The sheet anchor - in bygone days- was carried in the waist (between the forecastle and quarterdeck) and was the largest anchor and was always ready for use in an emergency. The term came to mean "security" among seamen.
There is a rare but extraordinary book called "The Young Officer's Sheet Anchor" which was often given to young officers setting out on their first voyage.
It contains, among other things, sail settings for various situations and was an invaluable guide for men in ships under sail.
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