The consideration of the comparative efficiency of the screw as a means of propelling, of course embraces the whole question, not merely of the effect produced, but also that of the proportionate power absorbed in producing that effect.
With respect to the mere effect of a screw, the performance of the ‘Archimedes’ has proved, in a satisfactory and undeniable manner, that a screw acting against the water with a surface even much smaller than that offered by the paddle-boards of a well-proportioned paddlewheel, will propel the ship at a very fair speed, but at what expense of power this effect has been produced is not so evident.
I shall first examine into the principal cause of what amounts practically to a loss of power, and which is common in a greater or less degree to all modes of propelling a vessel by exerting a pressure against the water as against a fixed point.
The resistance, whether to the surface of a screw, or of a paddle-board, or of the blade of an oar, or any other propelling body, offered by the fluid against which it acts, is of course not perfect, and there is a certain amount of yielding, commonly called the slip, of the paddlewheel; the amount thus slipped causes a considerable waste of power, inasmuch as the full power of the engine is expended through the entire space passed over by the paddles or other propelling surface, while the useful effect produced is only equal to the same power expended over the space through which the vessel passes: this loss frequently amounts to one-quarter, and even one-third, of the whole power employed. To investigate theoretically the amount of slip due to any given form and quantity of surface, involves much more complicated calculations than have generally been applied, and would indeed require data which we hardly possess; but fortunately we have had the means of making experiments, the results of which enable us to determine the comparative slip of the paddle and of the screw, with sufficient accuracy for all practical purposes.
The screw in use on board the ‘Archimedes’ is 5 feet 9 inches diameter, with a pitch of 8 feet—that is to say, in making one revolution the thread of the screw advances 8 feet; the area of the screw, considered as a disc of the same diameter, or the extent of the surface of water which is acted upon in the direction of the axis of the vessel, is therefore about 26 feet, without deducting the section of the shaft-bearing, &c. The midship section of the vessel when I experimented upon her was, according to Mr. Patterson’s estimate, 122 feet; the ratio of the resisting surface to the midship section being therefore as 1 to 4·7, which is a small proportion; and the form of the vessel is by no means peculiarly good as a steamboat. This proportion of propelling surface to midship section is much smaller—that is, the area of the screw is much less in proportion to the size of the vessel than is the area of paddle-boards immersed in steamboats generally.
The average paddle-board immersed and really effective is rather difficult to estimate, as allowances must be made for the disturbance of the water, when the wheel is in motion; but this average in the ‘Great Western’ measured perpendicularly—that is, allowing for the obliquity of the paddle—cannot be less than 180 to 200 feet, say only 180, while the midship section averages about 462 feet; the surface of paddle is therefore about 1/2·56 of the midship section.
I will now give the comparative effects of these different propelling surfaces in these two cases.
I have made very accurate experiments upon the comparative rate of the ‘Archimedes,’ and of the space passed through by the screw, and was enabled to determine this ratio with great certainty.
The average of a number of trials gave the following results:
Rate of ship, 50,867 feet per hour, or about 8⅓ knots.