Module 11 - Tides & Tidal Streams
Using Tide Tables
What you will learn
Extract the correct bracketing high and low waters from a tide product after checking port, date, time basis and datum.
A tide-table lookup is a source-control task before it is an arithmetic task. Confirm the publication, port, date, time zone and height datum, then copy the high and low waters that bracket the required time.
ADMIRALTY Tide Tables provide daily high- and low-water times and heights for standard ports, plus differences and methods for secondary ports. Almanacs and authorised digital services may present the same planning information in a different layout. Use a current product suitable for the area and follow its instructions.
First confirm the place and date. Then read the time basis exactly as displayed. UK paper tables commonly use UT (GMT), while a digital product may display a selected local or zone time. Add one hour for British Summer Time only when the source is in UT and BST applies; never add it to a value already displayed in local civil time.
For an intermediate height, copy the high and low waters that bracket the required time and use the port's supplied curve or method. Predictions describe astronomical water level, not an observation: pressure, wind, surge and waves can alter the level actually found.
Run a five-part source check
A digital prediction shows 0735 and the paper planning sheet says 0835, during British Summer Time.
1. Check both labels
Identify the port, date and time-zone display on each source before changing either number.
2. Convert once
If 0735 is explicitly UT and BST applies, 0835 is local clock time. 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 crew cannot convert it again.
Sense check: The passage plan, watch clock and tide calculation must all use the same declared time basis.
Common mistake or limitation
- Adding one hour automatically can create a two-hour planning error when a digital service already displays local time.
- A high water from the wrong port or date can look plausible, so the source labels must be checked before arithmetic.
Recap
- Check source, place, date, time basis and datum.
- Select the high and low waters that bracket the time required.
- Treat the prediction as a planning input, not a measured water level.
Optional quick check
Section 4 of 8
A tide service explicitly displays times in BST. What conversion is needed for a passage plan also kept in BST?
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.