64

ON THE PROPULSION OF STEAMERS.

The ‘ Gorgon, ’ an English steamer, had “ large wheels and little power,” so she'used oak or pine scantlings, 5 inches by 6, or 6 by 8, for paddles. Had her mana-gers been aware of the true effect of thick blades, they never would have adoptedthem with the view of economizing power.

Paddle planks vary in thickness from 1^ to 3 inches: no sea steamers havethem less than 2 inches. In the English vessels they are 2 \; in others, as the‘ Franklin,’ they are 2J ; in some of the largest class they are 3. The ‘ Atlantic ’ andthe ‘ Pacific,’ each of 3000 tons, for the Collins’ line, are to have them 3 inches.The former is to have twenty-eight blades; hence, united, they will form a solidmass, 7 feet thick in each wheel—just one-fifth of its diameter: they are tobe 12J feet long by 34 inches: those of both wheels will therefore contain nearly500 cubic feet of timber, and must displace that enormous volume of water at everyrevolution by their submersion alone ; and, as we have seen, not only uselessly, butwith a serious retardation of the vessel’s headway, and waste of her motive power.

The wheels of the ‘ Pacific ’ are 36 feet in diameter: each will have thirtyblades llj feet by 3 feet: the solid contents of her paddles will therefore equal 517cubic feet; her loss from the same source will consequently be greater. In everyrevolution of each of her wheels her paddles will lose 7|- feet of effective stroke, andthose of the ‘ Atlantic ’ 7 feet. Those of the ocean steamer ‘ United States ’ are 2^or 2 % inches thick ; they are thirty-six in number, but as they are split and attachedon both sides of the arms, there are really seventy-two. The effective stroke of herblades is certainly diminished from 10 to 15 feet in every turn of each of her wheels,startling as the assertion is.

Has the attention of engineers ever been turned this way ? or, have they for-gotten that a volume of water equal to that of a boat’s paddles, and every inch ofmaterial submerged with them, is neutralized as a resisting medium as often as it isdisplaced by their immersion,—that water is to them what steam is to pistons ? Themore space the latter occupy in cylinders, the shorter becomes their stroke, becausemetal then takes the place of steam; the object to be moved crowds out the mover.Thicken a piston till it fills its cylinder, and the motive agent being wholly kept out,all motion ceases.

It is much the same with the paddles of a wheel: let them fill up ^- 0 , or £of the circle they describe, and in those proportions they lose their virtue, because

do no harm, if they do no good: they add weight to the wheel, which is desirable, and their only dis-advantage is the additional load on the boat.” I believe this is the general, if not the universal, opinion ofengineers. But the experiments just referred to teach us, that if awheel require loading, the load shouldbe attached to those parts of the arms that revolve about the surface. They cannot enter the waterwithout becoming drags on the blades.