What is the sound signal for a vessel leaving a dock?

The sound signal for a vessel leaving a dock is one prolonged blast. This signal serves to alert other vessels and operators in the vicinity of the vessel's intention to depart. The prolonged blast is meant to provide clear notification in situations where visibility might be limited or where other vessels may not be aware of the departing vessel's movements.

In the context of navigational safety and communication, this sound signal is essential for preventing collisions and ensuring that all nearby vessels are properly informed of the departing action. The use of a prolonged blast reflects standard sound signaling practices outlined in the International Regulations for Preventing Collisions at Sea (COLREGs).

Other options, while representing different maneuvers or communications, do not pertain to a vessel's departure from a dock, which is specifically indicated by the prolonged blast.