TWO of the ships involved in the suspected smuggling of hundreds of mainland Chinese to the United States are believed to have rendezvoused in the South China Sea last month after leaving Hongkong to transfer their human cargo. It is believed that a third ship, impounded by the Marine Department last week, was bound on a similar trip until its crew complained about non-payment and the vessel suffered major mechanical problems. The East Wood and Eastern World 1, also known as the Fook Kee, are believed to have rendezvoused on January 18 and 131 people transferred to the former vessel. The black-hulled 4,399-tonne East Wood, which left Hongkong on December 27, was yesterday allowed to dock at a US military base in the Marshall Islands, after disease broke out among the 537 people on board. It is believed to have rendezvoused with the smaller, 489-tonne ship, off an island about 150 nautical miles northwest of Taiwan. This is close to Fujian province where most illegal mainland immigrants come from. A radio transmission between the East Wood and the Fook Kee, monitored on January 18, said: ''The cargo is all aboard.'' This is believed to refer to the movement of people from the Fook Kee, which was acting as a shuttle ship, taking the group from a departure point in China, to a prearranged site with the East Wood. The Fook Kee departed Hongkong on December 8 after being told by the Marine Department to dismantle 400 beds which had been fitted in its hold. Police believe a third vessel, the Sea Raider, was to take 300 people to the same rendezvous point last year but it broke down and the charterer, Mr Alvin Ng, ran into financial problems. International investigators are now focusing their attention on Hongkong, which has emerged as the central staging post for the scheme. Detectives believe small cargo ships, weighing under 1,000 tonnes, are chartered in Hongkong by new or fictitious firms. Others are bought through the ''bare boat'' system, which allows the buyer to use the vessel while paying off the purchase price. The cargo vessels are used to shuttle small groups of mainlanders to larger, mother ships, such as the East Wood, and then taken to the US, central or southern America. While in Hongkong, some of the vessels are fitted out with beds, provisions and new crews - none of which is illegal in Hongkong. They remain in port until informed of the shipment details. A ''snakehead'', most likely to be mainland Chinese, organises groups of people willing to risk the hazardous journey, after touring villages in Fujian. He arranges for the would-be aliens to gather at a pick-up point and confirms the date with the ship's captain, either in person or via a middleman. The snakehead also collects the payment of up to US$30,000 per person for the trip.