Which sound signal consists of three short blasts?

The sound signal that consists of three short blasts is used to indicate that a vessel is going astern or reversing. This signal is crucial for communication between vessels, especially in situations where visibility is limited or when maneuvering in tight spaces such as harbors or narrow channels. By sounding three short blasts, the operator conveys to other nearby vessels the intention to reverse direction, ensuring that everyone is aware of the vessel's movements and can take necessary precautions to avoid collisions. Understanding this sound signal is an essential part of navigation safety and helps promote appropriate responses from nearby vessels to maintain order on the water.

In contrast, the other options represent different intentions or actions. For example, "Going ahead" is signaled by one short blast, "Leaving dock" may not have a specific standardized signal in the context provided, and "Alter course to port" is signaled by two short blasts. Each sound signal plays a unique role in maritime communication, highlighting the importance of using the correct signal for the intended maneuver.