Page 88 - Introduction to Naval Architecture

FLOTATION AND STABILITY 7,5

Table 4.4

Space Permeability (%)

Watertight compartment 97 (warship)
95 (merchant ship)
Accommodation spaces 95 (passengers or crew)
Machinery compartments 85
Cargo holds 60
Stores 60

It is not easy to calculate the damaged waterline and successive
approximation is necessary. One reason is that the assumptions of small
changes will not apply. There are two common approaches: the lost
buoyancy method and the added weight method.

Lost buoyancy method
First the volume of the damaged compartment, Figure 4.34, up to the
original waterplane, and the area of waterplane lost, are calculated
making allowance for the permeability. Suppose the area of original
waterplane is A and the area lost is [A a, where ft is the permeability. Let
the lost volume of buoyancy bejuv. A first approximation to the parallel
sinkage suffered is given by:

A second approximation will almost certainly be needed because of the
variations in waterplane area with draught. This can be made by taking
the characteristics of a waterplane at sinkage z/2. The longitudinal
centre of flotation and the moment to change trim can be calculated

Figure 4,34 Lost bouyancy method