A boatswain's pipe or boatswain's call (pronounced "Bosun") is a whistle that is made of a tube, called the barrel, that directs air over a grape-sized metal sphere with a hole cut in the top. The player opens and closes the hand over the hole to change the pitch.
The historical use of the boatswain's call was as a signalling device on a ship. Because of its high pitch, it could be heard over the activities of the crew and bad weather. It is now used in the traditional color , sunset and other ceremonies of several navies, sometimes combined with other auditive features: ruffles and flourishes or even a gun salute.
It is also the Official badge of the Quartermaster in the Sea Cadet Corps
Whistling is forbidden onboard
Whistling is forbidden in most ships if only for the reason that it can often be confused with the sound of the boatswain’s call used for attracting attention before making a pipe. A former reason for the no whistling rule was that it was the custom to whistle a wind when becalmed in a sailing ship; if per¬chance a gale ensued the assumption was that they overdid it. So sailors, being superstitious, rigidly curtailed their whistling habits.
At the time of whistling for a wind it was customary to drive a knife into the mainmast on the bearing the wind was desired. Another strange and very ancient superstition for producing wind was the knotting of a short length of rope, a single knot for a light breeze, two for fresh breezes, and three for strong winds.
