- Difficult conditions are hampering search efforts
- Vessel was refitted upon arrival in Korea in February last year
- Rescue workers have been attempting to pump air into the ship
- Three giant "sea cranes" dispatched to the site
The South Korean ferry that sank dramatically on Wednesday with hundreds of passengers on board had been modified after coming to the country from Japan over a year ago, according to reports on CNN affiliate YTN.
The Sewol -- operated by Chonghaejin Marine Corporation -- regularly plied the Incheon to Jeju route, a journey that usually takes 13-and-a-half hours, according to information from the company's website.
The Sewol is listed at 146 meters long and with a gross tonnage of almost 7,000 tons. It boasts five cargo and accommodation decks.
It is listed as having a capacity of 921 passengers, but was only carrying a reported 429 when it unexpectedly started taking on water early on Wednesday. Within hours, all that was above the water surface was the bow of the boat.
The ferry was built in a Japanese shipyard 20 years ago and previously operated in that country's waters. It was refitted upon arrival in South Korea in February 2013, YTN reported. The ship's sleeping cabins were reconstructed after its arrival in South Korea, the network said.
Korea Diving Industry Institute president Jung Yong-hyun, a former general in the Korean Navy and former commander of the Ship Salvage Unit (SSU) told YTN that the refit may have affected the weight of the boat and could be a reason for the accident.
According to Jung, the reconstruction added around 239 tons of weight to the boat from its original form.
Investigators probing the incident will likely look at various aspects of the boat, including this reconstruction.
"As the center of the weight of the boat becomes higher, it becomes more difficult for the boat to balance," the expert told YTN.
There have been serious ferry accidents in South Korea before. The most recent maritime accident occurred in 1993. This was attributed to overcrowding.
The Sewol, however, was nowhere near capacity on the night of the 15th, when it left Incheon several hours late -- delayed because of heavy fog.
With difficult circumstances hampering rescue dives -- including tidal currents, choppy, frigid conditions, wind, rain and seabound fog -- more radical engineering solutions are being explored.
At around noon on Thursday local time, workers began trying to pump air into the boat, hoping air pockets might be expanded within the structure, helping to lift the ferry.
These efforts have proved unsuccessful so far. Authorities say that they will try again when the tide turns.
Analyst Jung Yong-hyun told YTN this method has been used previously, on the Costa Concordia and Cheonhanham warship wrecks, as example.
Three 3,600-ton "sea cranes" are due to arrive Friday to assist with efforts.
"The cranes are being moved as fast as possible, the focus is getting them there early," said Choi Myeong-beom, a Ministries of Oceans and Fisheries representative.
"What will be done with them will be determined by the Coast Guard on site.
"If the boat has not been stuck into the seabed then we will raise (it to) the surface with cranes and move them to a place where it will be easier to work on them."