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. Results Supply problems and technical difficulties caused some delays. More minelaying excursions to complete the barrage were cancelled when the approaching end of hostilities was recognized after the thirteenth minelaying excursion on 26 October 1918. The design of the minefield meant there was a theoretical 66 per cent chance of a surfaced U-boat triggering a mine and a 33 per cent chance for a submerged U-boat. On the basis of the number of effective mines observed while sweeping the barrage, the actual odds were assessed at being closer to 20 per cent for a surfaced U-boat and 10 per cent for a submerged one. As the final mines were laid only a matter of days before the end of the war, it is impossible to assess the success of the plan. Some contend the minefield was a major cause of the declining morale of the Imperial German Navy through the final months of the war, while others suggest Germany easily swept safe channels through the large, unguarded minefield. The official statistics on lost German submarines compiled on 1 March 1919 credited the North Sea mine barrage with the certain destruction of four U-boats, presumed destruction of two more and possible destruction of another two. 19 August 1918 unknown - possibly sunk by the North Sea mine barrage 9 September 1918 presumed sunk by the North Sea mine barrage area B (confirmed in 2007) 9 September 1918 sunk by the North Sea mine barrage