What sound signal indicates a vessel is constrained in restricted visibility?

The sound signal that indicates a vessel is constrained in restricted visibility is indeed three prolonged blasts at intervals of not more than two minutes. This specific signaling is in accordance with the COLREGs (International Regulations for Preventing Collisions at Sea), which provides guidelines for vessels navigating in conditions where visibility is reduced, such as fog or heavy rain.

When a vessel is constrained by its draft or other limitations and cannot maneuver as it would in clear visibility, it must communicate its status to other vessels nearby. The three prolonged blasts serve as a clear auditory signal that warns other vessels about the presence of the constrained vessel, allowing them to take appropriate precautions to avoid collision.

This signaling system is key to maritime safety, as it helps all vessels understand each other's constraints in challenging conditions, enabling safer navigation and reducing the risk of accidents.