While underway in fog, what should you do upon hearing two short blasts on the whistle from a vessel ahead?

When you hear two short blasts on the whistle from a vessel ahead, it indicates that the other vessel is signaling that it is operating in a restricted ability to maneuver, potentially because it is not able to comply with standard navigation signals or it may be constrained by its draft. The most appropriate action in this situation is to sound your own fog signals and continue to display your signals until you have the vessel in sight. Hearing two short blasts does not provide sufficient information to make assumptions about the intentions of the other vessel, so maintaining your own fog signal while remaining vigilant is critical for safety.

By signaling your presence, you enable the other vessel to better ascertain your position, thus minimizing the risks of collisions. Changing course without understanding the situation, increasing speed, or failing to indicate your presence could lead to dangerous misunderstandings in limited visibility conditions, which is why the choice to only sound fog signals until the other vessel is sighted is the best practice.