Three large salvage ships are expected to arrive at the scene of the sunken South Korea ferry " Sewol" early Friday morning to help raise the ferry, South Korea's West Sea Coast Guard said on Thursday.

South Korean coast guard and navy have dispatched a total of 29 aircraft and 171 vessels for rescue, the West Coast Guard said at a press briefing, adding that they will spare no efforts to conduct the rescue operation.

Military and civilian divers tried to enter the ferry for dozens of times on Thursday but failed finally. The rising winds, waves and rains led to huge obstacles for the rescue operation.

The passenger ship carrying 475 people, mostly high school students, capsized in waters off South Korea's southwest coast Wednesday. Local broadcaster YTN's latest reports said that at least 18 people were dead and 278 others still missing.

It was reported that the ferry was believed to have veered off course as it departed some two and a half hours later than scheduled due to a heavy fog. The coast guard said they are still analyzing the track of the ship's route supplied by marine company to determine whether "Sewol" is far off its intended route.

The 6,825-ton passenger ship, "Sewol," made an abrupt turn at around 8:48 a.m. local time Wednesday, according to the automatic identification system (AIS) data offered by the South Korean Oceans and Fisheries Ministry.

Why the captain suddenly made the turn had yet to be known, but the change in direction was believed to have moved some 180 cars and trucks and more than 1,100 tons of shipping containers on the deck of the ship to one side, driving the ship to lean to the port side gradually.

The coast guard said experts from South Korea's National Scientific Research Institute, National Marine Research Institute, Marine Science and Technology Institute and other agencies are investigating whether the ferry was overloaded or has some structural or systematic defects.

The ship's captain and crew members are also questioned by the police to find out what went wrong.

The West Sea Coast Guard also said the possibilities of the ferry hit an underwater rock remains low as the underwater image shows that there are few submerged rocks in the waters.

The final results of the investigation will be published after analyzing concrete evidences, official said.