A history of the growth of the steam-engine . economy and to the convenience of the service, to thosealready built. The forms of screws now employed are exceedinglydiverse, but those in common use are not numerous. Innaval vessels it is common to apply screws of two blades,that they may be hoisted above water into a well whenthe vessel is under sail, or set with the two blades directlybehind the stern-post, when their resistance to the forwardmotion of the vessel will be comparatively small. In othervessels, and in the greater number of full-power naval ves-sels, screws of three or four blades
A history of the growth of the steam-engine . economy and to the convenience of the service, to thosealready built. The forms of screws now employed are exceedinglydiverse, but those in common use are not numerous. Innaval vessels it is common to apply screws of two blades,that they may be hoisted above water into a well whenthe vessel is under sail, or set with the two blades directlybehind the stern-post, when their resistance to the forwardmotion of the vessel will be comparatively small. In othervessels, and in the greater number of full-power naval ves-sels, screws of three or four blades are used. The usual form of screw (Fig. 139) has blades of nearlyequal breadth from the hub t6 the periphery, or slightlywidening toward their extremities, as is seen in an exagger-ated degree in Fig. 140, representing the form adopted for 400 THE STEAM-ENGINE OP TO-DAY. tug-boats, where large surface near the extremity is moregenerally used than in vessels of high. speed running the Griffith screw, which has been much used, the hub. Fig. 139.—Serew-Propeller. is globular and very large. The blades are secured to thebub by flanges, and are bolted on in such a manner thattheir position may be changed slightly if desired. Theblades are shaped like the section of a pear, the wider partbeing nearest the hub, and the blades tapering rapidlytoward their extremities. A usual form is intermediatebetween the last, and is like that shown in Fig. 141, thehub being sufficiently enlarged to permit the blades to beattached as in the Griffith screw, but more nearly cylindri-cal, and the blades having nearly uniform width from endto end. MARINK ENGINES. 401 The pitch of a screw is the distance which would betraversed by the screw in one revolution were it to movethrough the water without slip ; i. e., it is double the dis-tance CD, Fig. 140. CD represents the helical path ofthe extremity of the blade B, and O EFH K is that ofthe blade A. The proportion of diameter to the pitch ofthe
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Keywords: ., bookcentury1800, bookdecade1870, bookidc, booksubjectsteamengines
