DISPLACEMENT AND CARRYING CAPABILITY.
A vessel floating at rest displaces a volume of water whose weight equals her own total weight.
For vessels floating in sea water the number of cubic feet of water displaced per ton of weight is, as nearly as possible, thirty-five. For vessels in fresh water—i.e., lakes or rivers—the cubic feet per ton of weight is thirty-six.
By calculating the volume of under-water portion of the vessel’s hull, the number of cubic feet displaced by the vessel when floating at any given draught is obtained. This result, divided by 35 or 36, according as the water is salt or fresh, gives the number of tons weight displaced, and consequently the total weight of the vessel.
Calculations being made of the volume of the vessel’s hull to intermediate distances between the keel and the maximum load line, it is thus possible to construct a “curve of displacement” from which the actual amount of displacement at any intermediate draught can be obtained.
From this curve a set of scales—usually set up alongside a vertical scale of feet and inches, representing the vessel’s draught-marks—are constructed, showing—1st, the tons “displacement” at any draught; 2nd, the tons of “dead-weight” capability—i.e., the tons displacement due to the weight of cargo, coal, ballast, stores, fresh water, spare gear, &c.—at any draught above the vessel’s light-draught: “light-draught” being that at which the vessel floats with holds clean-swept, bilges dry, water in boilers, and with such spare gear on board as is required by Board of Trade; and 3rd, the amount of “freeboard”—i.e., the distance in feet and inches from any particular draught line to the top of the deck amidships.