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

"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

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

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

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