Rate of Paddle, that of Ship being 1.
| Medea | 1·595 |
| Flamer | 1·483 |
| Firebrand | 1·501 |
| Columbine | 1·529 |
| Salamander | [200]1·200 |
| Dee | 1·366 |
| Firefly | 1·364 |
| Firebrand, as altered | 1·295 |
| Phito | 1·215 |
| Monarch | 1·323 |
| Magnet | 1·310 |
| Meteor | 1·490 |
| Carron | 1·287 |
| Average | 1·381 |
| Great Western | 1·27 |
| Archimedes | 1·29 |
This list shows that the result in the ‘Great Western,’ with which ship I have made the comparison, is in itself a favourable one, and that compared with many others the ‘Archimedes’ would stand much better.
This apparent superiority of the screw over the paddle as regards the resistance offered to it by the water may at first appear startling, but there is a great mistake committed in assuming that the action of the screw is a very oblique action, tending rather to drive the water laterally with a rotatory motion than to push it steadily backwards.
Having witnessed and carefully observed the degree and the nature of the disturbance in the water caused by the screw, and comparing this with the violent displacement of the water by the action of paddle-boards, even under the most favourable circumstances, I no longer feel surprised.
The mass of water pushed backwards by the action of the screw appears to be very large, spreading from the screw probably in the form of an inverted cone, but there is little or no appearance of any rotatory motion, and the surface of the water is not put into rapid motion as in the case of the paddlewheel, which may be observed to impart a considerable velocity to the water, probably for a small depth only, but over a very large space.
As regards the oblique action also, a great mistake appears to have been generally made, and very naturally made, by most persons when first considering the working of the screw. It is generally assumed that the inclined plane formed by the thread of the screw strikes the particles of water at that angle and with the velocity of the revolution of the screw, but it is forgotten that the screw is moving forward with the ship, and therefore that the angle at which the water is struck by the plane is diminished by all that much that the ship with the screw advances—indeed, it is evident that if the ship advanced the whole amount of the pitch of the screw, the screw, oblique as it appears, and rapidly as it revolves, would not strike the water at all, but simply glide through.
The angle at which any given part of the screw does in fact strike the water is only equal to the difference between the angle to which that part of the screw is formed and the angle or direction in which it moves by the compound motion of the revolution of the screw and of the forward motion of the ship and screw; and, contrary to one’s hastily imbibed notions of the action of the screw, this angle at which the plane of the screw is driven against the particles of water, is in such a screw as that of the ‘Archimedes’ very nearly equal over the greater portion of the surface, diminishing to nothing at the centre; and the motion imparted to the water, although perpendicular to the plane of the screw in point of direction, is small in extent or velocity, being also nearly the same over the whole surface of the screw, except close to the centre, where it is infinitely small.
In the ‘Archimedes’ screw, which appears to the eye so oblique, and the centre part of which would appear to act flat against the water, only causing it to revolve, the outer circumference being 18 feet and the slip 1 foot 8 inches, the angle at which this outer edge acts upon the water is only one in 11½.
The total amount of motion imparted to the water at right angles to the plane of the screw by one entire revolution even at the outer edge is not quite equal to the slip, being only 1·67 foot. The rotatory motion is still less, the total distance to which any particle of water is displaced laterally, or at right angles to the axis of the ship, by one entire revolution of the screw being at the outer edge only 0·69 foot, and the maximum distance being in any screw only equal to half the slip, and occurring at that part of the screw where the circumference is equal to the advance of the ship due to one revolution. This maximum of lateral motion is 0·9 foot, and takes place at 0·99 foot, or about 1 foot from the centre. In this mode of considering the direction at which the particles of water are acted upon by the plate of the screw I have taken no notice of the effect of the friction upon the surface of the screw, which, causing to be carried with it a film of water, will modify more or less according to the degree of smoothness of the surface the effect of the screw upon the water; and towards the centre this friction, however smooth the surface may be made, will gradually become equal to, and at last greater than, the propelling effect of that part of the screw; but this defect applies only to a very small portion of the whole area of the screw, and the absence of any very violent impulse to the water in a direction approaching to a right angle with the axis of the vessel, and which has always been assumed as an unavoidable evil in the screw, will account for the absence I have observed upon of any apparent rotatory motion.