When do the Rules state that risk of collision shall be deemed to exist?

The correct answer is that risk of collision shall be deemed to exist if the bearing of an approaching vessel does not appreciably change. This principle is rooted in the concept that when the relative positions of two vessels remain constant over time, it indicates that they are likely on a collision course. If the bearing is stable, it suggests that one vessel is approaching the other, leading to the conclusion that a risk of collision exists.

In maritime navigation, being aware of the changes in bearing is crucial for ensuring safe passage. Mariners are trained to keep track of how the angles of incoming vessels shift; if they note little to no change, it becomes a strong indicator that the vessels are closing in on one another.

The other options do not accurately define the concept of risk of collision. While the proximity of another vessel (within one nautical mile) can indicate a potential risk, it does not guarantee a collision course without the context of the bearing. Similarly, the failure to sound a horn does not inherently relate to collision risk, as sound signals are part of communication standards but do not determine the geometric relationship of the vessels. Lastly, while both vessels being on a collision course may indicate risk, the specific and clear guideline in the rules emphasizes the importance of the bearing as