One difficulty in the use of the screw as an auxiliary in sailing-ships is that in a good wind the screw seriously impedes the sailing. To prevent this, various devices ape resorted to. In some eases, the screw is disconnected from the shaft, and left to revolve freely; in others, as in most ships of war, it is disconnected and hoisted alto gether out of the water by means of an iron framework worked above the screw in a sort of well. Messrs. Maudslay have patented a " featbering-screw," which, by a simple apparatus, can. when the steam-power is not required, have the blades turned into a line with the ship's keel, and the screw (which must be two-bladed) fastened•n a verti cal pAsition. When thus ifeated, time screw is out of danger, and forms no impediment ;o time ship's progress.
The usual position for the screw is immediately before the stern-post, the shaft on it revolves passing, parallel to the keel, into the eitoine-room. Many vessel6 have been built, especially by Messrs. J. & W. Dudgeon, of London, with two screws, one under each quarter. These have independent action. and as one can therefore be reversed while the other goes ahead, great steering-power is imparted; so much so, that vessels constructed on this principle are said to be able to turn in their own length. For a given power, a twin-screw draws less water, owing to the lessened diameter of the propellers, an ordinary screw-steamer. As the action of the screw depends on the comparative immobility of the water in which it acts, it is necessary. for the ment of its full power, that it should be completely immersed, and that there should be nearly 2 ft. of water above the top of the upper blade. It follows from this that, eeeris paribus, the screw-vessel will draw more water thal the paddle-steamer; for in large steamers the screw is frolA 15 to 18 a. in diameter, and in the Great Eastern it reaches 24.
It now only remains to notice the comparative advantage of the paddle and screw. Under favorable circumstances, in ships or equal tonnage and power, there is little dif ference in speed or force. Before the wind, the paddle has a slight advantage; with the wind ahead, the resistance offered by the paddle-boxes transfers the advantage to the screw. Fastened stern to stern, the screw-ship drags the paddle-ship; but fastened bow to bow, the same result is not found. This is, however, rather to be attributed to
the less of power in a paddle ship when not in progress (see PADDLE than to any actual superiority of screw. In a long voyage, however, the gain is distinctly with the screw; because the weight of fuel borne at starting sinks the paddles too low in the water, and probably its exhaustion at the end of the voyage deprives them of their proper dip; whereas, with ordinary- management, the screw will always be immersed. Again, rolling deprives the paddle of much power; while pitching deprives the screw of its proper matrix; but the balance of loss in tempestuous weather is iu favor of the screw. It has been already shown that in men-of-war the seew is the most useful agent; and as an additional reason may be addnued the clear broadside which it allows for the guns. On the other hand, in point of comfort to the passengers, the advantage lies unquestionaffiy with the paddle; for the rapid revolution of the heavy screw on a shaft extending half the ship's length, produces a continuous and very unpleasant vibration; while the lower position of the engines and screw gives the vessel a deep roll. F or lakes and rivers, where the water is smooth and the voyage short, paddles are best, and more especially so when the water becomes often shallow or is choked with weeds, .which would soon clog the screw.
In scientific language the motion of a vessel by means of a screw, is said to be clue to the forward reaction of the water in which the propeller revolves upon the blades, and through them upon the whole vessel. In order that this useful reaction may bear the largest possible ratio to the work done by the engine, it is essential that the form of the ship aft should he such as will secure that plenty of water shall always have access to the forward side of the screw as the vessel goes along. This has been demonstrated by the experimental alterations in the after-body of the DIraV (1846), and still more strikingly by placing a disk of the same diameter as the propeller in front of it. If the propeller be worked in these circumstances, the vessel will not move forward at all, although the power given out by the engines remains as before.