There are mainly three departments on a cruise ship. The third department is the Hotel Department and it is the largest department on board.
Deck/Navigation Department: Captain is the head of this department and the whole ship. Staff Captain is 2nd in command and he is overall in charge of the deck department, including ship' maintenance, general discipline, security, safety and other matters. Ship's navigation is done by three watch-keeping officers doing four hours of navigation each twice a day. Safety Officer is in charge of safety of the ship and for training crew in safety matters. Navigation Officer (one of the watch-keeping officers) is in charge of ship's navigation plan, charts and publications. Other watch-keeping officers also look after environmental issues and assist the Staff Captain and Safety Officer. Ship's communication systems are looked after by Communication Officer who looks after Satellite communication systems, emergency communications (GMDSS) and ship's TV broadcast systems. (On some ships, this duty is shared between deck officers (GMDSS), Electronic Engineer / Broadcast Manager (for electronic equipments and broadcast systems) and Systems Manager. Then there is a medical department comprising of a Doctor and one or two nurses. Ship's maintenance work is carried out by Deck crew with Bosun as the department head. There are also carpenters and plumbers. Chief Security Officer, under Staff Captain, is in charge of ship's security, passenger and crew check-in/check-out and all other security related matters of the ship. He is assisted by three or more security guards.
Engine Department: The Chief Engineer is the department head and Staff Engineer is in charge of the Engine department. There are three watch-keeping engineers, keeping four hours of watch in each shift twice a day. The First Engineer is in charge of Main Engine and ancillaries. The Hotel Engineer looks after the Air-conditioning system, plumbing, galley equipment and fresh water distribution system. He is in charge of AC mechanics and plumbers. On some ships, this job is looked after by AC Engineer and the plumbing jobs are looked after by First Engineer. Chief Electrician is in charge of electrical department. He is in charge of electrical propulsion system (if the ship is fitted with such system), electrical distribution, electronic equipment, lightings, telephone system and all other electrical related equipment. He is in charge of First Electrician and a couple of second electricians. Electronic Engineer looks after all the electronic equipment on board including sound system, stage equipment, navigational equipment and other systems. Fitters and a team of engine crew look after the general maintenance. Technical Inventory Controller /Store keeper is in charge of all engine spare parts and consumables.
Hotel Department: This is the biggest department on the cruise ships. Hotel Director (also called Hotel Manager) is the department head. While deck and engine department looks after navigational and technical aspects of the ship, this department looks after all passenger services.
Food & Beverages Manager is overall in charge of this department, assisting the head of the department (Hotel Direcator). Cruise director is in charge of all entertainment aspects of guests. He has a team of singers, musicians, dancers, jugglers or magicians. Technical aspects of stage shows are looked after by the Stage Manager, assisted by light and sound technicians. The house keeping department headed by Chief House keeper or Chief Steward is in charge of house keeping aspects of passenger cabins and areas. All passenger tours and excursions are looked after by Tour Manager and his team. The Restaurant Department is in charge of all restaurant and room services of the passengers. The Bar Manager and his staff look after all the bar services on board. The Chief Purser is in charge of all financial services of the ship, assisted by First Purser, Second Purser and Assistant Pursers. The reception desk is handled by Front Desk Manager and receptionists. On board Casino is looked after by Casino Manager and his staff. IT Manager (Systems Manager or IT Officer) is in charge of computer network, on board billing systems and all other IT related matters. Executive Chef is in charge of food department. He has a team of Sous Chefs, Chefs, Pastry Chef, Baker and a big galley team. The stores department comprising of Store Manager, Provision Master, Store keeper and many other crew look after the enormous amount of food stores, beverages and general stores of the ship. Ship's laundry is looked after by Laundry Manager and a team of laundrymen. There is also a Liner Keeper and a tailor. On board Spa is run by Spa Manager, beauticians and masseurs. The Shop Manager looks after on board duty free shops. There is a Printer who looks after all printing jobs on board including brochures, invitations and menu cards. There are additional staffs like Fitness Instructor and others depending on the services provided on board. Overall, this is the biggest department on board.
All this information is gathered from the ship's I have been sailing. It may differ in other cruise ships.
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Holland America Line