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The investigation into the South Korea ship disaster focused Friday on a sharp turn the vessel made, as one expert warned that a design weakness common to car ferries likely contributed to the sudden sinking.
The Sewol made the sharp turn in the 10 minutes prior to its first distress call, but it's not known whether the maneuver was planned or caused by some external factor, said Nam Jae-heon, a spokesman for the Republic of Korea’s Maritime Ministry.
The turn could have sent unsecured cargo tumbling, causing the ship to list more than five degrees away from vertical — the critical point beyond which it is difficult to recover.
Water rushing into the vessel may then have sealed its fate, according to Carl Ross, a British marine architect and professor of engineering. He said the wide, open-plan vehicle decks of most car ferries could explain why the Sewol sank before hundreds could escape.
“If water gets into the vehicle deck, it can slosh from one side to the other, making the ship almost impossible to control,” he said. “You have to get off straight away or you’ve got no chance. The water will continue to come in and you’ll just get trapped.”
Ross said the sinking of the Sewol echoed a 1987 disaster in which the roll-on, roll-off car ferry Herald of Free Enterprise sank within minutes after taking on water as it set sail across the English Channel from Belgium to Britain.
“It happened so fast there wasn’t even time to raise the alarm,” said Ross of the 1987 sinking, in which 193 people died. “There was no time for an evacuation.”
Prosecutor Park Jae-eok said investigators were looking at whether the Sewol’s third mate ordered a turn that was so sharp that it caused the ship to list. The 26-year-old third mate, who had one year's experience steering ships and five months on the doomed vessel, was at the helm at the time it began to list sharply.
Yang Jung-jin, a senior prosecutor, said the 69-year-old captain Lee Joon-seok, was not present on the bridge as required when the ship was passing through an area with many islands clustered closely together.
Moon Serng-bae, professor of maritime-information engineering at Korea Maritime and Ocean University in Busan, told the Wall Street Journal that navigating through the area where the ship sank isn't straightforward and often requires vessels to travel in zigzag to avoid fishing boats or buoys.
Lee rushed to the helm and tried to re-balance the ship for up to 30 minutes before giving the order to evacuate, according to accounts from witnesses including one surviving crew member.
Prosecutors and police said Friday they have asked a court to issue arrest warrants for Lee and two other crew members. Officials were investigating witness claims that Lee was among the first to escape as the vessel sank under the surface, and whether the delay in evacuation led to the deaths of many on board.
Only 179 of the 475 passengers have been rescued since Wednesday’s sinking, with 28 confirmed dead. That leaves 268 missing, feared drowned, inside the submerged Sewol.
The ship had left the northwestern port of Incheon on Tuesday on an overnight journey to the holiday island of Jeju in the south.
The 6,835-ton ship had a capacity of 921 passengers, 180 vehicles and 152 regular cargo containers. Officials have not said how many containers or vehicles were on board, but local reports put the total cargo weight at between 600 and 1,200 tons.
A transcript of distress calls between the ferry and marine traffic officials suggests cargo on board may have shifted.
“What’s the current situation?” asked an unidentified official based in Jeju.
“Currently the body of the ship has listed to the left,” a Sewol crew member replied. “The containers have listed as well.”
Ross said tumbling cargo might have punctured the skin of the Sewol, sending water crashing in and making it impossible for the crew to order an evacuation.
“It’s a design failure on most car ferries — even modern ones,” he said. “The vehicle deck is open plan which means there’s nothing that can be done once enough water gets on board.”
A design modification — in which water would drain through a perforated deck into a zone divided by vertical bulkheads — would prevent water rushing across the width of the vessel, he said. However, the cost made the idea unpopular with operators.
He said reports that the captain told passengers to stay in place before eventually issuing evacuation orders likely contributed to the scale of the tragedy.
“These were mostly children who were listening to the instructions of adults,” Ross said. “They were just doing what they were told.”
NBC News' Arata Yamamoto in Jindo, South Korea, and The Associated Press contributed to this report.