While underway and in sight of another vessel a mile ahead, what is true regarding whistle signals if you put your engines on astern propulsion?

When a vessel is underway and in sight of another vessel, the rules regarding whistle signals are in place to help convey intentions and maintain safe navigation. If a vessel is putting its engines on astern propulsion, it is signaling its maneuvering status.

While it is customary to sound one short blast when a vessel is operating astern, the rules specify that under certain circumstances, such as if the vessels are not at risk of collision, no signal is required. The situation suggests maintaining an awareness of the other vessel’s position and actions while also considering the surrounding navigation circumstances.

This approach helps avoid unnecessary noise and ensures focused communication between vessels, particularly if there is already sufficient understanding of the movements being made. Thus, if you are at a safe distance and aware of the other vessel's presence, sounding no whistle signals is appropriate in this context.