« PreviousContinue »
Qualities sought in a ship.
THE qualities sought in a ship depend mainly on the fact, that the ship, with her burden, has to be safely and steadily carried by, and propelled through, the water, her movements being at all times under control. Setting aside, then, for the present, the qualities of strength and durability, the qualities of a ship which conduce to her efficient support by and propulsion through the water may be thus summed up
Buoyancy, to enable her to carry her burden without either sinking too deep in the water, or floating too lightly on it.
Stability, that she may tend to "right herself" when disturbed from an upright position, and may never, under the action of winds, waves, or other disturbing causes, deviate further from that position than is consistent with convenience and safety; and also, that her movements may neither be so extensive nor so abrupt as to strain or damage her structure or contents.
Speed sufficient for her purpose, with due regard to economy in the means whereby such speed is obtained.
The quality of working well, which it would be difficult to find any single word to express. In a vessel propelled by steam alone, it consists chiefly in ready and quick answering to the helm; in a sailing vessel, it embraces also "weatherliness," and the performing of various manoeuvres with promptness and certainty.
All those qualities depend mainly on forces exerted between the ship and the fluids by which it is surrounded-viz., the water and the air; and therefore the means of obtaining them depend to a great extent on principles belonging to the sciences of hydrostatics, or the balance of fluids, and hydrodynamics, or the motion of fluids. The practical application of those branches of science is commonly known by the term "hydraulics."
Besides knowing how to obtain separately each of the qualities that are sought in a ship, it is necessary that the naval architect should know how to combine those qualities in the manner best. suited for the use to which the ship is to be applied, and how to insure that the means adopted for obtaining one of them shall not be injurious to the others. This is the kind of knowledge whose application constitutes Design in naval architecture.
The following seven chapters will be devoted to a general account of the principles to be observed in designing a ship, leaving the details of their application to be explained further
THE buoyancy of a ship depends on the following principles : I. That in order that a body may remain steady in a given position, the forces acting upon it must be balanced; which, in the case of there being two forces, means that they must be equal, and directly opposed, to each other.
II. That a body plunged into a still fluid is urged downwards by its own weight, and pressed upwards by the fluid with a force equal and opposite to the weight of the volume of fluid which the body displaces.
III. That consequently, in order that a given body, such as a ship, may float steadily in a given position in smooth water, the weight of the volume of water displaced must be equal to the weight of the body; and the total upward pressure of the water,
which is equal and opposite to the weight of the water displaced, must be directly opposed to the weight of the body.
The quantity of water displaced by a ship is called her displacement, and may be expressed either by its volume (for example, as so many cubic feet), or by its weight (for example, as so many tons, a ton being the weight of 35 cubic feet of ordinary sea-water, or 35.9 cubic feet of fresh water).
The principle No. II. above stated is easily demonstrated by the following reasoning. The total pressure exerted on the solid body by the neighboring particles of fluid is the same with that which was previously exerted on the mass of fluid whose place
the solid body occupies; and that mass of fluid was in equilibrio; therefore the total pressure exerted on it was equal and directly opposed to its weight.
That the weight of the water displaced by a floating body is equal to that of the body and all its contents, may be experimentally proved by apparatus within the reach of every one.
Take two vessels, A and B, as represented in Fig. 1; place one within the other, and fill the upper one with water to the brim; then take another empty vessel, C, and lower it gradually into the water in B, until it is supported by the pressure of the water. When C is at rest, a volume of water equal to that displaced by it has run over into A; and if this water be placed in one side of a pair of scales, and the vessel C in the other, they will be found to balance each other. Replace C in the water, and gently drop some heavy material, such as sand or shot, into it, and more water will overflow; remove C with the material it contains carefully to one end of the balance, and add to the water before put in that which was caused to overflow by the introduction of the material into C, and, as before, the water will balance the vessel and its contents. This may be often repeated, until C sinks nearly to its upper part, and it will be found in every experiment that the weight of the water which has overflowed from B is always equal to that of C, and of the material it contains, provided great care be taken that none of the water is lost, and that none adheres to the outside of the vessels. From these practical proofs of the equality which always exists between the weight of the floating body with its contents and that of the water displaced, we also learn that for every weight put on board of a ship there is an equal weight of water displaced by it.
In consequence of the necessity for this equality of the weight of the ship and of the water displaced, we perceive that, if the water in which a ship floats at different times differs in density, there will be a corresponding difference in the immersion of the ship. Now, the weight of a cubic foot of sea water is a little more than 64 lbs., and the weight of a cubic foot of river water about one-fortieth less. A line-of-battle ship, when ready for sea, weighing about 5,000 tons, requires 26 tons to increase her immersion one inch; consequently, were such a ship to come direct from sea into the river, keeping precisely the same weights 5000 on board, she would sink inches, or 5 inches (very nearly) 40 × 26