The direct cause of the disaster was human error: the assistant bosun, responsible for closing the bow doors of this roll-on/roll-off ferry, had fallen asleep in his cabin and slept through the alarm telling crew that the ship was sailing.

The phrase "ships sink Zeebrugge" almost universally refers to the MS Herald of Free Enterprise disaster on March 6, 1987, which remains a seminal high-fidelity case study in maritime safety. The ferry capsized just minutes after leaving the Belgian port of Zeebrugge, killing 193 people. The immediate cause was a catastrophic human failure: the ship set sail with its bow doors wide open. Because the ferry was a "Roll-on/Roll-off" (RORO) design, it featured a large, open car deck with no internal partitions. As the ship increased speed, water rushed onto the car deck through the open doors—a phenomenon known as the Free Surface Effect. The weight of the water shifted rapidly, causing the ship to lose stability and capsize in seconds. The investigation revealed a "disease of sloppiness" within the operating company, as there was no indicator light on the bridge to tell the captain the doors were open. This tragedy led to immediate global changes in maritime law, mandating bridge indicator lights for all door closures and stricter protocols for deck crew communications, ensuring that such a "preventable" sinking could never happen in 2026.