An able seaman was injured when a weathertight door holdback arrangement failed as he was stowing stanchions into a locker. The seaman sustained an impact injury on his calf from the swinging door. An investigation found the holdback arrangement showed signs of wear and tear and was inadequate to secure the door. Weather conditions caused the securing hook to unlock from vessel movement. The company inspected all door and hatch holdbacks, fabricated a more robust arrangement for the door, and shared the learning with the fleet and industry to improve safety.