Brest's enclosed roadstead reached the sea through the Goulet and three exits obstructed by reefs, rocks, and strong tidal races. A squadron could not watch every route from one safe offshore position, while ships using the Raz de Sein had to time multiple narrow passages on the same tide. Unreliable positions in the official Neptune François charts made a direct approach to Brest dangerous, so inward-bound ships fixed their positions at safer landfalls before approaching through the Raz de Sein. Blockade therefore demanded local knowledge, careful stationing, and enough sea room to survive a westerly gale. Simply placing ships outside the harbour would leave them exposed to the same geography they were trying to control.
Chapter · 1:56-5:36
Brest's tides, reefs, and three difficult exits
A close blockade of Brest had to cover several tide-bound passages without letting Atlantic weather trap the watching ships against a dangerous coast.