To preserve the strength and watertight integrity of a main transverse bulkhead where a watertight door is fitted, the opening is bounded by a reinforced double-plate frame. Thicker compensating plates (or a vertical web frame) surround the aperture, with the door frame pieces doubling the plate thickness at the corners. The door opening edges are connected to strong horizontal and vertical stiffeners (intercostal framing) so that the bulkhead's ability to transmit shear and bending (it acts as a deep girder stiffening the hull) is not lost. The door itself is a steel watertight sliding or hinged door with a frame fitted with a resilient gasket/special joint; the edges are dressed and wired to the surrounding doubling plates, and the sill is raised above the deck to (in passenger ships) about a prescribed height. The framing around the opening is tied into the top and bottom stools/stringers, and where the aperture cuts the stiffening of the bulkhead, compensation is made by increased scantlings immediately adjacent.

• Only authorised personnel may operate them, and the closing mechanism (hydraulic or electric, power plus manual) must be tested before entering port/complex operations.
• Before closing, check the doorway and its rails/wheels are clear of cargo, stores, obstructions and personnel; sound a warning (alarm) before closing.
• The door must be fully open during cargo/engineering operations that require passage, and closed when the door is not in use, especially at sea when subdivision demands it.
• On closing (or opening) by power, monitor the travel; never operate a sliding door manually against a heavy load or with persons in the way.
• When a remote/central control is fitted, the interlock and indicator (open/closed/partly open) must be observed, and the action of the automatic watertight-closing (if fitted) verified.
• After closing, check the door is fully shut and the securing (wedges/dogging) engaged; the indicator should confirm closed position.
• Ensure the gasket is intact, the surrounding plates free of obstruction, and that the door is not used as a normal gangway when closed.
• Record routine tests and keep the operating instruction posted.

Watertight doors are categorised (per SOLAS Chapter II-1) by their type and location:

• Class 1 hinged watertight doors, fitted in bulkheads in machinery spaces and working spaces, normally operated manually (may be required to be closed before the vessel proceeds to sea).
• Class 2 sliding watertight doors, operated by power (remote) with local manual operation and capable of being closed from an accessible location above the bulkhead deck; these are fitted where frequent access is needed (e.g. main lobbies, machinery space trunks, cargo spaces in passenger ships).
• Class 3 hinged door or sliding door of a certain design in the lower portion of the hull in specific locations.

Regulation (SOLAS II-1/13 and Reg.22 etc.): watertight doors in subdivision bulkheads below the bulkhead deck, and their closing arrangements, must comply with the regulations; all watertight doors below the bulkhead deck must be kept closed except when actually used for passage, and sliding doors fitted with power closing, voice alarm and remote indicator are required in certain fire/passenger contexts. The number of watertight doors below the bulkhead deck is minimised, they must have a status indicator at a control station, and hinges/operating gear must be such that they can be closed even with the vessel heeled. Non-wateredef: performance, and that they be opened/closed from both sides and remotely.