Which signal is sounded ONLY when vessels are NOT in sight of each other?

The signal that is sounded only when vessels are not in sight of each other is four short blasts on the whistle. This signaling is used to indicate that a vessel is operating in restricted visibility conditions, such as fog or heavy rain, where the vessels cannot see one another.

In situations where visibility is limited, vessels must use sound signals to communicate important navigational intentions. The four short blasts specifically indicate that the signaling vessel is altering its course to starboard. This form of communication is vital in avoiding collisions by making intentions clear to other vessels that may also be navigating in low visibility.

Other signals, such as one short blast, two prolonged blasts, and three short blasts, have different meanings and are typically used under conditions where the vessels may be within sight of one another or may refer to other specific navigational intents. Understanding the correct use of sound signals is crucial for ensuring safety on the water, especially when visual communication is not possible.