sheet anchor 
noun
1An additional anchor for use in emergencies.
2A very dependable person or thing.As I said, it's the sheet anchor of what we've come to know in the last century as middle class life....
- Now I only know that a sheet anchor is a very good thing, and that Ancient India would have been in deep trouble if Alexander hadn't shown up.
- Wayne Hutchinson was again the sheet anchor of the defence with strong support from Johnny Kearney and Paul Hogan.
Origin
Late 15th century: perhaps related to obsolete shot, denoting two cables spliced together, later influenced by sheet2.