What does the signal of one prolonged blast followed by two short blasts indicate while underway in fog?

The signal of one prolonged blast followed by two short blasts while underway in fog specifically indicates a vessel that is engaged in towing operations. According to the International Regulations for Preventing Collisions at Sea (COLREGs), this particular sound signal is a means for vessels to communicate their presence and activities when visibility is poor, such as during fog.

This signal helps other vessels understand that they should exercise caution when navigating near the vessel that is towing, as the towing operation may affect the maneuverability and navigational aspects of that vessel. The significance of properly understanding and responding to these sound signals is crucial for safety on the water, particularly in low visibility conditions where the risk of collision is heightened.

In contrast, signals designated for different types of vessels—like those at anchor, fishing, or engaged in pilotage—have distinct sound patterns that are used to convey their specific situations. Each of these signals allows vessels to communicate their intentions and locations clearly to one another, thereby enhancing safety and navigation effectiveness on the water.