Wind against tide
When the wind blows against the direction of the tidal stream, the waves are squeezed: they become shorter, steeper and more confused than the wind alone would make them. When wind and tide run the same way, the sea is longer and flatter for the same wind. The stronger the stream, the bigger the difference.
Tidal races and overfalls
- Tidal races form where a strong stream is squeezed round a headland or through a narrow gap. The water runs fast and turbulent, often with steep, breaking seas, and much worse with wind against tide.
- Overfalls form where the stream runs over an uneven seabed, such as a ledge or bank, throwing up short, breaking waves even in light wind.
Both are marked on charts, usually with wavy lines or a note, and the pilot book and tidal atlas describe the worst ones and when they run. Well-known UK examples include the Portland Race off Portland Bill and the Alderney Race between Alderney and France.
Getting round safely
- Time it: pass at or near slack water, or when wind and tide are going the same way. The tidal atlas tells you when.
- Give it room: pass well outside the race, by the distance the pilot book recommends.
- Use an inshore passage only where the pilot book describes one, and only in suitable conditions. Some headlands have a narrow band of quieter water close inshore, and others don't.
- Prepare the boat: crew clipped on, hatches closed, and the engine ready in case the wind dies in the middle of it.
Tidal gates
A headland or channel with a strong stream becomes a tidal gate: a point you must pass at the right state of the tide. Plan the whole passage backwards from it, so you arrive when the stream is fair or slack. See passage planning and tidal stream calculation.