Which signal indicates a vessel is not able to maneuver?

A continuous blast of the horn is the designated signal indicating that a vessel is unable to maneuver. This sound signal is part of the International Regulations for Preventing Collisions at Sea (COLREGs), which dictate the appropriate sounds to use in various navigational scenarios. The continuous blast of the horn serves as a clear alert to nearby vessels that the signaling vessel has lost its ability to maneuver, signaling others to exercise caution and maintain a safe distance.

Other options do not fulfill the same function. A flag displayed at half-mast is typically associated with mourning or signaling respect rather than navigational status. Specific light blasts can convey various messages depending on the nature of the situation but do not specifically indicate an inability to maneuver. A combination of stationary and moving lights might be used for other signaling purposes but does not communicate that the vessel is not able to maneuver. Thus, the continuous blast of the horn is the proper signal to indicate this critical condition on the water.