What does one prolonged blast followed by three short blasts indicate?

One prolonged blast followed by three short blasts is a sound signal used to indicate that a vessel is being towed and that the tow is manned. This signal is specifically outlined in the Navigation Rules, particularly for situations involving vessels in conditions where visibility may be limited or other vessels may not be clearly seen.

This sound signal serves to alert nearby vessels of the presence of a tow, which is important for safety and navigational awareness. The prolonged blast serves as a warning, while the three short blasts indicate the specifics of the towing arrangement.

Understanding these sound signals is crucial for all mariners, as they facilitate safe navigation and prevent collisions, especially in busy or restricted waters.