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rectilinear edges A B and 6H, fig. 4, of the ordinary screws, are, in this respect, highly disadvantageous, since the whole of an edge enters and leaves!
at once the water on each side of the aperture; within which aperture the water is (Art. 32, p. 37) comparatively in a quiescent state; but if the leading edges of a screw-blade were curved, as Alp B, Qp'H.", they would slide obliquely and con-^ tinuously through the water, like a screw formed with an entire feather, so that, at no moment, would there be any shock or discontinuity of action. The curved edges have, besides, the advantage of readily throwing off any floating materials that may come in contact with them, and are not so liable to be broken by their oblique collision with large spars, as straight edges are by their direct blow; while the angular parts of the common screw are far more likely to be hitched by ropes than the rounded extremities of the other. If these curved parts could be furnished with sharp knife-edges, partly notched like saws, and made of a metal capable of resisting the corroding effects of sea-water, or could be frequently removed and cleaned, they would be capable of dividing any rope, spar, or other floating matter, like a powerful circular saw. C D represents the sternpost of a screw-steamer, B G the boss of the screw; and the direction in which the screw revolves is according to the order of the letters MANH."
in deep water. A similar accident occurred to the Peninsular mail-steamer 'Alhambra,' in her last voyage; the stuffing which closes the orifice at the stern of the ship, through which the shaft is connected with the screw, got away, and let in so much water as to render it necessary to keep the pumps going during the remainder of the voyage. Besides the severe and well nigh fatal catastrophe which happened to the 'Royal Albert,' the 'Crecy,' 'Colossus,' and many other screw steam-ships of the line had become so leaky in the after part of their clear runs, through the caulking being loosened by the constant tremor and vibration occasioned by the screw, as to require to be taken into dock.
It will be easily understood that until such a form shall be given to the screw as that it may exert a continuity of action, it will be in vain to expect that shocks should cease to take place. The author would not, at the sacrifice of speed, revert to a continuous featherscrew; but the form here recommended would, to a considerable extent, gain the proposed ends: if, also, the edges of the blades were chamfered, the shock produced as they pass through the water would be further diminished.
The curved lines in the above figure being traced within the straight lines A B, G H, which represent the edges of a common screw: it follows that a screw so formed will be of a somewhat diminished breadth ; but it is understood that a small diminution of breadth causes no sensible diminution of the propelling power of the screw. A loss of power might be obviated by tracing the curves so as to give the blades a greater breadth than that of the common screw; but this is not to be recommended; and it will be found advantageous to have the blades of a screw as narrow as possible, consistent with an adequate propelling power. The screw of a large ship weighs from 8 to 10 or 12 tons; and when a ship pitches much in a heavy sea, the screw is sometimes quite out of the water; in this case, the resistance being taken off, the screw revolves with such rapidity as to endanger the stability of the whole machinery, as often happens in merchant steamers forcing their way in long voyages against adverse winds and heavy seas. To provide against this evil—and the subject concerns as much the mercantile as the warlike navy of the country—the blades of the screw, and consequently the aperture in which it works, as well as the trunk through which it is occasionally drawn up, should be as much as possible reduced, and also the parts of the ship about the stern should be much strengthened, to enable them to resist the great strains to which they are liable. That this should be done effectually is plainly an affair of the highest national importance.
a The clotted line Ap p' H' will show that, with hlades so formed, there is a continuity of action on the water, though the screw itself be divided into two parts.
The screw lately invented by Mr. Griffiths, the blades of which diminish both in the fore and aft, and in the transverse direction, has some advantages over that which is in common use; it allows the aperture in which it works, and the trunk, to be of smaller dimensions, thus rendering the parts about the stern of the ship stronger; but it may be doubted whether the narrowness of the blades may not be the cause of a sensible diminution of the propelling power.
74. The screw is, in general, less exposed than a wheel to injury from objects floating in the water; yet in a crowded anchorage it is liable to become entangled with warps, nets, and the like; and any expedient by which the screw might be guarded from being so entangled and disabled, would obviously be of vast importance to the efficiency of a steam-ship.
