454 INICEOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
strength and to add to the safety of the vessel by subdividing the hull
into self-contained watertight compartments so that in the event
of one or more compartments being flooded there would still be left
sufficient reserve buoyancy to keep the ship afloat. They also
serve to reduce the risks from fire by confining an outbreak to one hold.
The Number of Watertight Bulkheads is regulated by the length of
the ship, but^steam vessels must have at least four, a collision bulkhead
placed at a distance of not less than 5 per cent., that is, one-twentieth
of "the vessel's length abaft the stem measured at the waterline; a bulk-
head before and another abaft the engine and boiler space and an after
peak bulkhead placed in a position to enclose the shaft tubes in a water-
tight compartment. Additional bulkheads- are fitted with increase
in the length of the vessel. When over 285 feet 5 bulkheads are fitted;
when over 335 feet, 6 are fitted; when over 405 feet, 7; over 470 feet, 8
and over 540 feet, 9.
Watertight bulkheads extend to the shell plating on each side and
from the floor to the upper deck. Such a large area of plating
must be efficiently stiffened, not merely to prevent it buckling under
pressures but more particularly to withstand the great pressure
of a body of water on one side only in the unusual event of the compart-
ment accidentally becoming flooded through stranding or collision.
The pressure of water would be greatest at the bottom, and so the lower
strakes of plating are increased in thickness and the relative thickness
of all the plating depends on the spacing and strength of the stiffeners
that are fitted. The stronger the stiffeners and the closer they are spaced
the thinner may the plating be.
Collision Bulkhead stifieners are stronger than for other bulk-
heads and the plating is made thicker to withstand the slashing
of free water should the compartment be laid open to the sea by collision.
Bulkheads make the section where they occur perfectly rigid and
overstrong so that the excessive local strength has to be distributed
by means-of brackets to the adjoining membeir of the hull, viz., to
stringers, keelsons, shell plating, deck plating, etc., and these components
pass it on throughout "the structure and maintain a continuity and
uniformity of strength.
Bulkheads .—Figure 43 illustrates a side view and front view of a
bulkhead. The plating A may be arranged transversely or vertically,
in this case transversely.
B indicates the vertical stiffeners with their top ends bracketed
to the watertight flat above, and their bottom ends to the inner