Sheet anchor

A sheet anchor is the largest and heaviest anchor carried by ships, especially sailing ships, and was used in an emergency, sometimes as a second or fail-safe anchor. It dates in this sense from the late 15th century. Sheet anchor came to used figuratively meaning someone or something on which one places total reliance, and dates in this figurative sense from the mid-16th century. Why 'sheet' anchor? The OED says that the 'sheet' here means 'sail' because sails were often referred to as 'sheets' in the days of sail, but adds that the connection is not obvious.