75. The instances of screws getting foul by roj:>es, nets, &c, wound up tightly on their bosses are too numerous to admit of notice in detail. The screw of the 'Exmouth,' ship of the line was fouled by her own sheet cable, in endeavouring to haul herself off a shoal, for which she had laid out an anchor; the cable was wound up so tightly that there was very great difficulty and much delay in clearing the screw.
The screw steam-ship 'Melbourne/ formerly the 'G-reenock,' on her voyage to Australia in 1852, was taken in a gale of wind, by which she was partially dismasted; her screw was fouled and disabled by the wreck of her own rigging; and being thus deprived of both powers of motion, sails and screw, she lay like a log on the sea, and put into Plymouth for repair.
When the 'Tribune' was docked at Sheerness on the 10th June, 1853, it was found that 11 fathoms of 3ife-inch rope had been wound up on the boss of her screw in such a manner as must have disabled the machine if the rope had been much longer. Before the 'Rattler' set out on her trial trip with the 'Alecto,' Art. 5l, it was found that a 7-inch hawser had been closely wound, together with some fishing-nets, about her screw, and it was several hours before a complete clearance could be effected. And numerous cases occur in which the screw-propellers of ships were discovered, when the ships were docked, to have wound up on their screws, ropes, fragments of nets or sails, lead and log lines.
76. The entanglements of screws in the open sea occur rarely compared with the foulings which take place in rivers, harbours, and roadsteads; but when steam-ships, in line ahead, are in action, the risk of, and the detriment arising from, their screws getting fouled by the rigging shot away, are very great; the wreck getting into the wake of the ships, or, passing these, floating into the course of those astern of them. On this account, any means by which the unimpaired action of screw-propellers may be insured is a matter of very high importance.
77. If there were no probability of a screw-propeller being disabled in action, and consequently no necessity for being prepared to resort to the sail, a great deal of the running rigging might be removed, the topgallant and royal yards, all the topgallant studdingsail and royal gearing sent down from the tops, and even the topgallant masts might be struck, in order to lessen the chance of rigging shot away, falling from aloft, and getting adrift in the sea; thereby fouling the screw, and rendering the vessel with all her armament immovable except by the sail.
78. After bestowing the most serious consideration on the subject of a fouled screw, and on the various expedients by which it has been attempted to remedy this great evil, the author has arrived at the conclusion, that the clearing of a screw can only be effected by some contrivance that may enable screw-propellers to clear themselves of any
ropes or other floating wrecks of rigging which may hitch upon a blade in its rotation, and which being drawn down to the root of the blade would be wound up on the boss so tightly as ultimately to disable the propeller, or derange the driving machinery, if the engine were not immediately stopped. For clearing the screw the author proposes to employ strong and sharp steel knife-edges firmly fixed to the metal trunk in which the screw works, and close to both edges of the blade, in such a manner that any rope that may have hitched on the boss would be acted upon, during the revolution of the screw, as a body revolving in a turning-lathe is acted upon by a chisel. Thus revolving with a force derived from the power of the engine, the rope must be drawn into and along the knife-edges, causing these to exert a drawing cut, sufficient to sever any rope, whatever be its thickness, and so clear the screw at once of any such entanglement as those shown in the annexed (fig. 5).
Should the force with which the rope so hitched is drawn under the knife-edges appear to create an injurious strain on the square end of the propelling shaft, the object may be accomplished by means of circular fixed cutters acting in a direction parallel to the shaft; the cylindrical parts of the boss, between the blades of the screw, being brought under the cutters as the screw revolves. The corrosive effects of salt-water on the cutters might be obviated by cleaning the edges, the screw being, for this purpose, temporarily removed.
Before he quits this subject, the author would suggest that a screw-propeller might be examined at any time, either at anchor or under way in a smooth sea, by sending an experienced diver in the diving-dress down the well on a step-ladder, to the boss; the blades