The weight of two adults was enough to dislodge loosely attached seats in the sunken Lamma IV, the commission of inquiry into the National Day ferry tragedy heard on Friday.

The commission was hearing evidence for the second day from Dr Cheng Yuk-ki, a forensic scientist at the government laboratory, who wrote a report after inspecting both vessels that collided on October 1 last year off Lamma Island.

Rescuers and survivors on the upper deck had earlier recalled that passengers standing on the seats waiting to be rescued in the vertically sunken ship had fallen after the seats were dislodged. Only one seat was left secured on the upper deck.

Cheng conducted experiments by pulling the frame of only remaining seat and determined that a force of about 230kg was enough to detach the seats.

If the force was evenly distributed, for example, a person sitting or hanging on the seats, the force needed to detach it will be halved to less than 115kg.

“Had two adults of normal build been grabbing the back of the seats hanging from them up or sitting on them when the Lamma IV sunk vertically, this could have provided sufficient force to detach the seats from the mount,” he said in his report. 

He also said it was more appropriate to use steel screws instead of aluminium to secure the seats because steel was resistant to corrosion.

Cheng also inspected the lifejackets under the seats of Lamma IV, the Hongkong Electric launch that sank after colliding with the public ferry Sea Smooth.

He found the jackets were put in tied garbage bags inside stowage areas attached to the seat frame by pieces of Velcro.

Passengers needed to unfasten the Velcro and untie the bag to retrieve a lifejacket.

Cheng said he found it easy to retrieve the life jackets during his inspection, but survivors had earlier complained of the difficulties in doing this after the accident.