The committee of the British Admiralty on designs of ships-of-war have reported recently: “The carrying-power of ships may certainly be to some extent increased by the adoption of compound engines in her Majesty’s service. Its use has recently become very general in the mercantile marine, and the weight of evidence in favor of the large economy of fuel thereby gained is, to our minds, overwhelming and conclusive. We therefore beg earnestly to recommend that the use of compound engines may be generally adopted in ships-of-war hereafter to be constructed, and applied, whenever it can be done with due regard to economy and to the convenience of the service, to those already built.”
The forms of screws now employed 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-power naval vessels, screws of three or four blades are used.
Fig. 139.—Screw-Propeller.
The usual form of screw ([Fig. 139]) 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. 140], representing the form adopted for tug-boats, where large surface near the extremity is more generally used than in vessels of high speed running free. In the Griffith screw, which has been much used, the hub is globular and very large. The blades are secured to the hub by flanges, and are bolted on in such a manner that their position may be 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 is like that shown in [Fig. 141], the hub being sufficiently enlarged to permit the blades to be attached as in the Griffith screw, but more nearly cylindrical, and the blades having nearly uniform width from end to end.
Fig. 140.—Tug-boat Screw.
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 C D, [Fig. 140]. C D′ represents the helical path of the extremity of the blade B, and O E F H K is that of the blade A. The proportion of diameter to the pitch of the screw is determined by the speed of the vessel. For low speed the pitch may be as small as 11∕4 the diameter. For vessels of high speed the pitch is frequently double the diameter. 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 of the diameter. A greater length produces loss by increase of surface causing too great friction, while a shorter screw does not fully utilize the resisting power of the cylinder of water within which it works, and increased slip causes waste of power. An empirical value for the probable slip in vessels of good shape, which is closely approximate usually, is S = 4M∕A, in which S is the slip per cent., and M and A are the areas of the midship section and of the screw-disk in square feet.