What sound signal is used by a vessel restricted in maneuverability?

A vessel restricted in maneuverability uses one long blast followed by two short blasts as its sound signal. This specific signal indicates to other vessels that the restricted vessel may not be able to comply with the standard navigation rules due to its limitations. The long blast serves as a warning of the vessel's presence, while the two short blasts inform others that it is restricted and unable to maneuver as a normal vessel would. This combination is standardized in the rules to ensure that all mariners are aware of the operational status of the vessel, allowing them to take appropriate actions to ensure safety on the water.

Other signals, such as one short blast or three short blasts, are used by different types of vessels and for different situations, making it crucial to recognize this particular combination as the correct sound signal for a vessel restricted in maneuverability.