In this story:

• Ferry sank in less than an hour
• Fishing boats, helicopters used
• Related stories and sites

MANILA, Philippines (CNN) -- A Philippine passenger ferry that sank with 454 people on board was carrying heavy cargo that may have shifted in strong waves, causing the ship to tilt and then sink, the chief investigator said Sunday.

Seventeen more people were rescued Sunday after spending more than a day in chilly, storm-whipped waters, bringing the number of survivors to at least 311, officials said. At least 39 people were dead.

Rough weather continued to hamper rescue operations. A navy ship was forced to turn back Sunday before reaching the accident area because of powerful waves.

Hopes were fading of finding alive more than 100 people still missing.

The ship's owner said 352 passengers and 102 crew were aboard the ship, revising earlier figures to account for people not listed in the manifest, the coast guard said. No non-Filipinos were reported on board.

Maritime officials puzzled over why the large Princess of the Orient began tilting in a tropical storm Friday night and then quickly sank in 360 feet (60 fathoms) of water.

The ship, with a capacity of 3,900 passengers, had been hit by strong waves and winds but should have been able to withstand much worse weather, officials said.

Survivors said the ship sank in less than an hour -- highly unusual for a vessel with no apparent damage.

Chief accident investigator Arnie Santiago said the ship was carrying 15 vehicles and 66 containers of truck parts and other heavy materials that may not have been properly secured.

"The way the ship tilted indicates that the cargo moved," he said.

He said some of the ship's water seals also may have been damaged by an onboard fire in 1997. After the fire the 24-year-old ship was repaired at a Singapore dry dock.

A surviving crew member, Vincente Go, said the ship began listing to the left and did not stabilize even after the crew began pumping ballast water to the right side.

Tropical Storm Vicki was beating the northern Philippines with 45 mph (75 km/h) winds when the ferry left Manila at about 8 p.m. (1200 GMT) Friday for Cebu, about a day's journey to the south.

A storm warning prohibited ships of less than 500 tons from sailing but did not apply to the 13,734-ton ferry.

The ship's owner, Sulpicio Lines, also owned the Dona Paz, a ferry that collided with a tanker in 1987, killing 4,341 in the world's worst peacetime shipping disaster. A year later, 250 people died in the sinking of another ship owned by the company, the Dona Marilyn.

For a second day, a large group of anguished relatives waited at a pier in Manila for the return of rescue ships.

They began crying as a navy patrol craft pulled up with 13 bodies, including two children.

"I hope my sister isn't one of them," said Praxedes Coronas, 54, as she stared at the pile of bodies. "We've been sleepless since yesterday, not being able to confirm whether she's dead or alive."

Survivors floated for hours in life preservers over the rough seas and were covered in black oil from the ship. Some were picked up by a passing fishing boat, while others were plucked from the water by helicopters or navy ships.

Gross overcrowding, lack of safety gear, aging ships and badly trained crews have been the usual causes of shipping disasters in the Philippines in recent years. But these factors seemed to be lacking in Friday's shipwreck, authorities said.

The Associated Press and Reuters contributed to this report.