Which signal indicates that a vessel is not able to proceed safely?

The indication that a vessel is not able to proceed safely is signified by a distress signal flag being displayed. This flag, commonly referred to as the code flag "N" or the International Code flag "N," signals to other vessels that immediate assistance is required. It is a universally recognized signal that alerts nearby vessels to the emergency situation at hand.

When a vessel displays a distress signal flag, it communicates the urgency of the matter clearly and effectively, prompting other vessels to provide the necessary help or to take appropriate action to ensure safety on the water. This type of signal is part of the established protocols under the COLREGs, ensuring that mariners are aware of dangers and issues faced at sea.

In contrast, safe passage indicators, casual flag waves, or light signals from the mast are not associated with a vessel's inability to navigate safely. These types of signals do not convey an emergency situation and are therefore not appropriate responses to a vessel in distress. Understanding these signals and their meanings is crucial for mariners to ensure safety and effective communication while at sea.