A reduction in the size of the righting arm usually
means a decrease in stability. When the reduction in GZ
is caused by increased displacement, however, the total
effect on stability is more difficult to evaluate. Since the
RIGHTING MOMENT is equal to W times GZ, it will
be increased by the gain in W at the same time that it is
decreased by the reduction in GZ. The gain in the
righting moment, caused by the gain in W, does not
necessarily compensate for the reduction in GZ.
In summary, there are several ways in which an
increase in displacement affects the stability of a ship.
Although these effects occur at the same time, it is best
to consider them separately. The effects of increased
displacement are the following:
1. RIGHTING ARMS (GZ) are decreased as a
result of increased draft.
2. RIGHTING MOMENTS (foot-tons) are
decreased as a result of decreased GZ.
3. RIGHTING MOMENTS are increased as a
result of the increased displacement (W).