Why did the Sewol ferry list so drastically to capsize? Videos recovered from black boxes loaded inside the ferry’s cargo deck indicated the starting point of when the cargos started sliding to the left and how much the ferry tilted by this moment.
The ferry listed very quickly: From 21 degrees to 47 degrees within just 23 seconds
The black box video collected from the Starex van, which was parked in the center front of Deck C, included a scene in which numbers of cars are pushed to the left corner to fall off and water stream start spilling off the ceiling. The water is estimated to have leaked from a sprinkler pipe, which was installed for cleaning purpose.
This water stream is falling off in parallel with the pillar right in front of it, forming an angle. The angle seem to have formed as the ferry already tilted a lot while the water falling vertically due to gravity.
This means, the angle formed between the pillar and water stream shows how much the ferry was listed at the moment.
When looking more closely, the water stream appears in the video for the first time at 8:49:36 a.m.
The Korea Center for Investigative Journalism (KCIJ)-Newstapa requested a video analyst for a measure. The analyst estimated that the angle between the pillar and the stream at this moment was about 21 degrees. This means, the ferry was already tilted by about 21 degrees to the left at the moment.
Since then, the angle gets larger very rapidly to reach 35 degrees at 8:49:43 a.m., the last moment when the water stream was seen in the video.
How much the ferry tilted since then was found in the black box video derived from the 1-ton truck parked right next to the Deck C’s wall near the portside.
The video shows that water streams into the deck fiercely, after numbers of cars inclining to the left at 8:49:59 a.m. The water forms almost a pool near the left corner.
KCIJ-Newstapa was able to estimate how much the ferry was listed at the time, by calculating the angle between the surface of water and the structures on the left wall. When materialized the moment with three-dimension (3D) analysis method, the angle was estimated to be 47 degrees.
The black box analysis proved that the the Sewol Ferry listed by 21 degrees by 8:49:36 a.m.; by 35 degrees within coming 7 seconds; and to 47 degrees within another 16 seconds.
In addition, the Sewol investigative committee, an independent investigative body consisting of vessel and legal experts, recently confirmed that the ferry was listed by 46.5 degrees at 8:50:36 a.m. after conducting a 3D analysis on the a video clip. The analysis was carried out on a clip recovered from late Danwon High School student Kim Si-yeon’s smartphone, by measuring an angle formed between the cabin wall and curtain hanging on this wall.
In conclusion, the analysis shows that the Sewol Ferry tilted 47 degrees to the left in a short period of time -- within a minute -- and then kept drifting from that angle.
Cargos seen to have started sliding rapidly 10 seconds before the ferry listed by 21 degrees
Then, how much has the ferry listed at the moment when cargos started sliding off?
The black box video from the Starex van, which was parked in the Deck C’s center front, had shown that the ferry was tilted by 21 degrees at 8:49:36 a.m.
However, in the black box clip from Matiz vehicle -- which was parked on the twin deck, a part near the ferry’s stern -- cargos start sliding to the left side to collide with each other 10 seconds earlier than that, at 8:49:26 a.m.
This means, cargos started sliding to move around even before the ferry listed by 21 degrees.
Based on past researches and experiments, the Korea Research Institute of Ships & Ocean Engineering (KRISO) estimated that cargos that can slip off and move around when the ferry listed less than 21 degrees are lighter ones like general container boxes.
This means, it is likely that the very first colliding sound captured at the Matiz vehicle had been created by container boxes that were sitting inside the Deck D.
However, it is still hard to completely trust the KRISO’s research result, particularly the angles it came up with -- the angle of when each type of cargos begins sliding off the spot.
Vehicles normally start slipping off when a ferry is listed by 34.6 degrees, the research supposed. However, cars in the Sewol Ferry were seen to start moving at 21 degrees in the black box videos.
After the black box analysis, Newstapa confirmed only few things to be certain at this moment: The cargos started sliding off to the left side at a much-smaller angle than in KRISO experiments, and this accelerated the ferry to list even faster to reach 47 degrees within a minute.