It is obvious that displacement will vary with the

depth of a ship's keel below the water line that is known

as draft. As the draft increases, the displacement

increases. This is indicated in figure 12-13 by a series

1'

of displacements shown for successive draft lines on

1'

1'

the midship section of a ship. The volume of an

6'

underwater body for a given draft line can be measured

1'

in the drafting room by using graphic or mathematical

means. This is done for a series of drafts throughout the

6'

probable range of displacements in which a ship is

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likely to operate. The values obtained are plotted on a

grid on which feet of draft are measured vertically and

tons of displacement horizontally. A smooth line is

faired through the points plotted, providing a curve of

displacement versus draft, or a DISPLACEMENT

It is obvious, then, that the volume of the submerged

CURVE as it is generally called. An example of this for

part of a floating ship provides the buoyancy to keep the

a typical warship is shown in figure 12-14.

ship afloat. If the ship is at rest, the buoyancy (which is

To use the sample curve shown in figure 12-14 for

the weight of the displaced water) must be equal to the

finding the displacement when the draft is given, locate

weight of the ship. For this reason, the weight of a ship is

the value of the mean draft on the draft scale at the left.

generally referred to as DISPLACEMENT, meaning the

Then proceed horizontally across the diagram to the

weight of the volume of water displaced by the hull.

displacement curve. From this point proceed vertically

Since weight (W) is equal to the displacement, it is

downward and read the displacement from the scale.

possible to measure the volume of the underwater body

For example, if you have a mean draft of 26 feet, the

(V) in cubic feet and multiply this volume by the

displacement found from the curve is approximately

weight of a cubic foot of seawater to determine what

16,300 tons.

the ship weighs. This relationship may be written as the

following:

1

The volume of the watertight portion of the ship

(1) W = V ×

above the waterline is known as the ship's reserve

35

bu oy a n cy. E x p r e s s e d a s a p e r c e n t a g e , r e s e r ve

(2) V = 35W

buoyancy is the ratio of the volume of the above-water

body to the volume of the underwater body. Thus

V = Volume of displaced seawater (in cubic feet)

reserve buoyancy may be stated as a volume in cubic

W = Weight in tons

feet, as a ratio or percentage, or as an equivalent weight

35 = Cubic feet of seawater per ton (For ships, the

of seawater in tons. (In tons it is 1/35 of the volume in

long ton of 2,240 pounds is used.)

cubic feet of the above-water body.)

DISPLACEMENT

WATERLINE

18,000 TONS

28 FEET

14,800 TONS

24 FEET

20 FEET

11,750 TONS

8,800 TONS

16 FEET

5,900 TONS

12 FEET

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12-6