You are underway in fog and hear a prolonged blast followed by three short blasts. What does this signal indicate?

The signal consisting of a prolonged blast followed by three short blasts specifically denotes a vessel engaged in towage operations. According to the International Regulations for Preventing Collisions at Sea (COLREGs), this sound signal is used by a towing vessel to communicate its presence and intentions to other vessels in limited visibility conditions, like fog.

When a towing vessel sounds this combination of blasts, it indicates that it is engaged in towing another vessel. The prolonged blast serves as a warning of the presence of a vessel that may not be easily seen, while the three short blasts provide further clarification that the vessel is performing towing activities. Understanding this signal helps to ensure safe navigation and awareness among all vessels in the vicinity.

In contrast, the other options represent different types of vessels that each have their unique sound signals, which do not match the prolonged blast followed by three short blasts. For instance, a sailing vessel would signal with different patterns, and so would a power-driven vessel and a fishing vessel. Recognition of these specific sound signals is vital for compliance with maritime navigation rules and for avoiding collisions in foggy conditions.