Three short blasts of a horn indicate which of the following?

Three short blasts of a horn are a universally recognized signal in maritime communication, specifically indicating that a vessel is using astern propulsion and is in the act of backing up. This signal is important for both the vessel that is backing up and for other nearby vessels to maintain safety and avoid collisions. By sounding three short blasts, the operator informs others that they need to be cautious as the vessel is moving in reverse.

Understanding this signal is crucial in navigation, particularly in crowded or narrow waterways where maneuvers can be limited. Those on other vessels can then take the necessary precautions to avoid getting too close or impeding the vessel that is backing up, thereby contributing to safer boating practices.