PHOPORTIONS OF SCREWS. 



example, in the case of a screw 12 feet in diameter, the pitch 

 would be from 12 to 14 feet, and the length about 2 feet. 



70. Screws are generally made with one uniform pitch, and 

 their blades are set at right angles to the shaft. A gradual 

 increase of pitch towards the leading end of the screw is, how- 

 ever, recommended. Thus, the pitch of the centre should be 

 about 10 per cent, less than at the circumference, for the centre 

 should merely screw through the water, without producing any 

 reaction or propelling force. The efficient part being near the 

 circumference, it is also recommended that the blades, instead of 

 being precisely perpendicular to the shaft, should be inclined a 

 little sternwards, so as to produce a tendency in the water which, 

 they drive backwards to converge to a point. It is assumed that 

 this convergent tendency may balance the divergent tendency due 

 to the centrifugal force attending the revolution : so that the two 

 forces being in equilibrium will cause the water to be projected 

 backwards from the screw in a cylindrical column. In the case 

 of the ordinary screw, with blades at right angles to the shaft, 

 the water projected backwards assumes the figure of the frustum 

 of a cone, and a certain proportion of the power is thereby lost. 



71. The relative advantages of screw and paddle-propellers 

 depend in a great degree upon the immersion. It appeared from 

 experiments made on a considerable scale with steamers of the 

 Royal Navy, that in deep immersion the screw has an advantage 

 over the paddle-wheel of one-and-a-half per cent. ; but that, 

 with medium immersion, the paddle-wheel had an advantage of 

 one-and-three- quarters per cent, over the screw, an advantage 

 which was augmented to four-and-three-quarters per cent, for 

 light immersions. It appears, therefore, that the screw-propeller 

 has a certain advantage over the paddle when the vessel is deep 

 in the water, and that, on the other hand, the paddle gains an 

 advantage over the screw in proportion as the immersion is less. 



72. In long sea voyages, where the immersion is liable to con- 

 siderable variation by reason of the lightening of the vessel owing 

 to the consumption of the fuel, the screw will have the advantage 

 over the paddle in the commencement of the voyage, and the paddle 

 over the screw towards the end of it. In rough weather, where, 

 by the rolling and pitching of the vessel, the paddle-wheels are 

 liable at one time to be deeply plunged in the water, and at another 

 to be raised out of it, the screw will have an obvious advantage. 



73. In his work upon the screw-propeller, Mr. Bourne has 

 given tbe details of a series of important experiments made with 

 H. M. steamers " Rattler " and "Alecto," to determine the 

 relative advantages of screw and paddle-wheels against a head 

 wind. The result of these experiments seemed to prove, that 



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