is the shock absorber for your mooring cables to protect the cables from the damage caused when waves move your dock and cause dockshock.
Dock Line Chafe Guard, Boarding Mat, Rope Protect
Competent crew skills: mooring lines - Safe Skipper Boating
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Mooring - Wikipedia
Competent crew skills: mooring lines - Safe Skipper Boating
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The secret to fast and easy mooring: the midships spring - boats.com