Under which condition must a vessel take avoiding action as per Rule 19d?

A vessel must take avoiding action under Rule 19d when it detects another vessel by radar alone and a close-quarters situation is developing. This rule is part of the COLREGs, specifically addressing situations where the use of radar is necessary to assess the proximity of other vessels, especially during conditions of reduced visibility.

In maritime navigation, relying solely on radar to detect other vessels requires the operator to be vigilant and ready to take action if a situation develops where collisions might occur. A close-quarters situation implies that the vessels are on a collision course or are dangerously close to one another, necessitating proactive measures to avoid a collision. The importance of this rule lies in the fact that visual detection may not always be possible, particularly in poor visibility conditions, making radar a crucial tool for safe navigation.

Taking action solely based on visual detection, overtaking situations, or waiting for signals from other vessels does not encompass the stipulations outlined in Rule 19d, as those scenarios do not adequately address the specific risk that arises from radar-detected close encounters in limited visibility. Therefore, the appropriate response is to use radar information actively to determine the need for action when faced with potential collisions.