Under what circumstance must a power-driven vessel sound a prolonged blast when departing a location?

A power-driven vessel is required to sound a prolonged blast when departing a location in the presence of other vessels that may be approaching. This action serves as a warning signal to indicate the vessel's intent to leave its berth and provides important information to nearby vessels about the location and movement of the departing vessel.

When other vessels are nearby, sounding a prolonged blast enhances safety by ensuring that other operators are aware of the departing vessel's presence and can take appropriate actions to avoid potential collisions. It emphasizes the responsibility of the vessel operator to communicate effectively in busy waterways. This practice is part of the broader maritime rules that aim to promote safe navigation and prevent accidents.

In contrast, the requirement to sound prolonged blasts in restricted visibility is more about the environment rather than immediate vessel interaction, and signaling while engines are in reverse or when simply leaving a dock may not hold the same urgency in terms of communication with other vessels.