Certain safety requirements have been imposed upon the normal naval architectural requirements by law, by official regulations, and by international convention. These cover a wide field involving health, hygiene, fire protection, lifesaving, and communications (radio and radar) as well as seasonal loading, watertight integrity, freeboard, subdivision, and other provisions to ensure that ships will remain upright and afloat. Provision for freeboard, subdivision and other major items in the present category must be made in the original layout and arrangement sketches as part of the preliminary design.
Freeboard
Aside from providing reserve buoyancy when rolling and pitching and keeping the upper decks free from green water and spray, freeboard is required to keep those decks above water when the hull is partly flooded. Adequate freeboard for running in waves must be larger in proportion to its length for a small ship than for a large one, because short waves are steeper than long ones. Freeboard must increase as the ship speed increases because of the greater pitching at higher speeds.
Subdivision and floodable length
Subdivision by watertight bulkheads is necessary to prevent extensive flooding after only local damage. A well-designed ship should, with some damage and moderate flooding, still be able to move, steer, and stay afloat. In recognition of this premise the major maritime nations of the world have approved international treaties and drafted rules specifying the minimum amount of freeboard and the extent of transverse watertight subdivision.
This subdivision is expressed as a function of the floodable length of a ship. A convenient method of relating floodable length to the ship is by a floodable-length curve. The curve is plotted on the profile of the ship so that the vertical ordinate at each point equals the portion of the ship, length, centred at that point, which can be flooded without immersing a margin line. The margin line is parallel to the uppermost deck to which the transverse watertight bulkheads extend and is several inches below it, so that in calm water the ship can sink to this line without water leaking or flooding into other compartments.
When plotting the floodable-length curve, allowance is made for trim but not for heel since the ship is assumed to be open to the sea from side to side. The curve is used by the naval architect to help determine where to place the transverse watertight bulkheads so they may be most effective in restricting the extent of flooding after damage.
The amount of water that can enter any compartment depends upon the ratio of the open-space volume to the total volume, known as its permeability. An empty hold can take nearly its entire volume of water, with a permeability of as much as 98 percent. If filled with coal, its permeability may be 20 percent or less. Current international standards of subdivision are given in the Safety of Life at Sea Convention.
Situation after damage
Any heel after damage makes a ship vulnerable. Excessive heel may be disastrous, as it was for the Andrea Doria in 1956 and the Costa Concordia in 2012. In a well-designed ship, subdivision is planned to ensure that the ship remains upright, or nearly so, no matter where it is opened to the sea or that heel can be corrected by counterflooding on the opposite side. Unless the flooding occurs amidships, the ship trims by the bow or stern. This may render it vulnerable in a heavy sea. Compartments at the ends are usually shorter than those nearer amidships.
Because of the intact water plane lost and the free surface in the flooded areas, the partly flooded ship almost invariably loses some transverse metacentric stability. Such a ship can survive if somehow it can be kept upright: Several ships bombed in the 1940s were saved by lashing them to adjacent piers and other craft to hold them upright until they could be pumped out.
The biggest problem in establishing international standards for ship stability after damage is in reaching agreement on the extent of damage to be assumed. In recent years the problem has been attacked from the viewpoint of probability. First, statistics were collected on the extent of actual collision damages, and then the compartmentation of individual ships was evaluated on the basis of probability of survival in case of damages like those occurring in the past. More rational international standards are evolving as a result of this work, under the auspices of the International Maritime Organization, a United Nations agency.