Page 87 - Introduction to Naval Architecture

74 FLOTATION AND STABILITY

Bulkheads cannot ensure complete safety in the event of damage. If
the hull is opened up over a sufficient length several compartments can
be flooded. This was the case in the tragedy of the Titanic. Any flooding
can cause a reduction in stability and if this reduction becomes great
enough the ship will capsize. Even if the reduction does not cause
capsize it may lead to an angle of heel at which it is difficult, or
impossible, to launch lifeboats. The losses of buoyancy and stability due
to flooding are considered in the following sections.

Sinkage and trim when a compartment is open to the sea

Suppose a forward compartment is open to the sea, Figure 4.33. The
buoyancy of the ship between the containing bulkheads is lost and the
ship settles in the water until it picks up enough buoyancy from the rest
of the ship to restore equilibrium. At the same time the position of the
LCB moves and the ship must trim until G and B are again in a vertical
line. The ship which was originally floating at waterline W 0Lo now floats
at Wi Lj . Should Wj LI be higher at any point than the deck at which
the bulkheads stop (the bulkhead deck} it is usually assumed that the ship
would be lost as a result of the water pressure in the damaged
compartment forcing off the hatches and leading to unrestricted
flooding fore and aft. In practice the ship might still remain afloat for
a considerable time.

Figure 4.33 Compartment open to the sea

Most compartments in a ship contain items which will reduce the
volume of water that can enter. Even 'empty' spaces usually have frames
or beams in them. At the other extreme some spaces may already be full
of ballast water or fuel. The ratio of the volume that is floodable to the
total volume is called the permeability of the space. Formulae for
calculating permeabilities for merchant ships are laid down in the
Merchant Ship (Construction) Rules. Typical values are presented in
Table 4.4. Although not strictly accurate, the same values of permeabil-
ity are usually applied as factors when assessing the area and inertias of
the waterplane in way of damage.