As the hull of an iron ship is both thinner and considerably lighter[122] than that of a wooden ship, an iron ship of the same external dimensions as a wooden one has both greater capacity for stowage and greater power to support the weights which may be put into her. These differences vary in some degree with the dimensions and form of the ship, being greater in proportion to the increased dimensions of the ship. They may, of course, be determined by computation; but, in all cases, an iron ship will carry considerably more cargo than a wooden one of the same external dimensions.
Again, the consideration of economy must not be omitted in any comparison of the merits of ships built entirely either of timber, or of iron. The economy begins with the construction, for the original cost of an iron ship is less than that of a wooden one, and, apart altogether from her superior capacity for cargo, it runs on with the course of the ship’s service as the result of several causes; as, for instance, the smaller amount and less expensive character of repairs: moreover, as it is not even yet known how long iron ships will last, the precise saving from their use cannot be estimated. On the other hand, the period of service of mercantile timber-built ships is defined. If they reach or exceed thirty years’ service, they must be ships of the very highest class as to quality[123], and must, indeed, within that period have undergone frequent and very expensive repairs. As iron ships are not subject to the same decay, at the same time that accidental damages are generally repaired at a much less cost, every item saved by the diminished charge for repairs is clear profit.
Admiralty slow to adopt iron for ships of war.
But with all these advantages, a considerable time elapsed before the Admiralty could be induced to consider the desirability of constructing any Government steamer of iron, or of even allowing the large private trading vessels engaged in the conveyance of the mails to be built of that material. They had objections of their own applying specially to the ships under their control, and very plausible objections too, in their opinion, compared with those originally raised by an ignorant public. A shot, they said, would penetrate an iron vessel with much greater ease than a wooden one, while the shot holes could not be as effectually plugged, if indeed they could be plugged at all. Wood, they argued, when pierced, would rapidly contract and leave a very small opening for water to get through, whereas a shot would make a clean cut through an iron plate which could not be thus expeditiously filled, and if it did not tear away the whole of the plate, would leave a gap as large as a “barn door.” However, a little experience[124] soon showed their arguments to be fallacious, and when they found that the engines of a paddle-wheel steamer, and, especially, the paddles themselves, offered conspicuous targets to an enemy, and that it was impossible to make the stern-frames of their wooden ships sufficiently strong to withstand, without serious leakage, the vibration of the screw, they abandoned, though reluctantly, the paddle-wheel, and at length gave up, also, vessels of wood for the purposes of war. These resolutions were, however, only carried into practice after vast sums of money had been expended on the “reconstruction” of a wooden British Navy, for which, in one year alone, and that so lately as 1861, when almost everybody except themselves saw that iron must supersede timber, they demanded from Parliament (and carried their vote) no less than 949,371l. to replenish the stock of wood in the dockyards: a sum far in excess of any previous vote for that material.[125]
Mr. Galloway’s feathering paddles, 1829.
While the art of steam navigation made rapid progress, the ingenuity of engineers had been constantly directed to the improvement of the paddle-wheels; and the above drawing of one, with “feathering paddles,” patented by Mr. Galloway in 1829,[126] represents the most perfect of any wheel in use at that period, and has not been materially improved on since then. But, at that time also, a substitute for the paddle was seeking practical solution. The screw, as a means of propulsion, had been suggested long before the steamboat had been brought into use. Indeed, its principle was known at a very early period in the use of an oar for sculling, and could, as already explained, be seen in the movements of the tail of a fish.
Story of the screw-propeller.
Though my faith in the reports of the genius and early inventions of the Chinese has frequently been rudely shaken in the course of my investigation of their reputed discoveries, I may remark that Mr. MacGregor, for whose opinions I entertain no ordinary respect, states, in the paper he read to the Society of Arts,[127] that “the use of the screw-propeller may be of an indefinite antiquity,” and adds that “a model of one was brought from China in 1680, which had two sets of blades, turning in opposite directions.” It was not, however, until 1729, that we have any authentic account of a plan of propulsion, in any way approaching the valuable invention now so largely in use. In that year an ingenious Frenchman, M. Du Quet, described a contrivance by which a screw turned by the water in a stream, wound up a rope for towing vessels, of which the annexed (p. 101) is an illustration.[128] In 1745, Masson describes an apparatus for working an oar at the stern of a vessel so as to give it a “sculling” motion; in 1746 Bougner mentions that “revolving arms, like the vanes of a windmill,” were tried for the propulsion of vessels, and, in 1770, as already incidentally noticed, the celebrated Watt speaks of using a screw-propeller, of which the annexed is a sketch, to be turned by a steam-engine.[129]
