What is the fog signal for a power-driven vessel underway but stopped and making no way through the water?

The fog signal for a power-driven vessel that is underway but stopped and making no way through the water is two prolonged blasts at intervals of two minutes. This signal is a specific requirement outlined in the COLREGs, which are the International Regulations for Preventing Collisions at Sea.

The rationale behind this signal is to effectively communicate the vessel's status to other nearby vessels in low visibility conditions, such as fog. By using two prolonged blasts, the stopped vessel is able to indicate its presence and position to others, thereby aiding in the prevention of collisions. This is crucial, as vessels that are not in motion may not be as easily seen or detected by other vessels, heightening the risk of an incident.

In contrast, other signals provided in the options serve different purposes: one prolonged blast indicates a vessel underway, one short blast is associated with direction on a two-vessel system, and three short blasts typically signify that a vessel is reversing. Understanding the specific contexts and meanings of these signals aids mariners in their navigational safety and compliance.