PROSPECTS OF STEAM-NAVIGATION. 



269 



It is generally known that within the last seven years a form of sub-aqueous 

 propeller placed at the stern of the vessel as a substitute for the paddle-wheels, 

 lias been invented and patented by Captain Ericsson. This contrivance has 

 now been in practical operation for so long a time, and in so great a number 

 and variety of vessels, that we must cease to regard it as an experiment. Its- 

 efficiency has been tested on an extensive scale. The propelling-wheel is 

 fixed upon an axis which is placed parallel to the keel, and which issues from / 

 the stern of the vessel ; the wheel therefore revolves with its face stern ward. > 

 In wheels of this form and construction, the principle of action is in general ' 

 similar to that of the common smoke-jack. The propelling surfaces have been 

 usually placed at an oblique angle to the course of the vessel, and have ex- 

 tended from the axle or nave to the outer edge of the wheel. Now, it will be 

 apparent, even to those who are least familiar with mechanical inquiries, that, 

 those parts of the blades which are near to the nave moving with the least ve- 

 locity, are the most inefficient for propulsion ; arid were it worth while, it would 

 be no very difficult matter to demonstrate that they are often an absolute ob- 

 struction. The outer ends of the blades, moving with greater velocity, act 

 with proportionately greater efficiency. 



These circumstances led Captain Ericsson to construct his wheel in such a 

 manner as to remove altogether those parts of the blades nearest to the nave, 

 and which were inefficient for propulsion, retaining only those which were most 

 remote and most effective. This he accomplished by forming a hoop of metal 

 concentric with the nave, and connected with it by two or more spokes, to 

 enable which to pass through the water with the le,ast possible resistance, he 

 gave them a twisted or spiral form, regulated with such mathematical precis- 

 ion, that, by the progressive motion of the vessel, combined with their own 

 rotation, they must always encounter the water edgewise. 



Drawings of this propeller, as applied to the Princeton, are given in figs. 

 1, 2, and 3. A section parallel to the face of the wheel is given in fig. 1 ; a 

 horizontal view is shown in fig. 2 ; and a section of the axle and hoop in fig. 3. 

 The nave in which the axle is inserted is at N, from which proceed six twist- 

 ed spokes R R, attached to and supporting the hoop H H H, bolted on to which 

 are six spiral propelling surfaces P P, &c. The axis inserted in the nave is 

 represented at A, fig. 2, where the obliquity and spiral form of the surfaces 

 are also shown, as well as the manner in which they are bolted on the hoop. 



In order to give to this wheel all the possible strength, six spiral spokes 

 were supplied, one for each propelling blade. The material of the wheel is 

 composition-metal, which resists oxydation. 



A propeller has been also supplied by Captain Ericsson for the United States 

 revenue-cutters Legare and Jefferson, represented in figs. 4, 5, and 6. The corre- 

 sponding parts are represented in the same manner as in the former diagrams, 

 and are marked by the same letters. In this wheel, the same strength not 

 being necessary, there are only four twisted arms supporting the hoop, and the 

 material of the propeller is wrought iron. 



Stern-propellers have been invented and patented of very various forms, 

 which, however, all agree in certain properties. When they are totally sub- 

 merged, with the face of the wheel presented backward, their revolution causes 

 a current of water to be projected backward from the stern, the reaction of 

 which is in fact the moving power. This effect is produced in all of them by 

 placing the surfaces of the radiating arms or plates i^a position inclined to 

 the course of the vessel. If these surfaces were placed at right angles to the 

 keei, the revolution of the wheel would make them cut the water edgewise, and 

 I-M reaction wou:a be obtained. If, on the contrary, they were parallel to the ( 

 keel, with taeir edges in the direction of the vessel's course, they would drive } 



