To design a ship in this case and every other is plainly a matter of compromise, a quest of the optimum by a balancing of demands for safety, strength, speed, capacity, handiness, good behavior in a sea-way, so that each invested dollar may in the long run earn the largest return possible. Excellent examples of judicious design are the best passenger steamers plying between Europe and New York. Usually their section amidships is like that of a cargo vessel, but for a special reason. Within the freighter’s walls the greatest feasible cross-section must be created; so that the shape is box-like; in a high-speed passenger ship the form is also square, because harbors are shallow; were they less shallow the designer would choose a midship section somewhat semicircular in contour. Were our harbors deepened, the easy sections of the first transatlantic steamers could be repeated in their gigantic successors of to-day, with increased speed for each horse power employed.
What a designer can do when his aim is swiftness at the expense of all other considerations, is shown in the lines of the torpedo-boat destroyer, page 62. Its length over all is 246 feet; length at water level, 240 feet, 10 inches; beam, 22 feet, 3 inches; mean draft, 6 feet, 11⁄2 inches; displacement, 489 tons; speed, 30 knots. It is interesting to contrast, on page 63, the [cross-section] amidships of this vessel, with similar lines of three other typical vessels described in this chapter.[6]
[6] In writing these pages on the forms of ships I have been much indebted to Mr. Harold A. Everett, Instructor in Naval Architecture, Massachusetts Institute of Technology, Boston.
G. I.
CHAPTER VI
FORM—Continued. SHAPES TO LESSEN RESISTANCE TO MOTION
Shot formed to move swiftly through the air . . . Railroad trains and automobiles of somewhat similar shape . . . Toothed wheels, conveyors, propellers and turbines all so curved as to move with utmost freedom.
Projectiles and Vehicles of Like Pattern.
While ships are much the largest structures built for motion, and therefore meet resistances which the designer must lessen as best he may, other moving bodies, small as compared with ships, encounter resistances so extreme that their reduction enlists the utmost skill and the most careful study. Speeds vastly higher than those of ships are given to projectiles. A ball leaving a gun muzzle with a velocity of 3,410 feet a second, as at Sandy Hook in January, 1906, suffers great atmospheric resistance, overcome in part by the shot having a tapering or conoidal form. Indians long ago stuck feathers obliquely into arrows so as to keep flight true to its aim by giving shafts a spiral motion; an attendant advantage being to lengthen flight. The same principle appears in rifling, that is, in cutting spiral grooves in the barrels of firearms large and small, a missile receiving a spinning motion through its base, a thin protruding disk of soft metal, forced into the grooves by the explosive. At first the grooves in firearms were straight with intent to preclude fouling; spiral grooves were introduced by Koster of Birmingham about 1620. Delvigne, a Frenchman, devised a lengthened bullet narrower than the bore so as to enter freely, under the pressure of firing it completely filled the bore, rotating with great velocity as it sped forth.