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A History of the Growth of the Steam-Engine

And probably in towage exceed 50 per cent


FIG. 141.--Hirsch 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 1-1/4 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_ = 4(_M_/_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.

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, whereby decreased slip is secured.

As has already been stated, the introduction of the compound engine has been attempted, but with less success than in Europe, by several American engineers.

The most radical change in the methods of ship-propulsion which has been successfully introduced in some localities has been the adoption of a system of "wire-rope towage." It is only well adapted for cases in which the steamer traverses the same line constantly, moving backward and forward between certain points, and is never compelled to deviate to any considerable extent from the path selected. A similar system is in use in Canada, but it has not yet come into use in the United States, notwithstanding the fact that, wherever its adoption is practicable, it has a marked superiority in economy over the usual methods of propulsion. With chain or rope traction there is no loss by slip or oblique action, as in both screw and paddle-wheel propulsion. In the latter methods these losses amount to an important fraction of the total power; they rarely, if ever, fall below a total of 25 per cent., and probably in towage exceed 50 per cent. The objection to the adoption of chain-propulsion, as it is also often called, is the necessity of following closely the line along which the chain or the rope is laid. There is, however, much less difficulty than would be anticipated in following a sinuous route or in avoiding obstacles in the channel or passing other vessels. The system is particularly well adapted for use on canals.

The steam-boilers in use in the later and best marine engineering practice are of various forms, but the standard types are few in number. That used on river-steamers in the United States has already been described.

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