The accompanying engraving represents an arrangement of a propeller on this principle, for which Dr. Spurgin obtained a patent in 1837. It possesses some advantages over preceding arrangements on the same principle, each paddle of the chain being made to enter and leave the water at the angle required to produce the most effective action. For this purpose, instead of employing only two pair of riggers to carry the endless chains, three pair are employed, the central pair being either of greater diameter than the pairs at each end, or having its axis placed lower than the axes of the other two, so as to cause the lower sides of the chain to stand obliquely to the water, instead of parallel thereto, as in all previous propellers of this class.; by this obliquity of the chain the paddles standing at right angles thereto are made to enter and leave the water at a corresponding angle.
The figure represents a side elevation of the apparatus. a a is a cast iron frame secured to the sides of the vessel by the bolts b b b b, c is one of a pair of spike wheels fixed on the engine shaft d, ee riggers or pulleys running upon the pins or studsff at each end of the frame, g g an endless flat chain passing round the riggers e e, and over and under the spike wheel c: in order to keep this chain tightly extended, the studsffmove in slots in the frame a, and are keyed up by any convenient method, so as to give the requisite tension to the chain; h h are the floats which are carried by the iron forks jj, which are forged in one with the middle links which support them.
Another improvement specified under the patent consists in the mode of constructing the chain. It is formed of single and double links alternately, the single links having on their backs a flange as broad as the double links, and projecting lengthwise half way over each double link, so that the flanges of the two contiguous single links abut against each other as shown in the figure, which represents a side elevation of a portion of the chain.
a a are the double links, b b the single links, and c c the flanges on the backs of the single links.
Stevens's Crank Axle Paddles, (1828.) - In this invention three paddles are attached separately to the arms of a three-throwed crank, and by means of radius and guiding rods connected with them, the paddles are each made to describe in the water the segment of an ellipse.
At a, Fig. 1, is the centre of the axis of a triple crank, the separate arms of which (marked c c c,) move between parallel bars on the side frame d d of the paddle-box. Ateee are the three paddles, connected by the guide rods /// to the radius rods g g g, the latter working from a fixed beam or centre h. At iii are "arched spreaders" to keep the paddles firm and steady. Fig. 2, which represents a section through the horizontal beams dd shown in Fig. 1, is added, for the better explanation of the parts than could be done by words. This invention was much admired for its ingenuity when brought out.
Nairn's Propelling Apparatus, patented in 1828. is an example of what is termed the "Ducksfoot motion," consisting of two, four, or more levers suspended over the sides of a vessel, descending nearly as low as the vessel's keel. These levers are made to vibrate by the engine, and in order that they may experience but little resistance from the water in their back stroke, they should be of such a shape as to present in their horizontal section a form like the adjoining Fig. 1. At each side of the lever, at its lower extremity, is attached a broad plate of iron a a, Fig. 2, by means of hinge joints, which, upon the lever being moved forward, close and offer no resistance, but when it is moved backward, they open or expand, and thereby propel the vessel forward. They are prevented from opening beyond their proper angle by arcs or chains.