What should a vessel do if visibility suddenly diminishes while underway?

When visibility suddenly diminishes while a vessel is underway, reducing speed and sounding appropriate fog signals is the best course of action. This action is in line with the principles of safe navigation and collision avoidance. Slowing down allows the operator to have better control of the vessel, reaction time, and the ability to assess the situation more effectively.

Sounding appropriate fog signals is also critical, as it helps communicate the vessel's presence to nearby vessels that may also be navigating in reduced visibility conditions. This is essential for ensuring that other vessels can detect your presence despite the lack of visual cues.

Increasing speed, using only radar, or maintaining the current course without changes may compromise safety. Increasing speed in poor visibility can lead to collisions, as the operator might not have sufficient time to respond to sudden obstacles or other vessels. Relying solely on radar can create a false sense of security, as it does not eliminate the risks associated with diminished visibility. Lastly, staying the course and making no adjustments could also lead to dangerous situations, especially if other vessels are also operating nearby.