What is the required sound interval for a power-driven vessel underway but not making way in restricted visibility?

The requirement for a power-driven vessel underway but not making way in restricted visibility is two prolonged blasts with a 2-second interval. This signaling method is established in the International Regulations for Preventing Collisions at Sea (COLREGs), specifically in Rule 35, which addresses vessels in sight of one another in restricted visibility.

By sounding two prolonged blasts at intervals of 2 seconds, the vessel effectively communicates its presence to nearby vessels, indicating that it is not making way and is therefore not under maneuvering power. This signals to others that the vessel is stationary in the water, which is crucial for navigation safety in conditions where visibility is compromised.

This particular signaling allows other vessels to understand the situation of the power-driven vessel, prompting them to take precautions as necessary since it is not currently moving through the water. The choice aligns with the established guidelines for sound signaling in fog or other visibility-reducing conditions, ensuring vessels can maintain safe distances from one another.