Page 66 - Introduction to Naval Architecture

FLOTATION AND STABILITY 53

If the breadth of the tank is originally B, the width of each of the two
tanks, created by the central division, is J5/2. Assuming the tanks have
a constant section, and have a length, 4 the second moment of area
3
without division is IB /12. With centre division the sum of the second
3
moments of area of the two tanks is (//12) (B/2) X 2 = 1&/48
That is, the introduction of a centre division has reduced the free
surface effect to a quarter of its original value. Using two bulkheads to
divide the tank into three equal width sections reduces the free surface
to a ninth of its original value. Thus subdivision is seen to be very
effective and it is common practice to subdivide the double bottom of
ships. The main tanks of ships carrying liquid cargoes must be designed
taking free surface effects into account and their breadths are reduced
by providing centreline or wing bulkheads.
Free surface effects should be avoided where possible and where
unavoidable must be taken into account in the design. The operators
must be aware of their significance and arrange to use the tanks in ways
intended by the designer.

The inclining experiment
As the position of the centre of gravity is so important for initial stability
it is necessary to establish it accurately. It is determined initially by
calculation by considering all weights making up the ship - steel, outfit,
fittings, machinery and systems - and assessing their individual centres
of gravity. From these data can be calculated the displacement and
centre of gravity of the light ship. For particular conditions of loading
the weights of all items to be carried must then be added at their
appropriate centres of gravity to give the new displacement and centre
of gravity. It is difficult to account for all items accurately in such
calculations and it is for this reason that the lightship weight and centre
of gravity are measured experimentally.
The experiment is called the inclining experiment and involves causing
the ship to heel to small angles by moving known weights known
distances tranversely across the deck and observing the angles of
inclination. The draughts at which the ship floats are noted together
with the water density. Ideally the experiment is conducted when the
ship is complete but this is not generally possible. There will usually be
a number of items both to go on and to come off the ship (e.g. staging,
tools etc.). The weights and centres of gravity of these must be assessed
and the condition of the ship as inclined corrected.
A typical set up is shown in Figure 4.18. Two sets of weights, each of
w, are placed on each side of the ship at about amidships, the port and
starboard sets being h apart. Set 1 is moved a distance h to a position