What sound signal indicates a vessel is constrained in restricted visibility?

A vessel constrained in restricted visibility uses three prolonged blasts at intervals of not more than two minutes as a sound signal. This signal serves as a warning to other vessels in the vicinity that the sounding vessel is unable to maneuver as it normally would, typically due to conditions such as fog, rain, or other circumstances that reduce visibility.

This sound signal is specifically defined in the International Regulations for Preventing Collisions at Sea (COLREGs) to enhance safety by providing other vessels with a clear indication of the constrained vessel's situation. By emitting three prolonged blasts, the vessel effectively communicates its presence and the limitations it faces in navigation due to visibility restrictions.