Page 78 - Introduction to Naval Architecture

FLOTATION AND STABILITY 65

Important factors in determining the range of stability are freeboard
and reserve of buoyancy.
The angle of deck edge immersion varies along the length of the
ship. However, often it becomes immersed over a reasonable length
within a small angle band. In such cases the GZ curve will exhibit a
point of inflexion at that angle. It is the product of displacement and
GZ that is important in most cases, rather than GZ on its own.

Example 4.3

The angles of inclination and corresponding righting lever for a
ship at an assumed KS of 6.5 m are:
Inclination (°) 0 15 30 45 60 75 90
Righting lever (m) 0 0.11 0.36 0.58 0.38 -0.05 -0.60
In a particular loaded condition the displacement mass is made
up of:
Item Mass (tonnes) KG (m)
Lightship 4200 6.0
Cargo 9100 7.0
Fuel 1500 1.1
Stores 200 7.5
Plot the curve of statical stability for this loaded condition and
determine the range of stability.

Solution
The height of the centre of gravity is first found by taking
moments about the keel:

The GZ values for the various angles of inclination can be
determined in tabular form as in Table 4.2. By plotting GZ against
inclination the range of stability is found to be 82°.