When navigating, what indicates that a vessel is backing up?

The correct answer is three short blasts. In boating navigation, sound signals are used to communicate the movements and intentions of vessels to avoid collisions and ensure safety on the water. Specifically, when a vessel is backing up, it is required to sound three short blasts of the horn. This signal informs other vessels in the vicinity that the boat is moving in reverse, allowing them to take appropriate action to maintain a safe distance and prevent accidents.

Understanding this signaling system is crucial for all boat operators, as it promotes clear communication in maritime traffic. The other sound signals have specific meanings; for example, one short blast indicates that a vessel is departing from the dock, two short blasts indicate that a vessel is moving forward, and five rapid blasts are used as a danger signal when there is an immediate risk or concern.