A mooring line leading forward or aft at a shallow angle to prevent the ship from moving ahead or astern.
In practice
Spring lines are used operationally as well as for securing. When singling up before departure, the master may instruct the deck officers to hold only a head and stern spring to allow the vessel to pivot away from the berth using engine thrust. Proper tensioning of springs is critical: over-tight springs in a tidal environment can cause the vessel to heel or to part the line as the tide falls. OCIMF's Mooring Equipment Guidelines recommend that all mooring lines in a standard arrangement are sized consistently so that load is distributed evenly across the berth, with springs typically making up a defined proportion of the total mooring complement.
Regulatory detail & full definition
A spring line is a mooring line rigged fore and aft at a shallow angle between the vessel and the berth, its purpose being to restrain the ship from surging ahead or astern in response to current, propeller wash from passing vessels, or the thrust of its own machinery. A forward spring leads aft from a point near the bow to a shore bollard abaft the ship, preventing headway; an aft spring leads forward from near the stern, preventing sternway. Both may be deployed simultaneously.
Spring lines are used operationally as well as for securing. When singling up before departure, the master may instruct the deck officers to hold only a head and stern spring to allow the vessel to pivot away from the berth using engine thrust. Proper tensioning of springs is critical: over-tight springs in a tidal environment can cause the vessel to heel or to part the line as the tide falls. OCIMF's Mooring Equipment Guidelines recommend that all mooring lines in a standard arrangement are sized consistently so that load is distributed evenly across the berth, with springs typically making up a defined proportion of the total mooring complement.