SEOUL • Part of the ill-fated Sewol ferry that sank in April 2014, killing 304 people, was made out of plastic or fabric instead of steel, according to crew member Oh Young Seok.

The information was revealed in a letter that the steersman wrote to a pastor seven months after the tragedy, detailing how water must have flooded the vessel very quickly, the Korea JoongAng Daily reported.

The ferry capsized within 101 minutes of tilting to its port side.

A part of an extension of the hull above the waterline "was made out of tent material", Oh wrote. "I apologise to the relatives of the dead and I promise to live my life with a penitent heart."

Oh was sentenced to two years in prison in November 2015, but died of lung cancer in April last year, according to the Korea JoongAng Daily.

The letter was addressed to pastor Jang Heon Kwon, who has been attending the trials of the crew members and captain of the Sewol, the Korea JoongAng Daily reported.

"At the time, I didn't think the letter was of value to investigations into the incident," Mr Jang told the Korea JoongAng Daily. "I recently met a reporter at a memorial site for the Sewol victims and handed over the letter then."

In his letter, Oh said the second floor of the ferry - the cargo section - was divided into two floors, and vinyl or cloth-like material, usually used in outdoor tents, was used in part of the external wall on the lower floor.

The blueprint of the ferry had stated that the external wall must be made with steel, the Korea JoongAng Daily said.

The ferry was successfully raised on March 23 and is expected to arrive at a port in Mokpo today, The Korea Herald reported, quoting the Ministry of Oceans and Fisheries.

The Sewol sank off Korea's south-western coast. Most of the victims were teenagers on a school excursion.

The bodies of nine people have yet to be accounted for.