Page:The New International Encyclopædia 1st ed. v. 17.djvu/788

* SCREW PROPELLER. 7U SCREW PROPELLER. roigli «atcr llii- vibialioii iiiiij- become exces- sive. I'oui-bliulcil siicnvs of largo tlianiftsr are •.;(iierally iisi-il ill the mercliant service tor slow- iiuiviug ongim'?. l'"or fast vessels, mercliant or naval, three bladeJ screws are the rule. The pilch of a propeller is its linear advance in one revolution, .supposing the water to be im- movable and the .screw to turn in it as i bolt turns in a nut. If we imagine the thread to ex- tenil sulliciently along the shaft to make one eoniplete revolution, the pitcli is equal to the length of shaft reipiired for this. In the true screw the pitch is cunslant at all points, but in propellers there are usiuilly some variations in this respect, particularly near the hub in those which are east and have small liubs. Many pro- pellers are designed to have slightly varying pitch at dillcrent parts of the blade, but the ad- vantages of this have never been conclusively de- termined. The hub, or boss, is now very com- monly s])lierical with a conical tail]iiece. Since the part of the blade near a relatively small hub is of little use, hubs are now made quite large, one-tifth to one-fourth the diameter of the screw. The diameter of the screw depends upon many things and no absolute rule can be laid down, though ap])roximate rules are given in some text books. It is now general practice to record ex- perimental data and design the screws in accord- ance with the results of actual practice, w'ith such variations as the particular characteristics of the ship and machinery seem to require. As it works in a yielding fluid, the propeller in ships of ordinary form has a greater speed than would be required if it turned in a solid nvit. The difTerence in the distance traversed in the two eases is called the 'apparent slip.' In all cases, however, the propeller acts upon water already in, motion, so that the real .s/i'/j. which represents the backward velocity of the water acted upon by the screw, may differ considerably from the iippurcnt dip. The speed of this fol- lowing water is difficult to ascertain, so that the slip ordinarily referred to is the apparent slip. If V represents the speed of the vessel, s the speed of the screw, and ir the forward speed of the water, then real slip _ .t — (r — iv) < + w Since a ship can only move by driving water astern, it is plain that negative real slip is im- possible; but from the formula given it is evi- dent that if )r is large, real slip might exist even if r exceeded s. In rare cases, with vessels of ex- ceptional form, negative slip has been observed; it always indicates a wasteful expenditure of power, for the force which gives forward motion to the water is derived from the ship in some way (bad shape of hull, frictional resistance, etc.). It must be noted that real slip — and therefore usually apparent slip — is a necessity of screw propulsion and does not of itself indicate loss of power. It is a necessary sequence of the action of a screw in a yielding fluid. The slip may be too great or too small, however: in the former case the pitch is probably (i.e. suppos- ing no other cause operative) too great: if it is too .small the pitch is probably too little. The efTicieiicy of difl'erent forms of ' propellers differs but little provided their pitch, blade area, etc., are suitable to the conditions of their use: but several changes have to be made in some in- stances before these details arc correctly deter- mined. The most important point to be consid- ered in propulsive elliciency is the shape of the vessel's hull. The shape of the bow (i.e. the entrance) is not so important, however, as that of the stern (i.e. the nm) ; the former nniy be quite full and blull' without greatly reducing the speed except at very high velocities, but the latter must be very hollow or lean or .the water will not How in solid to the propeller or pro- jjcllcrs except at low speeds. The screw is secured to the end of an iron or steel shaft called the propeller-shaft or tail-shaft, which connects to the line shafting, which in turn joins the crank-shaft at the engines. The pusli or thrust of the screw is received on the thrust bearing, which has a series of raised lugs or collars and grooves fitting over or into similar ones in the shaft. Slow vessels and small ves- sels usuall,v have a single screw. Large, fast ships are now generally fitted with twin screws, and a few are fitted with three. Some vessels having turbine engines have as many as nine screws, three on each shaft, and a Russian circu- lar armored ship has si.x screws, each on a sepa- rate shaft. The advantages of multiple screws are that the verv large power needed in modern fast vessels may be divided instead of being sup- plied by one ponderous engine, and the difficul- ties and dangers of breakdowns much reduced. One of the first definite proposals — if not the first — of using the screw for propulsion came from the great French mathematician Bernoulli, who. in 1752, received a prize from the French Academv of Sciences for an essay on the manner of propelling boats without wind, in which he proposed the use of a screw. During the I'evolu- tion David Bushnell, an ingenious and patriotic American, made a practical subnuirine boat pro- pelled by a screw turned by hand power and actuall,v used the boat in an attempt to blow up a British man-of-war. See Torpedo Boat, Sub- M.RIXE. Two Americans, Oliver Evans and John Fitch, experimented with screw propellers between 1780 and 1790. In 1801 or 1802 another American, John Stevens, built a screw-propelled steamboat which he successfully used. But it remained for Erics.son to develop the screw. His first successes were achieved in England in 1.837-38, but. getting little encouragement there, he came to the I'nited States in 1830. wlicre his plans were eagerly taken up bv Commodore Stockton and other officers of the navy. Through their efforts the United States steamship Princeton, of 1000 tons, was built under Ericsson's superintendence. She was the first screw man-of-war built in any country and the first to have her machinery wholly below the w-ater line. Her almost un- qualified success settled the question of the avail- ability of the screw for propulsion, particularly for war vessels. The use of paddle wheels in the merchant service continued for many .vears, but by 1870 the screw had evervwhere triumphed except in the navigation of shoal or interior waters. For further information, consult : Earna- by. Marine Propellers (London and New York, 1891) : Transactions of the Institiitioii of Naval .Architects (London, annual: different numbers contain many important papers on screw pro- pulsion) : A'nri/ Professional Papers (I'nited States NaA-y Department). "Screw Propulsion"; Information from Abroad (an annual: different