What is the additional sound signal used by an anchored vessel for warning?

The choice of two prolonged blasts followed by two short blasts is the correct sound signal for an anchored vessel to use as a warning. This signal is specifically prescribed in the International Regulations for Preventing Collisions at Sea (COLREGs) to alert other vessels in the vicinity that the anchored vessel is present and to help prevent potential collisions.

This signaling method serves as a clear and distinguishable warning, especially in conditions where visibility might be impaired. The use of two prolonged blasts followed by two short blasts is a widely recognized standard, allowing mariners to identify the type of warning being issued effectively.

Other signals listed do not pertain to the specific warning function required for an anchored vessel. For example, one prolonged blast is generally used by vessels to signal their presence while underway, not for an anchored position. Three short blasts usually indicate a vessel is going astern, and a continuous sound might suggest a boat in distress or one that is maneuvering in close quarters. Thus, the specific nature of the two prolonged blasts followed by two short blasts makes it the appropriate choice for an anchored vessel’s warning signal.