FIG. 1 WATERTIGHT DECK AT WATERLINE LIMITS INFLOW OF WATER
FIG. 2 HIGH BULKHEADS, WITHOUT WATERTIGHT DECK WOULD SAVE THE SHIP BUT PERMIT DEEP SUBMERSION
FIG. 3 SINKING BY THE HEAD; WATER FLOWING ALONG LOW BULKHEAD DECK AND ENTERING COMPARTMENTS THROUGH DOORS OR HATCHWAYS
FIG. 4 DOWN BY THE HEAD, BUT SAVED BY HIGHER BULKHEADS AND WATERTIGHT BULKHEAD DECK
FIG. 5 RELATIVE AREA OF FLOODING FROM SAME DAMAGE IN SHIPS, "A" WITH DOUBLE SKIN; "B" WITH SIDE BUNKERS; "C" WITH A SINGLE SKIN. TRANSVERSE BULKHEADS ON EACH SHIP
DIAGRAMS SHOWING PROTECTIVE VALUE OF TRANSVERSE AND LONGITUDINAL BULKHEADS, WATERTIGHT DECKS, AND INNER SKIN]
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.