Alpha Dive Flag
The international A/alfa/alpha flag
Most American divers and boaters are familiar with the red-and-white diver-down flag. Many have also seen a blue-and-white swallow-tailed one and think of it as the international diver-down flag.
Here's more of the story.
Over the past 200 years international maritime tradition and law has established a set of signal flags, including flags for each letter of the alphabet. The blue-and-white swallow-tailed flag is the signal flag for the letter "A." For clarity, this is pronounced phonetically as "alfa" or "alpha."
When used independently, most alpha-numeric signal flags also have a specific meaning. The alpha flag used to mean "I am undergoing a speed trial." Now the International Code of Signals has established it to mean:
"I have a diver down; keep well clear at slow speed."
The U.S. Coast Guard spells out the federal regulation for its use in Rule 27e - Vessel Engaged in Diving Operations:
Whenever the size of a vessel engaged in diving operations makes it impracticable to exhibit all lights and shapes prescribed in paragraph (d) of this Rule [i.e. most civilian vessels], the following shall be exhibited:
Three all-round lights in a vertical line where they can best be seen. The highest and lowest of these lights shall be red and the middle light shall be white; a rigid replica of the International Code flag "A" not less than 1 meter in height. Measures shall be taken to ensure its all-round visibility.
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