The southward expansion of the Han Dynasty led to new trade routes and diplomatic contact with foreign kingdoms. In 111 BCE, Emperor Wu conquered the Kingdom of Nanyue in modern Vietnam. He later opened up maritime trade to both Southeast Asia and the Indian Ocean. As a result of the trading potential, the Han Chinese began to heavily invest in ship-building. By at least the 1st century CE, the Han Chinese were able to brave distant waters with the usage of the new steering invention of the stern-mounted rudder. This replaced the much less efficient steering oar, which was unwieldy and required exhaustive labor. While Han China was home to various ship designs, including the layered and fortified tower ship meant for calm waters of lakes and river, the junk ship was China's first seaworthy sailing ship. Created by the 1st century CE, the typical junk has a square-ended bow and stern, a flat-bottomed or carvel-shaped hull with no keel or sternpost, and solid transverse bulkheads in the place of structural ribs found in Western ships. Since the Chinese junk lacked a sternpost, the rudder had to be attached to the back of the ship by use of either socket-and-jaw or block and tackle, a system of two or more pulleys used to lift heavy weights. The production of the junks began in factories during the early Western Han Dynasty. The factories had three large platforms capable of building wooden ships that were 30 m (98 ft) long, 8 m (26 ft) wide, and had a weight capacity of 60 metric tons. Other Han shipyards had a government-operated maritime workshop where battle ships were assembled. The widespread use of iron tools during the Han Dynasty was essential for crafting such vessels.