Three to five percent of the new mines dropped into the North Sea detonated as soon as the salt pellets dissolved; and hydrophones detected premature detonations continuing for a week after minelaying. These premature detonations were initially attributed to activation of the horn fuze detonation circuits by seawater leaking into the mines; and mine spacing was increased from on the first minelaying excursion to on subsequent excursions to minimize leakage caused by detonation of nearby mines. About one percent of the mines deployed during the first excursion broke free of their mooring cables and washed ashore in Norway within a month. Mines used for the last eleven excursions had springs installed at the mine mooring cable attachment points to buffer wave loading during storms. Premature detonations increased to 14 percent for the fourth minelaying excursion; because some mines had been assembled with the more sensitive antenna fuze relay settings made by the Bureau of Ordnance. The fifth minelaying excursion was halted when 19 percent of the mines detonated prematurely. "San Francisco" identified relay armature sensitivity as a major cause of premature detonations during a comparative field test minelaying excursion on 12 August. Subsequent investigations revealed copper sulfate deposits caused by antenna corrosion created a weak battery, increasing the probability of relay activation by shock wave accelerations when nearby mines detonated. Premature detonations dropped to four to six percent when sensitivity was adjusted to 30 to 45 millivolts for mines deployed by the last five minelaying excursions.