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.