While operating a power-driven vessel in fog, you see another vessel on radar that is half a mile distant on your port bow. You should _____________.

When operating a power-driven vessel in fog, your primary concern is safety and maintaining proper navigation. If you see another vessel on the radar that is close by, it's crucial to understand the protocols for navigating in reduced visibility.

In this scenario, if other vessels are detected, the appropriate action is often to maintain your course and speed, especially if you have determined that it is safe to do so. Sounding a fog signal or other communication signals may not be necessary if you are in a position to keep a safe distance from the other vessel. In many cases, it's preferable to avoid making unnecessary changes that could confuse other vessels or lead to a collision.

Sounding a danger signal, exchanging passing signals, or sounding one long blast are typically actions that would be taken in specific navigational situations or to indicate your intentions in more certain visibility conditions where visual communication can be accurately interpreted. In the fog, these actions could lead to ambiguity or misinterpretation among vessels, potentially increasing the risk of accidents.

Thus, making no change in your fog signal when navigating in fog, while monitoring radar and maintaining a safe distance, is the correct approach to ensuring safe operations under these circumstances.