A pilot vessel when on duty may sound an identity signal immediately after the required sound signals. What does this identity signal consist of?

The identity signal for a pilot vessel on duty consists of one prolonged blast followed by two short blasts. This particular sound signal is specifically defined in the International Regulations for Preventing Collisions at Sea (COLREGs), which establish a standardized method for vessels to communicate their intent and identify themselves in busy waterways.

The one prolonged blast serves as a clear indicator that the vessel is signaling its presence and indicating its role as a pilot vessel. The subsequent two short blasts further clarify that the vessel is indeed performing its duties of conducting a pilotage service. This method of signaling is essential for the safety of navigation, as it informs other vessels in the vicinity of the pilot vessel's intentions and role, thus helping to prevent misunderstandings and potential collisions.

Understanding the specific identity signal for a pilot vessel is crucial for maintaining safety and order in maritime traffic, especially in crowded or navigationally challenging waters. The other choices do not represent the standardized identity signal for pilot vessels as defined by the COLREGs, which is why they do not provide the correct indication of a pilot vessel’s identity when on duty.