Two watertight compartments at the rear of one of the vessels involved in the Lamma sea disaster probably ruptured and filled with water after it was hit by the other. This caused it to stand on its tail and sink quickly, a marine engineering expert said.
The expert, Louis Szeto Ka-sing, a member of the Hong Kong Institute of Engineers who specialises in vessel design and maintenance, said the sinking would have been accelerated if the watertight doors of the two compartments were open.
Survivors of the disaster, in which 38 people are known to have drowned, have told of their shock at how little time - about two minutes - they had to grab life jackets and escape as the water swirled up around them.
"It was life and death," Szeto said. "I can imagine that it was too rushed for them to do it."
Blueprints of a vessel similar to the sunken motor launch show that the two compartments, probably the engine room and the stern tube compartment, take up almost half the space of the below-deck area.
"[Compartments] filled with water, and the tail getting heavy suddenly will provide buoyancy for the bow. Then came what happened," Szeto said.
He suggested that the boat, being an older vessel, might also not have been fitted with a membrane that keeps newer vessels watertight when their hulls are ruptured.
Little information has been disclosed about the launch, which is believed to be made of fibreglass, a weaker material than the aluminium alloy of the public ferry, built by Cheoy Lee Shipyards in 2003.
Szeto estimated that the two vessels were probably travelling at their full speed of 15 to 20 knots immediately before they collided. It seemed the two skippers had not tried to avoid one another until the last minute, causing the ferry's bow to crash into the other boat's stern.
"The likely scenario was that neither of the vessels, travelling at full speed, gave way to the other, but when they came to realise the danger it was too late," he said.
Captain Tony Yeung Pui-keung, manager of the Maritime Services Training Institute, is of a similar view.
"If it was hit on the side, the boat should not have stood up as it did," he said. "But if it was hit on the back, water could get in rapidly, making the stern heavier."
The launch ended up half submerged with the bow pointing up at an angle close to 90 degrees.
Yeung said many crew had the bad habit of not closing the door of the engine room, near the boat's stern, because it was hot inside. This could have contributed to the speed of the sinking, because water would then flow into the room quickly, he said.
The fine weather and relatively good visibility might also have helped many people survive - while also prompting questions as to how the collision occurred.
Yeung said people might have gone to the boat's open area to see the scenery instead of staying inside and being trapped. "Many may have survived because of that. It's fortunate," he said.
Szeto urged Hongkongers to learn from the tragedy.
"Few people pay attention to where the life jackets are stored," he said. "While children should wear a life jacket on a trip that lasts for an hour or longer, it is also important to escape from the cabin before a ship is pulled down to the seabed by suction."