copy the HW and LW that bracket the required time and use the curve or method supplied for that port or exercise. The output is an astronomical prediction; pressure, wind, surge, river flow and waves can alter the water actually encountered.
| Check | Question | Failure caught |
|---|---|---|
| Source | Is this the current suitable publication or service? | Stale or incomplete data |
| Place | Is this the named standard/secondary port? | Plausible wrong-port value |
| Date | Are the bracketing waters on the intended day? | Midnight/date rollover |
| Time | UT, BST/local or another declared zone? | Double or missed conversion |
| Datum | What zero are the heights referred to? | Invalid depth combination |
Resolve a one-hour disagreement
A digital display says 0735 and a paper plan says 0835 during BST.
1. Read both labels
Check port, date and displayed time zone before changing either number.
2. Convert once
If 0735 is explicitly UT, it is 0835 BST. If 0735 is already local, no hour is added.
3. Write the basis
Record UT or BST/local beside the passage-plan time so the watch cannot convert it again.
Sense check: The passage plan, onboard clock and tide calculation must use the same declared basis.
Common mistake or limitation
- Adding one hour automatically can create a full-hour error when a digital source already displays local time.
- A direct secondary-port prediction must not receive the paper table's correction a second time.
Recap
- Control source, place, date, time and datum before arithmetic.
- Select the waters that bracket the time required.
- Treat the result as a prediction rather than a live measurement.
Optional quick check
Section 4 of 13
A service explicitly shows 0735 BST and the passage plan is kept in BST. What conversion is required?
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.