The barrage concentrated defenses between Dover and Calais, where the Western Front met the sea. German submarines discovered methods of passing it, including surfaced night transits, but each revealed method prompted British countermeasures. A 25-kilometre line of indicator nets laid in February 1915 caught U-8 by 4 March, and the first form of the barrage was largely complete by April. Later, more sensitive mines and stronger illumination increased losses, with UB-109 found broken in half along the barrage line. The barrier was neither impermeable nor static: its value lay in making passage difficult and adapting whenever a German method became visible.
Chapter · 6:42-14:19
The Dover Barrage as an adapting anti-submarine barrier
Mines, nets, searchlights, patrol craft, and changing tactics made the Dover Barrage difficult rather than perfectly impermeable to U-boats.