why stern ramp was left open, while at port no ingress of water then why at sea, what was the root cause.

Answer

The scenario of a stern ramp/bow door being left unsealed or open, showing no water ingress at port but severe, catastrophic flooding once at sea, is a classic stability and hydrodynamic problem in Ro-Ro (Roll-on/Roll-off) vessel operations (most famously seen in historical casualties like the MS Herald of Free Enterprise and MS Estonia).

1. Freeboard Differential (Static vs. Dynamic Water Line):

   • At Port: The vessel is stationary in calm, sheltered water. The height of the stern threshold/sill remains well above the static waterline, even with the ramp open or unsealed. Gravitational head pressure works against the water, so no sea enters.

   • At Sea: Vessel movement dynamically alters the effective waterline. Pitching, rolling, and squat (hydrodynamic sinkage at speed) drastically reduce the freeboard at the stern.

2. Hydrodynamic Bow/Stern Waves & Dynamic Pressure:

   • As the vessel moves through the water, it creates a stern wave system. At operational speeds, water mounds up at the transom/stern, creating dynamic pressure that easily exceeds the height of an unsealed stern sill.

3. Relative Wave Motion:

   • In open sea, ocean swells and following seas directly wash over the stern threshold. Even minor wave run-up drives hundreds of tonnes of water into the vehicle deck per minute.