Displacement. Since weight (W) is equal to displacement, it is possible to measure the volume of the underwater body (V) in cubic feet and multiply this volume by the weight of a cubic foot of sea water, in order to find what a ship weighs. This relationship may be written as:
| V = 35W. V = volume of displacement in cubic feet. W = weight in tons. 35 = cubic feet of sea water per ton. |
In dealing with ships it is customary to use the long ton of 2,240 pounds; in this text the word "ton" refers to the long ton exclusively.
Displacement varies with draft. As the draft increases, the displacement increases. This is indicated in figure 3-2 by the series of displacements shown for successive draft lines on the midship section of a cruiser. The volume of the underwater body for a given draft line can be measured in the drafting room, using graphic or mathematical means. This is done for a series of drafts throughout the probable range of displacements in which a ship is likely to operate. The values obtained are plotted on a grid, on which feet of draft is measured vertically, and tons displacement horizontally. A smooth line is faired through the points plotted, providing a curve of displacement versus draft, or a displacement curve, as it is generally called. The result is shown in figure 3-3 for a typical cruiser. It is one of the curves of form which are supplied to every Naval ship under the title displacement and other curves
To find the displacement when the draft is given, locate the value of mean draft on the draft scale at the left of figure 3-3. Then proceed horizontally across the diagram to the curve. From this point proceed vertically downward to the scale of tons, where the displacement can be read from the scale. For example, given a mean draft of 22 feet, the displacement found from the curve is approximately 13,400 tons. The curve also can be used in reverse. If given a displacement of 10,000 tons, the draft would be approximately 17.5 feet.
Reserve buoyancy. The volume of the watertight portion of the ship above the waterline is the ship's reserve buoyancy. Freeboard is a rough measure of reserve buoyancy, being the distance in feet from the waterline to the weather deck edge. Unless otherwise stated references normally are to mean or midship freeboard. As shown in figure 3-4, freeboard plus draft always equals the depth of the hull in feet.
When weight is added to a ship, draft and displacement increase in the same amount that freeboard and reserve buoyancy decrease. A substantial amount of reserve buoyancy is, of course, essential to the seaworthiness of a ship. Some approximate values of reserve buoyancy in Naval ships of various types are given below:
| BB (new) | 55% of displacement. |
| CV (large) | 130% of displacement. |
| CA, CL | 100% of displacement. |
| DD (2,100 ton) | 100% of displacement. |
| DD (1,630 ton) | 75% of displacement. |
| DE | more than 100% of displacement. |
