SAFETY LIES IN SUBDIVISION
Other things being equal, the protection of a ship against sinking is exactly proportionate to the number of separate watertight compartments into which the interior of her hull is subdivided. If she contains no watertight partitions whatsoever, her sinking, due to damage below the water-line, is a mere matter of time. If the inflow exceeds the capacity of the pumps, water will flow into the ship until all buoyancy is lost. Protection against sinking is obtained by dividing the interior of the hull into a number of compartments by means of strong, watertight partitions, or bulkheads. Usually, these are placed transversely to the ship, extending from side to side and from the bottom to a height of one or two decks above the waterline. They are built of steel plates, stiffened by vertical I-beams, angle-bars, or other suitable members. The bulkheads are strongly riveted to the bottom, sides, and decks of the ship, and the joints are carefully caulked, so as to secure a perfectly tight connection. In the standard construction for merchant ships, as used in the Titanic, the bulkheads are placed transversely to the length of the ship, and the number of separate compartments is just one more than the number of bulkheads, ten such bulkheads giving eleven compartments, fifteen, as in the Titanic, giving sixteen compartments, and so on. In the case of a few high-class merchant steamers, built to meet special requirements as to safety, bulkheads are run lengthwise through the ship. These longitudinal bulkheads, intersecting the transverse bulkheads, greatly increase the factor of safety due to subdivision; for it is evident that one such, running the full length of the ship, would double, two would treble, and three would quadruple the number of separate compartments.
The bulkhead subdivision above described is all done in vertical planes. Its object is to restrict the water to such compartments as (through collision or grounding) may have been opened to the sea. As the water enters, the ship, because of the loss of buoyancy, will sink until the buoyancy of the undamaged compartments restores equilibrium and the ship
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Hydraulically-operated, Watertight Door in an Engine-room Bulkhead
assumes a new position, with the water in the damaged compartments at the same level as the sea outside. This position is shown in Fig. 2, page 57. It must be carefully noted, however, that this condition can exist only if the bulkheads are carried high enough to prevent the water in the damaged compartments from rising above them and flowing over the tops of the bulkheads into adjoining compartments.
In addition to lateral and longitudinal subdivision by means of vertical bulkheads, the hull may be further subdivided by means of horizontal partitions in the form of watertight decks—a system which is universally adopted in the navies of the world. For it is evident that if the ship shown in Fig. 2, page 57, were provided with a watertight deck, say at the level of the water-line, as shown in Fig. 1, page 57, the water could rise only to the height of that deck, where it would be arrested. The amount of water entering the vessel would be, say, only one-half to two-thirds of that received in the case of the vessel shown in Fig. 2.
If ships that are damaged below the water-line always settled in the water on an even keel, that is to say without any change of trim, the loss through collisions would be greatly reduced. But for obvious reasons, the damage usually occurs in the forward part of the ship, and the flooding of compartments leads to a change of trim, setting the ship down by the head, as shown in Figs. 3 and 4. If the transverse bulkheads are of limited height, and extend only to about 10 feet above the normal water-line, the settling of the bow may soon bring the bulkhead deck (the deck against which the bulkheads terminate) below the water. If, as is too often the case, this deck is not watertight—that is to say, if it is pierced by hatch openings, stair or ladder-ways, ventilator shafts, etc., which are not provided with watertight casings or hatch covers, the water will flow aft along the deck, and find its way through these openings into successive compartments, gradually destroying the reserve buoyancy of the ship until she goes down. The vessels shown in Figs. 3 and 4 are similar as to their subdivision, each containing thirteen compartments; but in Fig. 3 the bulkheads are shown carried only to the upper deck, say 10 feet above the water, whereas in Fig. 4 they extend to the saloon deck, one deck higher, or,
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say, 19 feet above the same point. Now, if both ships received the same injury, involving, say, the three forward compartments, a loss of buoyancy which would bring the tops of bulkheads in Fig. 3 below the surface, would leave the bulkheads in Fig. 4, which end at a watertight deck, with a safe margin, and any further settling of the ship would be arrested.
Ordinarily, it would suffice to carry the first two bulkheads at the bow and the last two at the stern to the shelter deck, terminating the intermediate bulkheads one deck lower. But whatever the deck to which the bulkheads are carried, care should be taken to make it absolutely watertight. Otherwise, as already made clear, the so-called watertight subdivision of the ship may, in time of stress, prove to be a delusion and a snare.
Although the longitudinal bulkhead, which is employed below the water-line, and chiefly in the holds and machinery spaces, is the least used, it is one of the most effective means of subdivision that can be employed. A certain amount of prejudice exists against it, on the ground that it confines the inflowing water to one side of the ship, causing it to list, if not ultimately to capsize. But this objection merely points the moral that all things must be used with discretion. A single longitudinal bulkhead, built through the exact centre of a ship, would invite a speedy capsize in the event of extensive injury below the water-line. The loss of the British battleship Victoria emphasised that truth many years ago. But longitudinal bulkheads, carried through the engine and boiler spaces, at the sides of the ship, are a most effective protection. Not only is each of the large compartments in the wider central body of the ship divided into three, but along each side is provided a row of comparatively small compartments, several of which could be flooded without causing a serious loss of buoyancy.
