Which of the following is true about the sound signal of a vessel?

The correct choice highlights that the sound signal of a vessel consists of both short and prolonged blasts. This is a fundamental aspect of maritime communication that is crucial for ensuring safety on the water. Sound signals are used by vessels to communicate their intentions or to signal their presence to others, particularly in situations where visibility is limited, such as in fog or heavy rain.

Short blasts are typically used to signal a maneuver, such as when a vessel is about to alter course to starboard, while prolonged blasts can indicate more serious situations or assert the presence of the vessel. Understanding how to use these signals properly is essential for complying with maritime navigation rules and for enhancing safety among all waterway users.

The other options do not accurately reflect the requirements or practices regarding sound signals. For instance, saying it must be continuous misrepresents the nature of sound signals, as they are meant to be distinct and clear rather than continuous. Stating that it is necessary only in emergency situations overlooks the regular use of sound signals for everyday navigation and communication among vessels. Lastly, claiming it can be replaced with visual signals ignores the fact that sound signals serve a distinct purpose, especially in conditions where visual signals may not be effective.