This is the passage to the west of Brest, between the mainland and Ouessant.
For anyone travelling to Southern Brittany, this page may be useful, however, please refer to the Featherstone West France Cruising Companion (2nd edition) for more detailed information, especially if travelling by yacht.
Having studied the various tidal atlases, the slack water for the passages appear to be as follows:
Chenal du Four – High water Brest to HW +30mins and High water Brest +5.5 hours
We went past the Le Four lighthouse travelling South at HW Brest +5.5 hours and there was still some tide running.
As well as the waves and tides, there are often fog banks caused by the meeting of the currents
Raz de Sein – High water Brest -1 hour and High water Brest +5.5 hours. Slack water lasts for around 30mins
We arrived at the Raz de Sein at HW +5.5 hours and there was still a fair tide running to the North with overfalls. On our return journey, we were at the Raz de Sein at HW Brest + 4.25 hours and it was flat. Anyone have any experience they can share?
While discussing passages, remember, only go through the Ile de Batz passage when it is above half tide.