These bulkheads, built some 15 to 18 feet in from the side of the ship, not only form an inner skin for the ship, but they serve as the inner wall of the coal bunkers. They extend from the inner bottom to the under side of the lower deck, to both of which they are securely riveted, the joints being carefully caulked, to render them watertight. The space between the ship's side and the bulkhead is subdivided by transverse watertight partitions (see plan of Mauretania, Fig. 3, page 129), placed centrally between the main transverse bulkheads of the ship. A further and most effective means for protecting the buoyancy is to construct the ship with a double skin up to and preferably a few feet above the water-line. The inner skin should extend from the first bulkhead abaft the engine-room to the first or collision bulkhead, forward. This construction merely involves carrying the inner floor plating of the double bottom up the sides of the ship to the under side of the lower deck. As all merchant ships are built with a double bottom (see page 107), the cost of thus providing a double skin below the water-line is small in proportion to the security against flooding which it affords.
The description of the Titanic, published at the time of her launch, stated that any two of her adjoining compartments could be flooded without endangering the safety of the ship, and the question must frequently have occurred to the lay mind as to why the ability of the ship to sustain flooding of her interior was confined to two, and not extended to include three or even more compartments.
The ability to stand the flooding of two compartments only is not peculiar to the Titanic. It represents the standard practice which is followed in all passenger ships, the spacing and height of whose bulkheads is determined in accordance with certain stipulations of the British Board of Trade. These stipulations, as given by Prof. J. H. Biles of Glasgow University, in his book "Design and Construction of Ships," are as follows:
"A vessel is considered to be safe, even in the event of serious damage, if she is able to keep afloat with two adjoining compartments in free communication with the sea. The vessel must therefore have efficient transverse watertight bulkheads so spaced that when any two adjoining compartments are open to the sea, the uppermost deck to which all the bulkheads extend is not brought nearer to the surface of the water than a certain prescribed margin.
"The watertight deck referred to is called the bulkhead deck. The line past which the vessel may not sink is called the margin of safety line.
"The margin of safety line, as defined in the above report, is a line drawn round the side at a distance amidships of three-one-hundredths of the depth at side at that place below the bulkhead deck, and gradually approaching it toward the aft end, where it may be three-two-hundredths of the same depth below it."
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Closing, from the Bridge, All Watertight Doors Throughout the Ship by Pulling a Lever
Board of Trade rule called for the extension of the bulkheads amidships only to the upper deck, which, at the loaded draft of 34 feet, was only 10 feet above the water-line! Compare this with the safe construction adopted by Brunel and Scott Russell over fifty-four years ago, who, in constructing the Great Eastern, extended all the bulkheads (see page 83) to the topmost deck, fully 30 feet above the water-line.
Before leaving the question of bulkheads, the writer would enter a strong protest against the present practice of placing watertight doors in the main bulkheads below the water-line. They are put there generally for the convenience of the engine- and boiler-room forces, whose duties render it necessary for them to pass from compartment to compartment. As at present constructed, these doors are of the sliding type, and they can be closed simultaneously from the bridge, or separately, by hand. The safer plan is to permit no bulkhead doors below the water-line, and provide in their place elevators or ladders, enclosed in watertight trunks. Access from compartment to compartment must then be had by way of the bulkhead deck.
The advantage of lofty bulkheads was admirably illustrated in the case of the City of Paris and the City of New York, designed by Mr. Biles in 1888. Although these were small ships compared with the Titanic, their fourteen bulkheads were carried one deck higher. Biles laid down the rule that no doors were to be cut
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A Comparison of Bulkhead Protection in Some Notable Ships
through the bulkheads, and in spite of strenuous objections on the grounds of passenger accommodation and general convenience in the operation of the ship, he carried his point.
The wisdom of this construction was demonstrated years later, when, as a result of an accident to her engines, the two largest adjoining compartments of the City of Paris were flooded, at a time when the ship was 150 miles off the coast of Ireland. There was no wireless in those days to send out its call for help, and for three days the ship drifted in a helpless condition. Thanks to her lofty bulkheads, the good ship stood the ordeal and was finally brought into port without the loss of a single passenger.
| BULKHEAD SPACING ON NOTABLE SHIPS | |||||||
| NAME | Date of Building | * Registered Length, Feet |
No. of Main W. T. Bulkheads |
Average Length of Compartments |
Per cent. of Length | ||
|
1911 | 852.5 | 15 | 53 | 6.2 | ||
|
1907 | 762.0 | 16 | 45 | 5.9 | ||
|
1908 | 699.0 | 13 | 50 | 7.1 | ||
|
1854–59 | 680.0 | 9 | 68 | 10.0 | ||
|
1905 | 650.0 | 15 | 50 | 7.8 | ||
|
1893 | 601.0 | 8 | 67 | 11.1 | ||
|
1888 | 517.0 | 14 | 37 | 6.7 | ||
|
1894 | 270.7 | 11 | 23 | 8.3 | ||
| * Figures in this column represent the length between perpendiculars. | |||||||
An interesting study of bulkhead practice in some notable ships is afforded by the table and diagrams which are herewith reproduced by the courtesy of "Engineering." In the matter of height of bulkheads above the water-line, the Great Eastern stands first, followed by the Paris, the Lusitania, the Campania, and the Titanic.