Page 94 - Introduction to Naval Architecture

FLOTATION AND STABILITY 81

waterline is tangent to a line drawn 76mm (3 inches) below the
bulkhead deck at side. This line is called the margin line. The floodable
length at any point along the length of the ship is the length, with that
point as centre, which can be flooded without immersing any part of
the margin line when the ship has no list.
Take the ship shown in Figure 4.36 using subscripts 0 and 1 to
denote the intact ship data for the intact and damaged waterlines.
Loss of buoyancy = Vi - V 0 and this must be at such a position that
Bj moves back to B 0 so that B is again below G. Hence:

This then gives the centroid of the lost buoyancy and, knowing (Fj -
V 0) it is possible to convert this into a length of ship that can be
flooded. The calculation would be one of reiteration until reasonable
figures are obtained.
The calculations can be repeated for a series of waterlines tangent
to the margin line at different positions along the length. This will
lead to a curve of floodable length as in Figure 4.37. The ordinate

Figure 4,37 Floodable length

at any point represents the length which can be flooded with the
centre at the point concerned. Thus if / is the floodable length at
some point the positions of bulkheads giving the required compart-
ment length are given by setting off distances 1/2 either side of the
point. The lines at the ends of the curves, called the forward and after
1
terminals will be at an angle tan" 2 to the base if the base and
ordinate scales are the same.
The permeabilities of compartments will affect the floodable length
and it is usual to work out average permeability figures for the
machinery spaces and for each of the two regions forward and aft.