[Footnote 1: The constructors and engineers of the navy are unsurpassed in professional art or science, and when conjoined with naval officers—who should always determine the war-like essentials of ships—they are capable of producing a steam-fleet that would meet the requirements of all reasonable conditions. We venture to say, that the failures with which they have been charged would be found, on investigation, to be solely attributable to undue extraneous influences.]
Let it be distinctly understood what power is here meant. As the power applied to the propulsion of a vessel is only that which acts upon her in the direction of the keel,—and as, of the gross indicated power developed by her engine, one portion is absorbed in working the organs of its mechanism, another in overcoming the friction of the load, while still other proportions are expended in the slip of the propeller and in the friction of its surfaces on the water,—only that portion of the gross power which remains is applied to propulsion; and it is this remainder which varies in the ratio of the cube of the speed.
Hence a steamer, that with five hundred horse-power can make eight knots per hour, will require rather more than one thousand horse-power to drive her at the speed of ten knots,—the law being thus modified by the increased resistance consequent upon the greater weight of the large engines; and thus a limit to speed is imposed, depending upon the weight of machinery which, relative to her dimensions, a ship can carry. A ship, that at the rate of ten knots under steam may run twelve hundred miles, can, at the speed of eight knots, and with the expenditure of rather less fuel, run the distance of eighteen hundred miles; and therefore it is, many contend, that a man-of-war for distant service should not be laden with large engines, whose full power can rarely be wanted, and which monopolize so great a space and displacement as to render it impossible to carry fuel for their proper development.
It is true, that, with large power of engine, the vessel may command, so long as her coals last, the advantage of high speed, and her large cylinders will enable her, by working the steam very expansively, to use her fuel with great economy; but there still remains the disadvantage of the increased first cost of the machinery, and its greater weight and bulk, to be permanently carried, whether used or not, and which, by increasing the displacement of the vessel, proportionally diminishes her speed.
The last great improvement in connection with the screw remains to be noticed, namely, lining the "bushings" and "bearings" with lignum-vitae,—the invention of Mr. Penn, of Greenwich, near London.
The lignum-vitae is introduced in the manner shown in the drawing. In connection therewith, it must be said, that the length and diameter of bearings has been increased far beyond the proportions of former years. The "brasses" are bored out about three-sixteenths of an inch larger than the shaft; then the recesses are slotted out for the reception of the wooden strips. If care be taken with this part of the operation, any number of strips can be supplied ready fitted, and to put in a set of spare strips becomes a short and simple operation.
[Illustration]
Strange as it appears, these wooden bearings are far more durable than those of metal, and in some ships they have endured for years without any perceptible wear in those parts which, previously to this invention, had occasioned so much trouble and expense. But for this important discovery, it is thought by some of the most competent engineers that they would have been compelled to abandon the use of the screw in heavy ships.
The Napoléon, the type of the new steam-ships of the line in the French navy, is a good illustration of a first-class, full-powered steamer.
Her dimensions are as follows:—