A vessel should alter course primarily when another vessel poses a risk of collision. This is a fundamental principle in maritime navigation and safety. The International Regulations for Preventing Collisions at Sea (COLREGs) emphasize the importance of taking evasive action to avoid collisions. When a vessel detects that another is on a potential collision course, the responsible operation of the vessel requires the watchstander to take preemptive actions to ensure the safety of all vessels involved.
Altering course in response to a risk of collision enhances maritime safety, promotes good seamanship, and fulfills a vessel's legal obligations. It’s crucial for vessels to assess their environment continuously, and to determine the safest course of action when threats to navigation arise.
The other circumstances listed do not directly relate to immediate safety concerns. For instance, altering course due to low fuel is more about fuel management strategies than navigational safety and does not indicate an immediate risk to the vessel. Entering calm waters or navigating only at night does not inherently necessitate a course change regarding collision avoidance; rather, it pertains to navigational comfort or visibility conditions.