Page 357 - Practical Ship Design

3 14 Chapter 11

Even the deterministic rules do not provide any direct guidance on what
freeboard should be provided in a design. Instead, they set a standard of sub-
division which must be achieved. If the freeboard has been set “too low”, the
number of bulkheads necessary will be excessive and the space between them such
that the development of a satisfactory design will be difficult or impossible. A wise
designer should therefore select an initial freeboard ratio or drafddepth ratio based
on that of a ship having approximately the same factor of subdivision and an
arrangement reasonably similar to that intended for the new design.
In choosing a basis ship for this guidance, attention should be paid to the block
coefficient and the sheer of the bulkhead deck of the basis ship and the corres-
ponding values intended for the new design as these affect the floodable lengths
which can be obtained with a given freeboard ratio, particularly for compartments
towards the ends of the ship.
It is also wise to think ahead to damaged stability, which not infrequently makes
it desirable to have more freeboard than is needed purely to meet the subdivision
requirements.
If initial subdivision calculations show an improvement in subdivision is
required, the most usual ways of obtaining of achieving this are by the respacing of
bulkheads, adding a bulkhead, reducing the draft and/or increasing the depth to the
bulkhead deck.

11.4 DETERMINISTIC RULES FOR PASSENGER SHIPS

This section gives a condensed version of the main provisions of these rules but
reference to the rules themselves remains essential as there are many secondary
provisions.
Reference should be made to a standard textbook on naval architecture for a
description of the first principles method of calculating floodable length. Regret-
tably, there does not seem to be any quicWapproximate method suitable for
inclusion in this book.

General definitions

The floodable length at a given point is the maximum length of compartment,
having its centre at that point which can be flooded without the margin line being
submerged.
The margin line is 76 mm below the top of the bulkhead deck at side.
Uniform permeabilities as determined by the following formulae are to be used
for the machinery space and the spaces forward and aft respectively of the
machinery space: