moment, use the ship's baseline, or keel, as the axis.
In one sense, a ship may be considered as a
Figure 12-11 shows the calculation of the vertical
system of weights. If the ship is undamaged and
moment of a 5-inch gun on the main deck of a ship. The
floating in calm water, the weights are balanced and
gun weighs 15 tons and is located 40 feet above the
the ship is stable. However, the movement of weight
keel. The vertical moment is thus 15 x 40, or
on the ship causes a change in the location of the
600 foot-tons.
ship's center of gravity, and thereby affects the
stability of the ship.
15 TONS
F i g u r e 1 2 - 9 s h ow s h ow a n I N C L I N I N G
MOMENT is produced when a weight is moved
outboard from the centerline of the ship. If the object
weighing 20 tons is moved 20 feet outboard from the
centerline, the inclining moment will be equal to
400 foot-tons (F x d, or 20 x 20).
d
BASE LINE
ORIGINAL
w
NEW
POSITION
POSITION
DCf1211
Figure 12-11. Vertical moment.
BUOYANCY VERSUS GRAVITY
"Buoyancy" may be defined as the ability of an
DIRECTION
OF MOMENT
object to float. If an object of a given volume is placed
DCf1209
under water and the weight of this object is
Figure 12-9. Inclining moment produced by moving a weight
outboard.
GREATER than the weight of an equal volume of
water, the object will sink. It sinks because the
Figure 12-10 shows how a forward (or aft)
FORCE that buoys it up is less than the weight of the
movement of weight produces a TRIMMING
object. However, if the weight of the object is LESS
MOMENT. Let's assume that a 20-ton weight is
than the weight of an equal volume of water, the
moved 50 feet forward; the trimming moment
object will rise. The object rises because the FORCE
produced is 20 x 50, or 1,000 foot-tons.
that buoys it up is greater than the weight of the
object; it will continue to rise until it is partly above
the surface of the water. In this position the object will
float at such a depth that the submerged part of the
t
object displaces a volume of water EQUAL to the
w
weight of the object.
NEW
ORIGINAL
As an example, take the cube of steel shown in
POSITION
POSITION
figure 12-12. It is solid and measures 1 foot by 1 foot
DIRECTION
by 1 foot. If you drop the steel cube into a body of
OF MOMENT
water, the steel cube will sink because it weighs more
DCf1210
than a cubic foot of water. But if you hammer this
cube of steel into a flat plate 8 feet by 8 feet, bend the
Figure 12-10. Trimming moment.
edges up 1 foot all-around, and make the corner seams
watertight, this 6-foot by 6-foot by 1-foot box, as
shown in figure 12-12, will float. In fact, it will not
It is also possible to calculate the VERTICAL
only float but will, in calm water, support an
MOMENT of any part of the ship's structure or of any
weight carried on board. In calculating a vertical
additional 1,800 pounds.
12-5
