Correct name for the pipe is a Bosun's Call, the act of piping someone aboard is called 'piping the side'. The most common pipes are The Still, Piping the Side, the Carry On, the Hail and the General Call. The Still is commonly used prior to Colours, the act of raising the White Ensign at sunrise and lowering at sunset. The Carry On is sounded after Colours has occured. There are many others, some no longer used, the call was originally used in the days of sail to communicate with topmen, wheras shouts could be carried away by wind, the high pitched shrill of the bosun's call carries much better. I have included a link which contains various examples of RN pipes...
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