SCREW-PROPELLER, ME, is of the same construction as the common screw (q.v.), but with the narrow thread exaggerated into a broad, thin plate, and the cylinder dimin ished to a mere spindle. One complete turn of such a screw is shown in fig. 1. Now if a screw of this form were turned round in an unyielding substance, as wood, it would for each turn advance as much as the center of the blade (or thread) had moved along the spindle in forming the screw, i.e., the distance ab. If, on the other hand, the screw itself were prevented from moving longi tudinally, and the piece of wood not fixed, the latter would be compelled to advance along the screw the same distance ab. When the screw is fixed beneath a ship, and made to revolVe in the water, the case lies between the two just supposed, the screw moves forward, and with it the ship, and the water in which it has been working moves backward. The backward motion should only be small proportionately, and the ratio between it and the sum of the backward motion of the water and the forward motion of the ship is called the slip, which in well-designed vessels has a value of from 0.1 to 0.25.
It is obvious also that on the same spindle there may be more than one blade, pro vided that all the blades have the same pitch or rate of progression along the spindle (in fig. 1 ab is the pitch of the screw). Screws have thus been formed with two, three, four, and six blades or arms, but the form most commonly used is two blades for ships of war. and three or four blades in the merchant service.
H the screw be cut off before attaining the length ab of a whole convolution, as at e, the portion ac will still retain all the properties of the screw. In the earlier attempts screws were tried of the length of a whole convolution, or even two whole turns; but experiment has since shown that this length is a disadvan tage. The best results are obtained when the sum of the lengths, measured parallel to the center line of the shaft of all the blades, is equal to about 0.4 of the pitch. This holds equally good for two, three, or four bladed propellers, so that if n equals the number of blades, then the length of one blade, or ac, would be expressed by the equation ae = ab . A four bladed screw of this kind, and of a form very generally used in the mer chant navy, is shown in fig. 2.
The following are the technical terms applied to the screw-propeller: The shaft is the cylindrical axis on which the screw revolves, and is the medium for communicating to it the power of the steam-engine; the blade is the thread of the screw; the pach, the length of shaft on which the blade would make one complete turn; the diameter is the distance between Uri tips of opposite blades; and the length is the distance from the front to the back edge of blade projected upon a fore and aft plane.
The application of the screw to the propulsion of a vessel through the water is not nevi. In 1802 Dr. Shorter, an English mechanician, produced motion by its agency; but his discovery was valueless, as the steam-engine had not then been practically r )plied to navigation. Those who first employed Watt's engine on board ship adopted .e paddlewheel, the success of which turned attention from the screw for nearly thirty 1.-ars. At length, in 1832, Mr. B. Woodcroft patented a screw-propeller with an increas ing pitch; and four years later, Mr. F. P. Smith patented a screw making two whole
turns, which he reduced in 1839, to one whole turn. In 1S37 he and cap. Ericsson, an American inventor, brought the matter practically forward on the Thames, where a small screw-steamer, 45 ft. long, 8 ft. broad, and of 27 in. draught, towed the Toronto of 630 tons against tide at 4 knots an hour. In 1839 an American gentleman had the Robert Brockton built for him by Messrs. Laird, with which he reached America. The British admiralty, however, refused any support to the new propeller, until the success of the Archimedes, built in 1838, of 232 tons and 80 horse-power, which was exhibited at the principal ports, rendered opposition no longer possible. The admiralty, then, as an experiment, constructed the Rattler, from the trials of which vessel many valuable data for the screw-propeller have been derived. Meanwhile, in 1838, Mr. James Lowe had shown that the length of the screw should not exceed 3th of the pitch; and after actual and successive trials, the screw of the Rattler was cut down from 5 ft. 9 in. to 1 ft. 3 inches. These experiments established the screw as a rival to the paddle-wheel; and its advantage for ships of war became incontestable, as, from the entire submer gence of the propeller, and consequent lowness of its engines in the ship, the chances of injury from an enemy's shot were reduced almost to nothing. Some of the great steam-companies—notably the Peninsula and Oriental company—also patronized it, and it was found of great value as, an auxiliary in Sailing-vessels. The result is that, at this time, its use in the British navy is almost universal, except in cases where want of sufficient depth of water, or other special circumstance, causes the paddle-wheel still to be employed.' Several varieties of screw have been introduced, each finding many supporters. The one which was for many years used in the British navy wits invented by Mr. B. Grif minis. In it the blades, in place, of rising from a small boss, as in fig. 2, spring from a 1 ollow sphere occupying the screw's diameter. This arrangement was adopted because experiment proved that the central portions of the blades of the ordinary screw absorb about 20 per cent of the propelling power, while they prcduce little useful effect, from the circumstance that at that part (especially in screws of a coarse pitch) the blade is nearly in a line with the shaft, and acts at right angles on the water, causing only a disturbance of that portion on which time outer and more powerful end of the blade operates. The globe, on the other hand, revolves with little friction. A further im provement was effected by bending the tips of the blades a little over backward, so that the face of the blade striking the water was partly convex. The older propellers had blades which increased in width uniformly from boss to tip. These were found to create much vibration in the ship, and the "leading" corner is therefore rounded away. as shown in fig. 2. This is also done in Griffiths's propeller, but he probably carries the principle to excess in cutting away also the "following" corner, and so lessening the effective surface of the blade. A propeller invented by Mr. Hirsch, and known by his name. has been lately successfully tried by the Admiralty, and may probably be much used by them in future.