A boatswain formerly and dialectally also bo's'n, bos'n, or bosun, also known as a petty officer, deck boss, or a qualified member of the deck department, is the most senior rate of the deck department and is responsible for the components of a ship's hull. The boatswain supervises the other members of the ship's deck department, and typically is not a watchstander, except on vessels with small crews. Additional duties vary depending upon ship, crew, and circumstances.