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A powerboat less than 39.4 feet long is anchored at night white light must be visible from all directions.
I believe it is a white masthead light.
If you are operating a powerboat at night your green side light must be visible to boats approaching over an arc of the horizon 112.5 degrees and so fixed as to show the the light from right ahead to 22.5 degrees abaft of the beam on its respective side.
Visible light causes reflection of light in all directions
A white light on a powerboat (that obviously doesn't have a mast) must be displayed so as to be visible from 360 degrees.
This white-colored light that must be visible from all angles is called an anchor light. It is usually situated atop the mast.
The masthead light should be visible in a 225 degree arc centered on dead ahead.
It is called the Tyndall effect.
Visible light causes reflection of light in all directions
A white light used by itself which can be seen in all directions is the anchor light, which means you are anchored. A white used in conjunction with bow lights (green and red) are your navigation lights. The white light indicates your stern ( back of boat). If all you can see is the white light, a boat is either anchored or underway going away from you.
A white masthead light must be shown when a powerboat is operating at night.
This white-colored light that must be visible from all angles is called an anchor light. It is usually situated atop the mast.