What sound signal must a vessel aground sound in fog?

The correct sound signal for a vessel aground in fog is three strokes of a bell, followed by rapid ringing of the bell, and then three more strokes of the bell. This specific sequence is designed to alert other vessels to the presence of a grounded vessel, ensuring that they can take appropriate action to avoid a collision. The use of a bell as a sound signal indicates the vessel's situation clearly, as it is specifically tailored for situations involving vessels that are unable to maneuver due to grounding.

This protocol is rooted in the regulations set forth in the International Rules and is critical for safety in foggy conditions, where visibility is low. The rapid ringing in the middle serves to emphasize the condition of being grounded. Other sound signals, like a single long blast or short blasts, are designated for different situations, such as signaling intentions or alerting other vessels of trouble, but do not convey the specific situation of being aground, which is why they are not appropriate in this case.