424 On the Various Modes of Propelling Vessels. 



answer much better : and he placed the screw in the dead wood. 

 Ericsson used two propellers, each consisting of short spiral 

 plates, attached to the periphery of a broad thin hoop, which 

 was fixed on arms radiating from the axle. Both propellers were 

 behind the rudder, and revolved round a common centre, the 

 shaft of one being within that of the other, and one being* in 

 front of the other. The hinder screw revolved with a greater 

 velocity than the one in front of it, to enable it to act on water 

 already in motion. But an equal advantage would be attained 

 by the use of one screw of a larger diameter. 



The slowness of the ordinary marine engine opposed a 

 serious difficulty in the earlier attempts to apply the screw pro- 

 peller. To overcome this, gearing was used, a very large wheel 

 being made to work into a pinion fixed to the screw shaft. 

 But there are great objections to such an arrangement. Inde- 

 pendently of the intolerable noise, the teeth wear out rapidly, 

 and are liable to sudden fracture with any violent strain of the 

 sea. At present, a sufficiently rapid motion is obtained directly 

 from the engines ; nor is there any objection to this, since the 

 supposition that the best speed for the piston is precisely that 

 which is best for a canal horse, namely, 220 feet per minute, 

 has for a considerable time been known to be a fallacy. 



The screw shaft exerts an enormous thrust, in the place at 

 which it abuts within the vessel, the whele force of impulsion 

 being imparted there; the plate against which it works has 

 been rendered white hot, although a stream of water was 

 constantly flowing over it. Various means have been used to 

 overcome this difficulty. Thus, in some cases, the end of the 

 shaft is made to work against a disc of hardened steel, fixed 

 eccentrically with reference to the shaft, and having a slow 

 motion communicated to it ; and in others, against rolling sur- 

 faces. In others, steel collars are placed on the end of the shaft, 

 and being immersed in oil, are little liable to heat. Should, 

 however, undue friction arise between the actual rubbing sur- 

 faces, new ones come into play, since all the collars are 

 moveable. Other expedients also have been employed for the 

 same purpose. 



In the early days of the screw propeller, it was used only 

 as an auxiliary to the sails. When not in use, if left in its 

 ordinary position, it would retard the vessel, and other incon- 

 veniences would arise from it. To obviate these, means were 

 used for raising it when desirable, and even for closing the 

 aperture in the dead wood, which if left open must seriously 

 interfere with the steering. 



Centrifugal force disperses the Water, causing the screw to 

 throw it off in the form of a cone ; it is far better that it should 

 assume the shape of a cylindrical column. This has been 



