Twistlock

A twistlock is a standardised mechanical device that engages with the corner castings of ISO shipping containers—castings built to ISO 1161 specification—and locks by rotating 90 degrees to secure two containers together or to a ship's cell guide fitting. The device is the primary vertical load-bearing connection in container stacking, transferring the weight of upper tiers through the corner castings to the ship's structure. Semi-automatic twistlocks lock automatically as the container lands and are released manually from a safe position; manual (or dumb) twistlocks require operation at container level. The Cargo Securing Manual specifies the stacking weight and racking load limits for approved twistlock types. The chief mate oversees the twistlock deployment plan before loading commences, ensuring that the correct type and sufficient number of twistlocks are available and that stevedores use them correctly. Improperly seated or accidentally unlocked twistlocks have caused catastrophic stack collapses at sea, resulting in loss of life and environmental damage. Post-loading checks by the ship's officers verify that all twistlocks are engaged and that lashing rods and turnbuckles are tightened to specification. SOLAS Chapter VI, Regulation 5 requires that securing operations be completed satisfactorily before departure.