MARINE Department officials are baffled by the collision between two cargo ships in open sea off Hongkong early yesterday. The 3,961-tonne Sun Kung No 6 and the 42,414-tonne Mackinac Bridge collided in clear conditions just off Po Toi Island, according to a Marine Department spokesman. A full investigation has been launched into the incident, which forced the crew of one to abandon ship. The Panamanian-registered Sun Kung No 6 is now listing heavily in Junk Bay with a five-metre wide gash in the side of its hull. However, it was no longer in danger of sinking as it had been when its 24 crew sought help shortly after midnight. The Japanese-registered Mackinac Bridge suffered only slight damage to its bow and will leave port today for the United States. ''Until our surveyors finish their work, and we fully analyse the reports from both captains, it's too early to say exactly what went on,'' the spokesman said. The ships were well out of major shipping lanes at the time. The agents for both ships are also holding inquiries to prepare for possible litigation in Japan and Panama - the flag states of the vessels. The master of Sun Kung No 8, Captain Kwok Choi-san, 50, of Hongkong, has yet to submit a complete report, according to a spokesman for his agent, Po Hui Shipping. The ship had just left Hongkong for Indonesia when it had the collision with the Mackinac Bridge, which was arriving from Singapore. Yim Kan-wing, assistant general manager for Kawasaki (Hongkong), agents of the Mackinac Bridge, said he could not explain the accident.