CHAPTER V
FORM—Continued. SHIPS
Ships have their resistances separately studied . . . This leads to improvements of form either for speed or for carrying capacity . . . Experiments with models in basins . . . The Viking ship, a thousand years old, of admirable design . . . Clipper ships and modern steamers. Judgment in design.
Forms of Ships Adapted to Special Resistances.
In giving form to a ship a designer has a three-fold aim,—strength, carrying capacity and speed. Strength is a matter of interior build as much as of external walls; it is conferred by girders, stays and stiffeners which we have already considered, so that we may here pass to the general form of the hull, which decides how much freight a ship may carry, and, to a certain extent, how fast she may run. A ship is the supreme example of form adapted to minimize resistance to motion; its lesson in that regard will be the chief theme of this chapter. Until the close of the eighteenth century the resistance to the progress of a ship was regarded as a single, uncompounded element, plainly enough varying with the vessel’s speed and size. It was Marc Beaufoy, who first in 1793 in London, pointed out that a ship’s resistance has two distinct components; first, friction of the shell or skin with the water through which the vessel moves, dependent upon the area of that skin; second, resistance due to the formation of waves as the ship advances, dependent upon the speed of the vessel and the shape of her hull. Other resistances have since been detected, but these two are much the most important of all; each varies independently of the other as one ship differs from another in form, or as in the same ship one speed is compared with another. To take a simple case: a ship’s model of a certain form, of perfectly clean skin, is towed at various speeds and the pull of the tow-line is noted; then the same model with its skin roughened and covered with marine growths is towed at the same speeds, and much greater pulls are observed in the tow-line. The wetted surface is the same in the two series of experiments, the speeds correspond throughout, and the increase of resistance due to a roughening of surface can only mean that the friction between the water and the submerged skin has increased. Next we take a model of certain form and definite size, and a second model having the same area of wetted surface but a different form; we tow both models at the same speed to find that one requires a decidedly stronger pull than the other. This difference cannot be due to frictional resistance of surface, for this is the same in both models, therefore it must be due to the increased resistance offered by the water as it is pushed aside, a resistance measurable in the created waves. Mr. Edmund Froude, an eminent English authority, says:
“For a ship A, of the ocean mail steamer type, 300 feet long and 311⁄2 feet beam and 2,634 tons displacement, going at 13 knots an hour, the skin resistance is 5.8 tons, and the wave resistance 3.2 tons, making a total of 9 tons. At 14 knots the skin resistance is but little increased, namely 6.6 tons; while the wave resistance is nearly double, namely, 6.15 tons. Mark how great, relatively to the skin resistance, is the wave resistance at the moderate speed of 14 knots for a ship of this size and of 2,634 tons weight or displacement. In the case of another ship B, 300 feet long, 46.3 feet beam, and 3,626 tons displacement—a broader and larger ship with no parallel middle body, but with fine lines swelling out gradually—the wave resistance is much more favorable.[4] At 13 knots the skin resistance is rather more than in the case of the other ship, being 6.95 tons as against 5.8 tons; while the wave resistance is only 2.45 tons as against 3.2 tons. At 14 knots there is a very remarkable result in this broader ship with its fine lines, all entrance and run and no parallel middle body:—at 14 knots the skin resistance is 8 tons as against 6.6 tons in ship A, while the wave resistance is only 3.15 tons as compared with 6.15 tons. The two resistances added together are for B only 11.15 tons, while for A, a smaller ship, they amount to 12.75 tons.”
[4] The entrance is that part of the ship forward where it enters the water and swells out to the full breadth of the ship; the run is the after part from where the ship begins to narrow and extending to the stern. A ship may consist of only entrance and run; it may have a middle body of parallel sides between the entrance and run. Such a middle body is discussed by Lord Kelvin in “Popular Lectures and Addresses,” Vol. III, Navigation, p. 492.
Experimental Basins.
These figures show that a designer must bear in mind the speed at which this ship is to run; they prove that he may choose one form to minimize friction, or another form if he particularly wishes to bring wave-making resistance to the lowest possible point. Forms of these two kinds are readily studied when represented in models 12 to 20 feet in length towed through tanks built for the purpose. Experiments of this kind were undertaken as long ago as 1770, in the Paris Military School; the methods then inaugurated and copied in London at the Greenland Docks were greatly improved by Mr. William Froude in a tank which he constructed at Torquay in England, in 1870. His modes of investigation, duly adopted by the British Admiralty, and after his death continued by his son, Mr. Edmund Froude, have created a new era in ship design. To-day in Europe and America there are eleven such tanks as Mr. Froude’s, all larger than his and more elaborate in their appliances. In addition to learning the behavior of models diverse in type, Mr. Froude worked out the rules which subsist between the performance of a model and that of a ship of like form; these he brought to proof in 1871 when he towed Her Majesty’s Ship Greyhound, and verified his estimates in towing its model. The rules concerned, known as those of mechanical similitude, are given in detail by Professor Cecil H. Peabody in his “Naval Architecture,” page 410. While experiments become more and more valuable as one refinement succeeds another, there is always much well worth knowing to be learned from the actual behavior of a vessel as she takes her way through a canal, a shallow river, or the storm-beaten stretches of the sea.
The experimental tank of the United States Navy at Washington, is 470 feet long, 44 broad, and 141⁄2 deep; it is arranged for models 20 feet in length. See the page opposite. The towing carriage is a bridge spanning the tank just above the water; it is a riveted steel girder. The towing mechanism, of massive proportions, is driven by four electric motors of abundant power. A double set of brakes brings the carriage gradually and quietly to rest from a high speed. A self-acting recorder measures both speed and resistance. Ship builders may have models built by the Bureau in charge, that of Construction and Repair of the United States Navy Department, and have these models towed at any desired speeds, paying simple cost.