The first five boats sailed independently in October 1939 from bases in NW Germany, sailing northabout around the coast of Scotland in order to reach the Western Approaches. The last boat, U-40, which was delayed in departing, was ordered to take the southabout route through the Channel; this was more dangerous, but was deemed practicable. In the event U–40 became trapped by the Allies' Channel defences, the Dover Barrage; on 13 October she struck a mine and was destroyed.