Downflooding is the flooding of a vessel's hull or compartment resulting from water on deck. In stability, downflood points are evaluated to meet regulatory criteria, typically presented in a downflood angle curve or as downflood heights.
Downflooding points, critical points, or key points are used in stability evaluation to delineate extrema at which the vessel is considered foundered. Depending on the conditions being applied, the downflood points can be required at any openings (such as ventilation openings or exhaust casings), to the edge of coamings for watertight compartments (such as the edges of holds, scuttles or emergency exits), or to watertight fittings which may be opened as sea (such as portholes, deadlights, and watertight doors).
Downflood height is the heighth above baseline of the lowest downflood point for a given condition of trim and heel. This may alternatively be given as the heighth above the waterline to the downflood point.
For the safest operation of a vessel, it is ideal to have no downflooding points, though this is generally not practical outside of submarines. Where possible, downflooding points should be eliminated or minimized in size. Where downflooding points are unavoidable, they should be placed as far from the vessels extremities as possible. Hatches should be on suitably sized coamings, and ventilation/feed air openings should be as high as practical above the waterline. These steps all seek to increase the angle of trim/heel at which they would be immersed.
Care should be taken to ensure that fixtures, such as watertight doors or manhole covers, are adequately tested for leaks, strength, and adequate structural support. The risks to mariners of inadequate closures are highlighted by the loss of the trawler Hope Bay and historical losses of liners to open portholes such as the Empress of Ireland.
Reducing the risk of downflooding is approached by three principal methods - the placement of flooding points, the type of openings allowed for, and operational regimes which favour watertight configurations. Good design practice maximizes the height of openings which could lead to flooding of the vessel to the greatest extent possible, however it's not always practicable where openings are required for access, escape, air flow, or the passage of lines/equipment. Exposed floodpoints, such as ventilation pipes, can be fit with hydrostatic automatic closures - such as a ball check-valve - which automatically close off when submerged.
Hatches and doorways present a particular risk, as these may be purposely opened or left open while at sea. For example, hatches might be opened in hot weather to reduce underdeck temperatures, and doorways or hatches might be opened for access and passage of crew or passengers. Where-ever possible, it is preferential to minimize the number and duration of accesses open. From a basic design stance, unprotected accesses should be minimized (if possible, interior access from higher decks are preferred). So much as possible, hatches should be on adequate coamings. Exposed doorways should be fitted with high sills. Ventilation should be trunked above the weather deck, should be adequate for operational service conditions. Deck coatings, particularly in tropical conditions, should minimize solar loading (or insulated, such as with a layer of wooden sheathing).
If possible visual confirmation of the status of openings should be maximized - either by remote indicators, direct observation from the bridge, or remote viewing aids (such as closed circuit television). Non-essential openings should be indelibly, and in high-contrast, marked as "NOT TO BE OPENED AT SEA". Hold-back provisions should be carefully considered. Downflooding integrity is important to maintain the watertight integrity of the vessels envelop, it is closely related to issues of floodable length, fire division, subdivision, damage control and vessel survivability.
These fittings are expected to withstand hydrostatic heads and accelerations and must therefore be of simple, robust, and reliable construction; as well as have adequate structure integrity.