What is the fog signal for a power-driven vessel underway but not making way?

The fog signal for a power-driven vessel that is underway but not making way is indeed two prolonged blasts every two minutes. This signal serves as a means of communication to alert nearby vessels of its presence, indicating that the vessel is in motion but not actively progressing through the water.

The use of two prolonged blasts works effectively in foggy conditions, providing a clear indication to other vessels of the power-driven vessel's status. The signaling protocol is established to enhance safety on the waters, allowing other mariners to gauge the situation and react accordingly.

In contrast to this, other signaling options do not align with the established regulations. For instance, a single prolonged blast indicates a vessel that is either leaving or entering a berth but does not convey that the vessel is underway without making way. The three short blasts occur in different contexts, often associated with maneuvering signals, not fog warnings. Lastly, a short blast does not suffice to indicate that a power-driven vessel is not making way, as this signal is generally used for other purposes.