STEAM ENGINE 349 contact with the surrounding mass of water. The currents so produced, in the case of screw vessels, impinge upon the screw, which works immediately astern of the vessel, and com- municate to it a portion of that energy which would otherwise be lost in the creation of suh currents. Screws work far below the surface of the water, and lose less by slip than the paddle wheel. Screw engines are quick- working, compact, and light. Their higher piston speed, their smaller size, and especially their more uniform action upon the propeller and the water, produce greater economy in the use of steam and a more effective appli- cation of power than is obtained with the paddle wheel. Incidentally, by permitting the replacing of a considerable weight of ma- chinery and fuel by paying freight, they add greatly to the commercial value of the steam vessel. The forms of screws are exceedingly diverse, but those in common use are not numerous. In naval vessels it is common to apply screws of two blades, that they may be hoisted above water into a "well" when the vessel is* under sail, or set with the two blades directly behind the stern post, when their resistance to the forward motion of the vessel will be comparatively small. In other vessels, and in the greater number of full-pow- ered naval vessels, screws of three or four blades are used. The usual form of screw has blades of nearly equal breadth from the hub to the periphery, or slightly widening toward their extremities, as is seen in an exaggerated degree in fig. 15, representing the form adopt- ed for tug-boats, where large surface near the extremity is more generally used than in ves- sels of high speed running free. In the Grif- fith screw, which has been much used, the hub is globular and very large. The blades are se- cured to the hub by flanges, and are bolted on in such a manner that their position may be FIG. 15. Tug-boat Screw. FIG. 16. Hirsch Screw. changed slightly if desired. The blades are shaped like the section of a pear, the wider part being nearest the hub, and the blades tapering rapidly toward their extremities. A usual form is intermediate between the last and that shown in fig. 15, the hub being suffi- ciently enlarged to permit the blades to be at- tached as in the Griffith screw, but more near- ly cylindrical, and the blades having nearly uniform width from end to end. The Hirsch screw, fig. 16, is used on the steamship City of Peking. The pitch of a screw is the distance which would be traversed by the screw in one revolution were it to move through the water without slip; i. e., it is double the distance D, fig. 15. C D' represents the helical path of the extremity of the blade B, and E F H K is that of the blade A, The proportion of diameter C' to the pitch of the screw is de- termined by the speed of the vessel. For low speed the pitch* may be as small as one and one fourth the diameter. For vessels of high speed the pitch is frequently double the diam- eter. The diameter of the screw is made as great as possible, since the slip decreases with the increase of the area of screw disk. Its length is usually about one sixth the diameter. A greater length produces loss by increase of surface causing too great friction, while a shorter screw does not fully utilize the resist- ing power of the cylinder of water within which it works, and increased slip causes waste of power. Negative slip occurs when the vessel moves at a higher speed, than it would attain were the screw to work in a solid nut ; it is sometimes observed in badly formed vessels. The slip is decreased by increasing the diameter, and also by increasing the length of the screw. The increased friction above referred to prevents the latter process from being economically carried beyond the maxi- mum given. An empirical value for the prob- able slip in vessels of good shape, which is closely approximate usually, is given by Prof. Thurston as S=4 ^, in which S is the slip per cent., and M and A are the areas of the mid- ship section and of the screw disk in square feet. The most effective screws have slightly greater pitch at the periphery than at the hub, and an increasing pitch from the forward to the rear part of the screw. The latter method of increasing pitch is more generally adopted alone. The thrust of the screw is the pressure which it exerts in driving the vessel forward. in well formed vessels, with good screws, about two thirds of the power applied to the screw is utilized in propulsion, the remainder being wasted in slip and other useless work. Its efficiency is in such a case, therefore, 66 per cent. Twin screws, one on each side of the stern post, are sometimes used in vessels of light draught and considerable breadth, where- by decreased slip is secured. The following are the dimensions of some of the largest ma- rine screw engines of the well known types. The engines of the British iron-clad Monarch, a vessel of over 8,000 tons displacement, have given an "indicated power" of 8,528 horses at 65 revolutions a minute, when making a speed
