A leading idea in the conception of these ships was to reduce the number of gun-decks from two and three to a single deck, and, consequently, the space in which shells could be lodged. This is a consideration which must, it is believed, sooner or later govern in naval construction; although France and England, long accustomed to measure the power of ships by the number of gun-decks, may be more slow in following our lead in this respect than in imitating the increased calibre of our ordnance.
The new classes of steamers preparing for sea, of which the Hartford and Iroquois are types, promise to be most efficient ships, and to reflect much credit upon our naval authorities for their bold, yet judicious departure from traditions which had long hampered the administration of this important branch of the public service. Although the reflection is seldom made, it is nevertheless true, that much of the reputation enjoyed and of the influence exercised by the United States is due to the efficiency of her navy; and if these are to remain undiminished, then it is of the utmost consequence that the national ships should always represent the highest advancement of nautico-military science.
[Footnote 1: A series of experiments with the screw were made on board the Dwarf in 1845, and on board the Minx in 1847 and 1848, but the results did not materially differ from those previously obtained. In the Rattler, Dwarf, and Minx twenty-nine different propellers were tried.]
The efficiency of the screw having been demonstrated, it was seen that the next requirement for a war-steamer was to place her machinery below the waterline; and hence arose a demand for an entirely new description of engines, which it was clear would make a great change in all the labors of the engineer and machinist. Such change it was evident would greatly enhance the risk of failure, and therefore it was determined by the Admiralty to insure success in this very difficult task by enlisting all the best talent of the country. Accordingly, for the twenty-three ships an equal number of screw engines were ordered; and as with the constructors, so with the engineers, each was required to comply with certain conditions, yet each was permitted to put forth his own individuality, and each has illustrated his views of what was required by a distinct plan of engine.
The wise and liberal action of the British Admiralty, which faltered at no expense, and made trial of every improvement in machinery that gave assurance of good performance and promised in any way to increase the efficiency of the fleet, produced no less than fourteen distinct varieties of the screw engine. Among them all, Penn's horizontal trunk-engine appears to be the favorite, and had performed so well in the Encounter of fourteen guns, the Arrogant of forty-six, the Impérieuse of fifty, and the Agamemnon of ninety, that two years ago it had been placed, in about equal proportions of two hundred, four hundred, six hundred, and eight hundred horse-power, on board of forty ships and many smaller vessels of the British navy; it had fulfilled all the promises made for it, without in any instance requiring repairs. These engines comply with all the conditions reasonably demanded in the machinery of a man-of-war; they lie very low, and the fewness and accessibility of their parts leave scarcely anything to be desired;—a lighter, more compact, or more simple combination has yet to be conceived.[1]
In all the ships above referred to the connection of the engines is direct, and many of them are driven at rates varying from fifty to seventy-five revolutions. This point is dwelt upon because it is observed that many engineers find difficulty in freeing themselves from early impressions made by long-stroke engines, express apprehensions at fifty and sixty revolutions, and stand ready to obviate the difficulty by gearing,—which it is hoped may not henceforth be adopted in our national ships. Geared engines are much heavier than those of direct connection, and occupy more space,—a great consideration in ships where room for fuel is in such demand, besides making it more difficult to place them below the waterline,—a consideration which in men-of-war should be regarded of paramount importance, as the engines of a war-steamer should be as secure from shot as her magazine. Experience has shown that the apprehensions entertained from the quick stroke of direct engines were without foundation; and that, in auxiliary ships, with a properly modelled propeller, there will be no necessity for a very high speed of piston.
The form of engine generally adopted with great success in the later screw-ships.
[Footnote 1: "Its large amount of friction" is an objection often speciously urged against the trunk-engine, although the friction diagram shows it to be actually less in this than in most other engines.] of the United States navy is the "horizontal direct action," with the connecting-rod returning from a cross-head towards the cylinder; these engines make from sixty to eighty revolutions per minute. The steam-valve is a packed slide with but little lap, and the expansion-valve is an adjustable slide working on the back of the steam-valve. The boilers are of the vertical water-tube type, with the tubes above the furnaces, and are supplied with fresh water by tubular surface-condensers, which, together with the air-pumps, are placed opposite the cylinders.
While the vessels ordered by the Admiralty were on the stocks, it was suggested by Mr. Lloyd that the model of their after-bodies was not that most favorable to speed,—that they were too "full," and that a "finer run" would be preferable. To settle this question, the Dwarf, a vessel of fine run, was taken into dock, and her after-body filled out by three separate layers of planking, so as to give it the form and proportions of the vessels then building. These layers of planking could be removed in succession, and the effects of a fuller or finer run upon the speed of the vessel easily ascertained. A trial was then made, and the result proved the correctness of Mr. Lloyd's opinion; the removal of the different layers of planking increasing the speed from 3.75 to 5.75, to 9, and finally to 11 knots. A trial between the Rifleman and the Sharpshooter, vessels of four hundred and eighty tons and two hundred horse-power, and the Minx and Teaser, of three hundred tons and one hundred horse-power, gave similar results,—the speed in each trial being twenty-four per cent. in favor of the finer run.
Although great efficiency and economy had now been attained, there was still an important defect to be remedied, namely, the impediment to speed and to evolution under sail presented by the dragging propeller; which was accomplished by the invention of the "trunk" or "well," into which the propeller can be raised at pleasure; and there is no longer anything to prevent the construction of a screw-frigate which shall be fit to accompany, under canvas only, a fleet of fast sailers, with the assurance that she may arrive at the point of destination in company with her consorts, having in reserve all her steam-power.