Some of the ship's students react upon their arrival to a naval base in Rio de Janeiro, three days after the SV Concordia capsized. (Felipe Dana/Associated Press)

Toronto student Christine Palonek said she didn't sleep the night she found out the SV Concordia tall ship sank off the coast of Brazil — her younger sister Samantha had been on board.

Samantha, 16, and dozens of other Canadian students, were in a special program that allowed them to take high school courses while sailing to Europe, Africa and South America.

The ship got caught in a thunderstorm last Wednesday. A rare microburst — a sudden, vertical downdraft — slammed into the Concordia and caused it to capsize. As soon as she heard the news, Christine, 19, immediately thought the worst.

"I thought my sister had died, because when you say the ship went down, I thought it went down," she said.

Samantha and the 63 other people on the ship, managed to get on the life-rafts. But it took a few days for Christine to get in touch with her sister and hear first-hand exactly how harrowing the experience was.

"She was crying," Christine said when her sister finally called her. "She said that she didn't think that she was going to make it out alive."

Christine said she broke into tears herself as Samantha described the moment the Concordia began to capsize. She had been in a classroom with several other students when the ship suddenly flipped onto its starboard side.

"[The students] all flew to the side," Christine said. "She said within 15 seconds she had water up to her neck already."

The 57.5-metre steel vessel sank within 20 minutes. The inflatable life-rafts were pinned under the water, so the passengers had to use a kitchen knife to cut the straps while the ship sank.

Everyone on the Concordia, including eight teachers and eight crewmembers, got out in time, but the danger wasn't over. Conditions on the water were rough and the life-rafts were crowded.

Samantha told her sister how people were dehydrated and vomiting. Some of the life-rafts were punctured, and the survivors spent 40 hours bailing water out.

The Concordia sank about 550 kilometres southeast of Rio de Janeiro, the Brazilian navy said. The search started Thursday after a distress signal from the ship was picked up at about 4 a.m., according to officials.

The survivors were located by a Brazilian military helicopter and picked up by merchant ships and Brazilian naval vessels.

Christine said she's concerned her sister has been traumatized by the event. She also wonders why it took nearly two days for rescuers to find them.

"I personally never thought this could even happen with technology nowadays," she said. "I thought within an hour of that boat going down that someone would have known. Like, that just scares me — she was out there for so long, and if the navy hadn't picked up this distress signal she wouldn't be alive today."

Samantha and the other survivors are due to arrive at Toronto's Pearson airport from San Paulo, Brazil early Monday morning.