DISPLACEMENT AND BUOYANCY The displacement of a vessel is the quantity or bulk of water (generally represented by a measure of weight) which a vessel displaces or pushes away when she is put into the water. This quantity of water is always equal to the whole weight of the vessel and everything that she contains ; that is to say, the vessel will sink into the water until she has displaced a quantity of the fluid equal to her own weight and the weight of everything that she contains.
If the weight of water displaced is also exactly equal in bulk to the bulk of the vessel, then the WWs will sink in the fluid until her entire bulk is immersed; or, in other words, if the body immersed be a solid of the specific gravity of the water, then will the solid sink into the fluid until it is entirely immersed. For example, a cubic foot of African oak weighs 621b., a cubic foot of fresh water weighs 6241b., and consequently, if a cubic foot of African oak were placed in fresh water, it would nearly sink to the level of the surface ; but a cubic foot of sea water weighs 641b., and consequently, if a cubic foot of African oak were placed into sea water, it would sink until 621b. of the fluid were displaced (which would be less than a cubic foot), and would sink no deeper, so practically 21b. of the oak cube would remain above the surface.
This well illustrates the meaning of the term " a vessel's displace ment." A vessel weighs, we will say, with all her ballast, spars, sails, gear, stores, crew, and everything belonging to her on board, one ton ; then, if she is put into the water, she will displace exactly one ton of the fluid. Now a ton of sea water in bulk contains 35 cubic feet; consequently, if the bulk of the vessel only equalled 35 cubic feet, she would sink into the water until entirely immersed. But a vessel that weighed one ton would contain in bulk a great deal more than 35 cubic feet, measuring her actual body on the outside from keel to deck as if she were a solid ; that is, the whole body or bulk of the vessel so measured would probably equal 50 2 cubic feet. The result would be that the vessel would sink into the water until 35 cubic feet of the hull became immersed, and sink no further; this would leave 15 cubic feet above water.
The buoyancy of a vessel may be taken as a force equal to the weight of water it displaces ; or, in other words, any given weight of fluid will support a similar solid weight of equal bulk. The quantity, or bulk, of a fluid which a vessel will displace depends upon the density of that fluid, as previously explained. Thus sea water is denser, or more buoyant, than fresh water ; and, consequently, a cubic foot of sea water will support a greater weight in the same bulk than a like quantity of fresh water.
Mercury is a fluid so dense that even iron will float in it with only a little more than half its bulk immersed, for the reason that a cubic foot of mercury weighs 8491b., whereas a similar bulk of iron only weighs 4801b.
Thus the displacement of a vessel is always equal to her own weight, including everything and everybody on board ; and providing that the bulk or size of the body of water displaced is smaller than the bulk or size of the vessel (regarding her from deck to keel), then a portion of the vessel will always be above the surface of the water, and this portion of a vessel is called her freeboard, and sometimes " surplus buoyancy." The truth of the foregoing can be demonstrated by a simple experi ment. Take a large basin, such as A (Fig. 1), and fill it carefully to the brim with water, and stand it in the saucer, C. Then take a smaller basin B, and put it into the water, which of course will overflow into the saucer. If the water that so overflows and the small basin be afterwards put into a scale and separately weighed, they will be found to be exactly equal; and, further, if shot or other substance be put into the small basin B whilst it is floating, still more water will overflow, and if the whole of the water which so overflows be weighed, and the small basin and its contents be weighed, their respective weights will be proved equal.
This experiment can be utilised to arrive at the displacement of the ship from that of the model. Thus say a model of the Kriemhilda is made to half an inch scale (or one twenty-fourth of her real dimensions), and put into a trough filled with salt water to the aperture of a waste pipe ; then as the model became immersed, the water would escape by the waste pipe into some vessel, say a large bucket. The escaped water in this particular case would weigh 18.651b. Now the displacement of the ship to that of the model is simply as the cube of the difference in the dimensions ; or say the scale for the ship is twelve times greater than that for the model; then the displacement or weight of the model multiplied by the cube of 12 would give the displacement of the ship. In the case given above, the scale for the real yacht was to be twenty-four times greater than that of the model, the weight of which was 18651b. ; the cube of 24 is 13,824, and 18.651b. multiplied by 13,824 is equal to 257,8171b. There are 22401b. to one ton; then 257,817 divided by 2240 gives a quotient of 115.59 tons, the exact displacement of Kriemhilda. If the model is made to other scales the displacement can be found by a similar process.