What direction do unbound vessels check in on channel 13?

Unbound vessels check in on channel 13 in the downbound direction because this channel is primarily designated for communication between vessels and maritime traffic control in these specific contexts. In navigation terminology, "downbound" refers to vessels traveling with the current or flow of the water, typically moving toward the open sea or a larger body of water, which is relevant for coordinating traffic on busy waterways.

This practice ensures that downbound vessels can communicate effectively about their positions, intentions, and any information regarding navigation hazards or traffic updates. By doing so, they help maintain a safe and orderly flow of vessel movement, especially in areas where traffic can be congested. The other options pertain to different directions, where eastbound and westbound would typically be more relevant in coastal or riverine contexts, while upbound refers to vessels going against the current, which wouldn't be using channel 13 for check-ins in the context of this question.