Page 95 - Introduction to Naval Architecture

82 FLOTATION AND STABILITY

Figure 438 Floodable length with permeability

This leads to three curves for the complete ship as shown in Figure
4.38. The condition that a ship should be able to float with any one
compartment open to the sea is a minimum requirement for ocean
going passenger ships. The Merchant Shipping Regulations set out
formulae for calculating permeabilities and a factor of subdivision
which must be applied to the floodable length curves giving permis-
sible length. The permissible length is the product of the floodable
length and the factor of subdivision. The factor of subdivision
depends upon the length of the ship and a criterion of service numeral
or more simply criterion numeral. This numeral represents the criterion
of service of the ship and takes account of the number of passengers,
the volumes of the machinery and accommodation spaces and the
total ship volume. It decreases in a regular and continuous manner
with the ship length and factors related to whether the ship carries
predominantly cargo or passengers. Broadly, the factor of subdivision
ensures that one, two or three compartments can be flooded before
the margin line is immersed leading to what are called one-, two- or
three-compartment ships. That is, compartment standard is the inverse of
the factor of subdivision. In general terms the factor of subdivision
decreases with length of ship and is lower for passenger ships than
cargo ships.

SUMMARY

The reader has been introduced to the methods for calculating the
draughts at which a ship will float, and its stability for both initial
stability and stability at large angles of inclination. Standards for
stability have been discussed. Both the intact and the damaged states
have been covered. These are fundamental concepts in the design
and operation of ships. A more detailed discussion on stability at
about this level, with both worked and set examples, is to be found
4
in Derrett.