If you are approaching a narrow channel and you hear a continuous sound signal from a vessel that is not visible, what is the best course of action?

When approaching a narrow channel and hearing a continuous sound signal from an unseen vessel, reducing your speed and proceeding cautiously is the most prudent course of action. This sound signal typically indicates that the other vessel is either constrained by its draft or restricted in its ability to maneuver. By reducing speed, you allow yourself the time to assess the situation further, maintain control of your vessel, and be prepared for any sudden maneuvers that may be necessary to avoid a collision.

Cautiously proceeding will also help to ensure that you do not inadvertently encroach upon the other vessel's navigation space or compromise the safe transit of both vessels through the channel. Maintaining a vigilant lookout and listening for further audio cues will be crucial in this scenario.

The other choices would likely create unsafe conditions. For instance, speeding up could compromise your ability to respond if the unseen vessel is in a vulnerable position or is approaching rapidly. Altering your course to avoid the channel could lead to unintended consequences, such as running aground or entering an area of higher risk. Continuing at the same speed and sounding your horn without knowing the exact situation might confuse other vessels and increase the risk of collision.