An additional anchor carried in the largest ships for security should the bower anchors fail to hold the ship. Originally two additional anchors were carried, one at either chesstree abaft the fore rigging, one anchor being termed the sheet, the other the spare.
The term sheet anchor is also used as a synonym for security generally. ‘I never knew an appeal made to them for honour, courage or loyalty,’ said Admiral of the Fleet Lord St Vincent (1735–1823) of the Royal Marines during the Napoleonic War (1803–15), ‘that they did not more than realise my highest expectations. If ever the hour of real danger should come to England, they will be found the country's sheet anchor.’
Subjects: Maritime History.