Module 11 - Tides & Tidal Streams
What Causes Tides?
What you will learn
Explain why astronomical forcing produces a local tide rather than a universal timetable, and select the correct dated prediction for a coastal decision.
The Moon and Sun supply the main predictable forcing. Ocean basins, depth, coastlines, friction and resonance turn that forcing into the local times and heights a skipper must use.
The gravitational effects of the Moon and Sun, together with the motion of the Earth-Moon system, create the principal astronomical tide-producing forces. The familiar two-bulge picture is an introductory model, not a harbour timetable: real tides travel through ocean basins as long waves.
Basin shape, water depth, coastal geometry, friction and resonance change the response. Nearby ports can therefore have different high-water times and ranges, while some locations have double high waters, double low waters or prolonged stands. The applicable prediction is local to a named place and date.
Many UK ports are mainly semi-diurnal, with two high and two low waters in a lunar day of about 24 hours 50 minutes. A comparable phase is often about 50 minutes later on average, but the interval varies. A coastal plan uses the current prediction and keeps the broad pattern only as a transcription check.
Reject a copied timetable
Yesterday's high water was near 1100. A watchkeeper proposes using 1150 today for a shallow harbour without opening the current tide source.
1. Name the location
Confirm the standard or secondary port and whether the berth has a separate local note.
2. Read the dated prediction
Copy the high- and low-water times and heights for the correct date and declared time basis.
3. Use pattern as a check
Challenge a gross transcription error with the lunar-day pattern, but never replace the port prediction with it.
Sense check: If the value differs sharply from adjacent days, recheck the port, date and time zone before assuming the observation is wrong.
Common mistake or limitation
- The two-bulge diagram explains forcing but cannot predict a real harbour's time or range.
- High water is not exactly 50 minutes later every day; the actual interval varies by port and tide.
Recap
- Astronomy provides forcing; the basin and coast create the local tide.
- Semi-diurnal is common in UK waters but not universal.
- Use a dated local prediction for navigation.
Optional quick check
Section 1 of 13
Why is yesterday's high water plus 50 minutes unsuitable as today's harbour prediction?
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.