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 water-line. 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.
[Illustration: HYDRAULICALLY-OPERATED, WATERTIGHT DOOR IN AN ENGINE-ROOM
BULKHEAD]
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
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, 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.
[Illustration:
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.
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."
By referring to the diagrams on page 66 showing the disposition of
bulkheads on certain notable ships, it will be seen that, in the case of
the _Titanic_, the application of the 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.
[Illustration: CLOSING, FROM THE BRIDGE, ALL WATERTIGHT DOORS THROUGHOUT
THE SHIP BY PULLING A LEVER]
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 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.
[Illustration: COURTESY OF ENGINEERING
OLYMPIC AND TITANIC 1912
LUSITANIA 1906
GREAT EASTERN 1858
CAMPANIA 1893
PARIS 1868
A COMPARISON OF BULKHEAD PROTECTION IN SOME NOTABLE SHIPS]
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
======================================================================
|Date of |Registered| No. of | Average | Per
NAME |Building| Length, |Main W. T.| Length of |cent. of
| | Feet [1] |Bulkheads |Compartments| Length
-----------------+--------+----------+----------+------------+--------
Titanic | 1911 | 852.5 | 15 | 53 | 6.2
Lusitania | 1907 | 762.0 | 16 | 45 | 5.9
George Washington| 1908 | 699.0 | 13 | 50 | 7.1
Great Eastern | 1854-59| 680.0 | 9 | 68 | 10.0
Carmania | 1905 | 650.0 | 15 | 50 | 7.8
Campania | 1893 | 601.0 | 8 | 67 | 11.1
New York | 1888 | 517.0 | 14 | 37 | 6.7
Alma | 1894 | 270.7 | 11 | 23 | 8.3
-----------------+--------+----------+----------+------------+--------
[1] 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_.