When During periods of restricted visibility vessels must emit 1 prolonged blast immediately followed by 2 short blasts every two minutes?

During periods of restricted visibility, vessels must emit one prolonged blast followed by two short blasts every two minutes to signal their presence and intentions to other nearby vessels. This sound signal helps ensure safety and awareness in conditions where visual communication is limited. The prolonged blast alerts other vessels of the ship's position, while the two short blasts indicate that the vessel is underway. These signals are part of the International Regulations for Preventing Collisions at Sea (COLREGs).