What sound signal is used for going astern?

The sound signal used for going astern is represented by three short blasts. This specific signal is part of the International Regulations for Preventing Collisions at Sea (COLREGs) and serves as a clear communication method among vessels on the water. When a vessel emits three short blasts, it conveys to others that it is preparing to move backward. This is particularly important in ensuring safety and preventing collisions, as it alerts nearby vessels to the vessel's intended maneuver, allowing them to take appropriate action if necessary.

In contrast, the other options represent different signals: one long blast is typically used to indicate a vessel's intention to overtake or pass other vessels, one short blast indicates a vessel's intention to alter course to starboard, and five short blasts are used to signal danger or that a vessel is unsure of the intentions of another vessel nearby. Each signal serves a distinct purpose, ensuring effective communication on the water.