A vessel leaving a dock must sound a prolonged blast of the whistle only if what condition is met?

The requirement for a vessel to sound a prolonged blast of the whistle upon leaving a dock is primarily associated with certain conditions to ensure safety and communicate intentions to other nearby vessels. One critical condition is when the vessel is a power-driven vessel. This is significant because power-driven vessels typically have more maneuverability and thus need to indicate their operations clearly to avoid accidents.

When a power-driven vessel departs from a dock, sounding a prolonged blast serves to alert other vessels that it is maneuvering away from its position, especially since it may not be able to maneuver as easily in certain situations compared to non-motorized vessels. This action is part of the navigation rules that help maintain safety on the water, particularly in areas where other vessels might be present.

In cases where other conditions apply, such as visibility being restricted or other vessels approaching, those factors are relevant but do not specifically dictate the need for a prolonged whistle blast by a power-driven vessel as it maneuvers away from the dock. The design of navigation rules aims to accommodate various vessel types and their operational capabilities, reinforcing why the nature of the vessel is a key determining factor in this scenario.