Clean sweep (naval)
A "clean sweep" for a naval vessel refers to having "swept the enemy from the seas," a completely successful mission. It is traditionally indicated by hanging a broom from a mast or lashing it to the periscope of a submarine.
It is said the use of brooms in this respect originated during the 1650s, when the Dutch Admiral Maarten Tromp, after a decisive victory in the First Anglo-Dutch War, hung a broom from his mast to indicate he had "swept the British from the seas" - his opponent Admiral Blake is said to have responded with the hoisting of a whip, indicating he would whip the Dutch into submission.
The United States Submarine Service during World War II generally considered a patrol a "clean sweep" if the sub sank every target she engaged. (Individual torpedoes might miss, and convoys usually had far too many ships for all to be sunk by a single boat, but these unavoidable inefficiencies did not mar a "clean sweep.")
Few wide-ranging war patrols have been conducted since World War II, so commanding officers have taken other opportunities to fly brooms. For example, in the year 2000 the Military Sealift Command hung a broom from the flagpole yardarm outside their headquarters to symbolize its "clean sweep" of the Y2K bug on all the command's ships.
In 2003, under circumstances perhaps closer to the traditional context, after USS Cheyenne (SSN-773) launched her Tomahawk missiles during Operation Iraqi Freedom, her commanding officer decided that placing all missiles on target, with no duds or failures, was a modern "clean sweep."
In contrast, the commanding officers of USS Ohio (SSGN-726) in 2005 and USS Virginia (SSN-774) in 2006 felt that completion of Alpha sea trials, indicating those vessels' basic seaworthiness, qualified them to fly the broom.
In Naval Mine Warfare: After sweeping through a minefield, a mine sweeper would hoist the broom on the yardarm to signify their mission accomplished. Finding and destroying any mines found along the route, clearing the way for later ship's safe passage through the field. The crew's on these ships called themselves "The Last of the Iron Men on Wooden Ships."
Ref: USS Adroit - MSO-440
In the Israeli navy the custom of PT boat crews is to fly an upside down broom upon return from a successful mission.