Sideboys are a part of the Quarterdeck ceremonies when an important person or Officer arrives on board or departs a ship. Large ships have sideboys detailed to the quarterdeck from 0800 to sunset. When the side is piped by the Boatswain's Mate of the Watch (BMOW), from two to eight sideboys, depending on the rank of the Officer, will form a passageway at the gangway. They salute on the first note of the pipe and drop their salute together on the last note.
In the days of sail, it was not uncommon for the Commanding Officers of ships sailing in convoy to convene aboard the flagship for conferences. It was also not uncommon for COs to invite each other to dine aboard their vessels. Unfortunately, there was no easy way to bring visitors on and off a ship while underway. Also, there was no dignified may for a high ranking officer to scurry up or down a rope ladder hanging down the side of a ship.
Often the boatswain's chair, a rope and wood sling, would be used to hoist the guest onto and off of the ship. The Boatswain's Mate would control the heaving by blowing the appropriate commands with a whistle known as a Boatswain's Pipe. The number of "strong backs" needed to bring the visitor aboard depended upon the size of the "load" being hoisted. Somewhere along the line, it was noted that the more senior the visitor's rank, the more sailors were needed to "man the side." Over time, the need to hoist visitors onto and off of Navy ships went away, but the custom of mustering the sideboys and piping distinguished visitors aboard ship remains.