A traditional Boating & Sailing Whistle
A boatswain's call, pipe or bosun's whistle is a pipe or a non-diaphragm type whistle used on naval ships by a boatswain.
It is pronounced, and sometimes spelled, "bosun's call".
Historically the boatswain's call was used to pass commands to the crew when the voice could not be heard over the sounds of the sea. Because of its high pitch, it could be heard over the activities of the crew and bad weather. It is now used in traditional bugle calls such as Evening Colors/Sunset, and in other ceremonies in most modern navies. It is sometimes accompanied by other auditive features such as ruffle and flourishes, voice commands and announcements, or even a gun salute.