Page 367 - Practical Ship Design

324 Chapter 11

compartments, each of which lie inside both A.T. and F.T. - all as shown in Fig.
11.3. A further spreadsheet can then be used for the calculation of groups of
compartments, also as shown in Fig. 11.3. Shown at the foot of Fig. 11.4, but not
used in this case, are lines for the tabulation of si values leading on to the pi . si
values and their aggregation to give the attained subdivision index A.
In the simplistic ship shown in Fig. 11.4 there were neither longitudinal
bulkheads nor watertight decks introducing complications in the subdivision.
These are probably undesirable features from a subdivision and damaged stability
point of view, but other reasons may nevertheless make them essential. Figure 11.6
shows all the possible structural variants that can arise out of combinations of these
features and the different damage possibilities.
Longitudinal subdivision is dealt with by the use of a reduction factor r applied
to the pi value of the compartment concerned. Values of r are given in Fig. 1 1.7.
Horizontal subdivision is on the other hand treated as a factor affecting s - the
probability of survival, already mentioned but not yet defined.
The formula for s, together with graphs of its two components SIC and c are
given as Fig. 11.8. Both require an input from damaged stability calculations for
the compartment concerned; the former the GZ and the range and the latter the
final angle of heel.
It is a requirement that calculations of s are made at both the deepest subdivision
loadline and at a partial loadline set at 60% of the difference between the light draft
and the subdivision draft, with the results averaged.
The permeabilities to be used in the damaged stability calculations are:
stores 0.60 accommodation 0.95
machinery 0.85 void spaces 0.95
dry cargo 0.70 liquid tanks 0 or 0.95, whichever results in the more
severe requirement.
Horizontal subdivision within the compartment brings in a reduction factor v
which represents the probability that spaces above the division will not be flooded.
A graph of vi is given as the third graph in Fig. 11.8 and should be studied in
association with the sections on Fig. 1 1.6.
As was said earlier, the rules are very complicated and demand the use of a
computer program.
It has been said that very few existing ships would meet these rules, so some
guidance for new designs is clearly desirable. Unfortunately practical designers
have so far had little experience of the implications of the rules to guide them and
until this is forthcoming must go cautiously and carry out full calculations before a
design becomes fixed.
Some good immediate guidance could be obtained by carrying out calculations
for one or two existing ships and seeing by how much these fail and how they
might best be modified to pass.