Module 11 - Tides & Tidal Streams
What Causes Tides?
What you will learn
Explain why astronomical forcing produces tides while local coastlines and water depth determine the times and heights used aboard.
The Moon and Sun provide the main predictable forcing, but the sea does not behave like two simple bulges. Ocean basins, coastlines and depth turn that forcing into a local pattern that must be read from predictions for the place and date.
The gravitational effects of the Moon and Sun, together with the motion of the Earth-Moon system, create the main astronomical tide-producing forces. A simple equilibrium-tide diagram with two bulges is useful for introducing the cycle, but it is not a timetable for a real harbour.
Tides travel through ocean basins as long waves. Basin shape, coastal geometry, depth, friction and resonance change their timing and size. That is why nearby ports can have different high-water times and why some places have double high waters, double low waters or prolonged stands.
Many UK ports are mainly semi-diurnal, with two high waters and two low waters in a lunar day of about 24 hours 50 minutes. A comparable phase is therefore often about 50 minutes later on average, but the interval varies. For passage planning, use the published prediction rather than advancing yesterday's time by a rule of thumb.
Start with the harbour, date and source
Yesterday's high water was close to 1100, and a crew member proposes using 1150 today without opening the tide product.
1. Identify the place
Confirm the intended standard or secondary port and whether the berth has a local tidal peculiarity.
2. Use the dated prediction
Read the published high- and low-water times and heights for the correct date and time basis.
3. Treat patterns as checks
Use the roughly 50-minute lunar-day pattern only to challenge a transcription error, never to replace the prediction.
Sense check: If the copied time differs sharply from the surrounding days, recheck the port, date and time zone before assuming the sea is wrong.
Common mistake or limitation
- The familiar two-bulge picture explains astronomical forcing but does not predict local harbour times.
- High water is not exactly 50 minutes later every day; the actual interval varies by port and tide.
Recap
- Astronomy supplies the forcing; the coast and basin shape the local tide.
- Many UK locations are semi-diurnal, but local regimes can be more complex.
- Use a dated local prediction for navigation.
Optional quick check
Section 1 of 8
Why should yesterday's high-water time plus 50 minutes not be used as today's prediction?
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.