Brest's tidal passages and the problem of keeping watch

Brest's dockyard stood on the Penfeld River inside a large enclosed roadstead, reached through the Goulet. Beyond it, reefs and islands left three passages into open water: the Iroise to the west, the Four to the north, and the Raz de Sein to the south. Tidal streams reached about three knots in the Goulet, four and a half in the Four, and seven in the Raz. A ship using both the Goulet and the Raz had to time a passage of roughly twenty-five miles to catch the same favorable tide, or wait at anchor. A fleet could watch all the exits only by standing dangerously close in, while observers on the French mainland could not see far enough offshore to locate a more distant blockade squadron.