What Is a Spring Line?
Cross lines that stop a boat creeping fore and aft along the berth — the difference between a mooring that just holds and one that holds properly.
A spring is the cross-running line used alongside the bow and stern lines when a boat is berthed alongside the quay: it runs aft from the bow (fore spring) and forward from the stern (aft spring). Bow and stern lines hold the boat in close to the berth; springs stop it moving fore and aft along the quay.
In a choppy marina or on a berth affected by ferry wash, a boat that is allowed to work fore and aft puts the whole load on the bow and stern lines if there are no springs, and fenders shift out of position and stop doing their job.
Four-line mooring arrangement
Standard: bow + stern + two springs, each led to separate points.
Length: ideally a spring should be close to the boat’s length and led as nearly horizontal as possible; a short spring will not do the job properly.
Chafe: a spring often passes at fender height, and a line that runs unprotected will come ashore at the end of the season worn right through.
When a blow is forecast, the first extra line to add is still a spring: locking the boat along the berth halves the load on the other lines. It is also a skilled manoeuvring aid; taking the load on a single spring under power is the classic way to ease a boat off a berth by moving the bow or stern out in tight spaces.