What does five or more short blasts on a vessel's whistle indicate?

Five or more short blasts on a vessel's whistle are used to indicate doubt about the intentions or actions of another vessel. This sound signal serves as a warning, suggesting that the vessel making the signal is uncertain whether it can safely maneuver or pass the other vessel. This communication is crucial in maintaining safety and avoiding collisions, especially in situations where visibility may be poor or when other vessels are close together.

While other whistle signals have specific meanings, such as indicating that a vessel is anchored or confirming safe passing, the five or more short blasts squarely convey a lack of confidence in the situation at hand, prompting the need for vigilance and potentially further action to ensure safety.