When is a risk of collision indicated if the bearing to another vessel is constant?

A constant bearing to another vessel indicates that the two vessels are on a collision course, assuming they are also moving. If the range to the other vessel is decreasing while maintaining a constant bearing, this clearly signifies that the vessels are getting closer together, which indeed represents a risk of collision.

In maritime navigation, the concept of "constant bearing, decreasing range" is critical. It means that you are not changing your angle towards the other vessel but are getting closer, which necessitates action to avoid a collision, such as altering course or speed. In contrast, if the range were increasing, it would suggest that the vessels are moving apart rather than toward each other, thus reducing or eliminating the risk of collision.

When both vessels are moving at equal speeds, it does not provide a definitive indication of collision risk unless the range is also analyzed, and simply stating that no risk exists does not apply in this scenario when the range is decreasing. The key takeaway is that a constant bearing with a decreasing range indicates an imminent collision risk.