about the ship's centerline, the ship will undergo
LONGITUDINAL DIMENSION scale (B); this line
parallel sinkage to a new waterline such as W1l1. Since
will intersect the pivot scale. Then draw a second
the weight is off center, however, the ship assumes an
straight line from the point of intersection on the pivot
inclined position with a waterline similar to W2L2.
scale (C) to the appropriate point on the displacement
scale (D). The point at which this second straight line
intersects the GM reduction scale (E) gives you the
reduction in GM (in feet) that is caused by the free
C
L
L4
surface.
L3
For example, assume that you want to find what
L2
reduction in GM is caused by free surface effect in a
L1
W1
b
W
L
partially flooded compartment that is 35 feet athwart
a
W2
ships and 20 feet fore-and-aft, in a ship of 10,000 tons
W3
W4
displacement. Draw the first straight line from the
35-foot point on the athwartship dimension scale to the
20-foot point on the longitudinal dimension scale.
Then draw the second straight line from the point of
DCf1231
intersection on the pivot line to the 10,000-ton point on
the displacement scale. The point at which the second
Figure 12-31. Free communication effect in off-center
straight line intersects the GM reduction scale
compartment.
indicates how much reduction in GM has occurred
because of free surface effect in the partially flooded
If the water in the compartment is now returned to
compartment. In this example, GM has been reduced
its fluid state, it will have a waterline a-b that is parallel
0.2 foot.
to (but below) the ship's waterline W2L2. Immediately,
Free Communication Effect
however, additional water will flow in from the sea and
flood the compartment to the actual level of the ship's
Thus far, the stability changes caused by the effect
waterline W2L2. The ship will therefore sink deeper in
of free surface and by the addition of the weight of the
the water and will assume a greater list; the waterline
flooding water have been considered. In certain
will reach a position such as W3L3. Again, additional
instances, it is also necessary to make allowance for
water will flow in from the sea and flood the
stability changes that occur when an off-center
compartment to the level of the ship's waterline W3L3;
compartment is in free communication with the sea.
this will cause the ship to sink even deeper in the water
If a boundary of an off-center compartment is so
and to assume an even greater list. These interactions
extensively ruptured that the sea can flow freely in
will continue until the waterline is at the position
and out as the ship rolls, the FREE
represented by W4L4.
COMMUNICATION EFFECT will cause a reduction
Note that stability is not usually reduced by free
in GM and in GZ. Note that the free communication
effect on stability is IN ADDITION TO the effect of
communication if the compartment is symmetrical
free surface and the effect of added weight. To
a b o u t t h e s h i p 's c e n t e r l i n e . U n d e r c e r t a i n
understand the free communication effect, consider
circumstances, free communication in a centerline
an off-center compartment partially full of water and
compartment may increase the free surface effect, and
in free communication with the sea, as shown in
thereby reduce stability. However, it is important to
figure 12-31. (Note that this compartment is free to
remember that this reduction in stability occurs from
vent at the top.)
the increased free surface effect, rather than from any
Before the hull is ruptured, the ship floats on an
free communication effect.
even keel at waterline WL. Then the compartment is
Summary of Effects of Loose Water
partially flooded and left in free communication with
the sea. Assume that the water enters the compartment
The addition of loose water to a ship alters the
instantaneously (up to the level of the ship's original
stability characteristics by means of three effects that
waterline WL) and is instantaneously frozen solid. If
the weight of the frozen water is distributed equally
must be considered separately: (1) the effect of added
12-18
