When must a vessel sound a whistle signal?

A vessel must sound a whistle signal primarily in situations of limited visibility or when operating near other vessels to ensure safe navigation and to communicate its presence to others. This requirement is outlined in the International Regulations for Preventing Collisions at Sea (COLREGs), which emphasize the importance of sound signals as a means of avoiding collisions when visual contact is compromised.

In limited visibility, such as fog, rain, or night-time conditions, sound signals alert nearby vessels of one’s position and intentions, helping to prevent accidents. Additionally, operating near other vessels necessitates the use of sound signals as a way to indicate maneuvers, such as altering course or stopping, which is crucial for maintaining safety on the water.

The other choices do not align with the established requirements for sound signals. Mechanical failure does not inherently mandate a whistle signal unless it affects navigational safety, docking typically involves direct visual communication rather than sound signals, and emergency situations might require a whistle signal, but this is not the only circumstance that necessitates its use. Thus, the focus on operating in limited visibility or near other vessels encapsulates the fundamental rationale for sounding a whistle signal.