Page 362 - Practical Ship Design

Freeboard and Subdivision 319

1 .o

R 0.8 R = (0 002 + 0 0009 LS)’”
Required
subdivision
index 0.6

0.4

0.2

0
0 100 200 300 400
Ls in metres
Fig. 11.2. Probabilistic subdivision of cargo ships - Required Subdivision Index.

Subdivision length has a new definition: the greatest length of the ship at or
below the deck which limits the vertical extent of flooding.
To assist understanding, graphs have been drawn of each of the factors involved,
with the corresponding equations alongside. Figure 1 1.2 gives a graphical repres-
entation of the Required Subdivision Index R. The fact that this is related only to
length has been criticised by those who think that the crew numbers should be a
factor. An answer to this criticism is that if increasing crew numbers resulted in a
higher value of R and therefore in a higher building cost, it would have the
undesirable effect of increasing the pressure to reduce crew numbers. In any case
cargo ship crew numbers vary little with size of ship.
The next factor is the Attained Subdivision Index A which is:

where
i represents each compartment or group of compartments (see Fig. 11.3). This
shows a ship with six single compartments, five groups of two compartments,
four groups of three, three groups of four, two groups of five and one comprising
the whole ship.
pi represents the probability that the compartment or group under
consideration may be flooded, disregarding any horizontal subdivision;
si represents the probability of survival after flooding of the compartment or
group, including the effects of horizontal subdivision.

The calculations are made at level trim. The summation is made for all cases of
single compartment flooding plus all cases of flooding of two or more adjacent
compartments that the ship can survive.