On the Various Modes of Propelling Vessels. 425 



effected by setting the blades, not at right angles to the screw 

 shaft, but in such a way that they point outwards from the 

 stern. Their tendency is, then, to concentrate to a point the 

 water thrown off by them ; and centrifugal force corrects this, 

 so as to give the water the form of a cylinder. Such an 

 arrangement gives excellent results. 



The action of the screw is greatly affected by adverse winds, 

 currents, variation of the depths of immersion of the vessel, 

 etc. But, as the velocity with which it revolves is almost in- 

 variable, whether the progress made is great or little, the 

 consumption of power is, in nearly all cases, the same. It is 

 otherwise with the paddle wheel, which in similar circum- 

 stances revolves more slowly. To meet this difficulty, means 

 of altering the pitch of the screw blades, according to circum- 

 stances, have been devised ; and of the plans proposed for the 

 purpose, those of Bennet Woodcroft are the most remarkable 

 for ingenuity and effectiveness. 



To obviate the loss of power from portions of the blades 

 effecting very little more than a dispersion of the water, the 

 leading edge of the screw has in some instances been made 

 nearly at right angles with the axis of the shaft, the pitch 

 increasing slowly at first, and then rapidly, so that the trailing 

 edge should stand on a line with the axis of the shaft. But the 

 large amount of rubbing surface thus produced neutralizes the 

 theoretically excellent qualities of this form of blade ; and it 

 has been greatly improved by cutting away a considerable 

 amount of its leading edge near the periphery. 



Not only has any interference of the screw with the steering 

 been prevented, but it has been made a most effective auxiliary 

 to, and even substitute for, the rudder. An application of the 

 screw propeller in this way was suggested so early as 1800 by 

 Shaler, and was carried into effect in 1803 by Dallery, who 

 arranged it in such a way that it was turned with the ruddei 

 without its revolution being interfered with. Bennet Wood- 

 croft, in 1851, devised a means of manoeuvering the vessel, by 

 causing the blades to feather, so as to pass edgeways through 

 the water during one part of their revolution, and sideways 

 during another. The application of the twin screw affords so 

 excellent a steering apparatus, that by means of it a ship, even 

 in still water, may be made almost to revolve on its centre. 

 The twin screw has, besides, other advantages of so important 

 a kind, that it is likely to be exclusively employed hereafter in 

 the navy, and very generally in the mercantile marine. It 

 affords a perfect substitute for the rudder, should the latter 

 get out of order : if one screw is disabled by accident, there is 

 still a propelling power : and it allows the vessel to be moved 

 with equal velocity ahead or astern. It has been brought 



