Page 349 - Practical Ship Design

306 Chapter I I

The freeboard rules currently in force are embodied internationally in the
provisions of the International Convention on Load lines 1966 and for British ships
in The Merchant Shipping (Load Line) Rules 1968. The freeboard rules are
divided into two sections:
(i) a section which lays down ship construction requirements which deal with
structural strength, stability, the watertight integrity of the ship, the safety
of the crew, and in some cases the ability of the ship to withstand flooding
of specified compartments; and
(ii) a section dealing with the calculation of freeboard, based on the geometry
of the ship.
It is not proposed to elaborate on the first section here, although the stability
requirements and the ability to withstand flooding are dealt with in a later section
of this chapter.

11.2.1 Different types of freeboard
There are a number of different freeboards which apply to different types of ship
and each of these has some special construction requirements.
Each ship also has a number of different freeboards which apply under different
circumstances of sea area, time of year and water density. The basic freeboard for
any ship is its “summer” freeboard. “Summer” here is a technical term defined in
the rules as covering particular periods of the year in particular sea areas.
Reverting to the different types of freeboard applicable to different ship types,
these are as follows:

(1) Type A
This is a reduced freeboard permitted for tankers designed to carry liquids in bulk
and which have certain features. Since the rules were written new requirements
introduced by IMO for segregated ballast tanks have so changed the design of oil
tankers that their design is no longer weight based and currently this type of
freeboard is rarely used.
(2) Type B
This is the standard freeboard which applies to the majority of ships.

(3) Type B-60 and B-100
These are reduced freeboards which can be given to ships, which can be shown to
have an ability to withstand damage of an extent required by the rules. This type of
freeboard is particularly advantageous for large bulk carriers and especially those
intended for the iron ore trade.
The “100“ in B-100 refers to the difference between a type A and a type B
freeboard. The ”60“ refers to 60% of this difference.
The damage which these ships must be able to withstand is dealt with in 0 12.4.