What signal must a vessel wishing to overtake another in a narrow channel sound?

When a vessel wishes to overtake another in a narrow channel, the appropriate signal to sound is two prolonged blasts. This is in accordance with the International Regulations for Preventing Collisions at Sea (COLREGs), specifically Rule 9, which deals with narrow channels. The purpose of signaling with two prolonged blasts is to indicate to other vessels that you intend to overtake them safely.

In this situation, the overtaking vessel communicates its intentions clearly to the vessel being overtaken, as well as to any other vessels in the vicinity. The two prolonged blasts alert the other vessel to your maneuver, facilitating safe navigation and minimizing the risk of collision in the confined space of a narrow channel.

The nature of the other signals—such as one prolonged blast or various combinations of short blasts—serves different purposes, typically relating to other maneuvers or communication needs within different contexts, but they do not specifically convey the intention to overtake in a narrow channel like two prolonged blasts do.