Query: In what areas were Han ships able to be sailed in part due to the stern rudder?
Context: Junks featured a square-ended bow and stern, a flat-bottomed hull or carvel-shaped hull with no keel or sternpost, and solid transverse bulkheads in the place of structural ribs found in Western vessels. Moreover, Han ships were the first in the world to be steered using a rudder at the stern, in contrast to the simpler steering oar used for riverine transport, allowing them to sail on the high seas.