Screw Propellers
A screw propeller is a similar construction to the common screw, except that the thread enlarges to a plate as the cylinder diminishes to a spindle. It acts much as a bolt in a fixed nut. When placed under the ship and revolved, the screw advances, pushing the ship, and the water is thrust backwards. John Stevens, of Hoboken, employed a screw propeller in 1804. In 1836 patents were granted to Capt. J. Ericsson, United States of America, and Francis P. Smith, England, which resulted in their final adoption as a regular mode of propulsion for steamships. Propellers are cast from bronze, steel, and cast iron--some whole and others with boss and blades as separate castings. The latter may very readily be all made in greensand. Large numbers of small wheels are made from entire patterns in greensand. An improved method of moulding small wheels in greensand from one blade and an equal section of hub secured to a nowel-frame of wood, cut to the shape of the joint, allows of the pattern being rammed therein, the joint made, and an impression of the upper side being taken in a close-fitting cope, also of wood. This whole process, being performed upon a lifting-plate that stands within the nowel-frame, permits the blade-moulds when placed together to be arranged in their respective positions upon a level bed, and all rammed with a containing flask or curb. The covering-plate, weights, and runners complete the operation. Larger wheels are made in dry-sand from one-blade pattern with hub attached, the latter being made to fit a central spindle that rests on a foundation-plate on which as many nowels are fixed as there are blades to the wheel. The copes, being separate ones, are rammed in succession over the pattern, as the lower surface of the blades are alternately formed in each nowel. When all the blades have been moulded by this means, the whole mould is finished, blackened, and dried for casting. When small and medium-sized wheels are made in loam, the whole of the blades are formed upon one foundation-plate; very large ones have a separate foundation-plate for each blade. An outer swept-bearing, just beyond the brickwork of the blades, serves as a rest for the inclined plane on which the sweep-board, attached to the free arms of the spindle, must be made to travel, and which gives the pitch of the wheel. Bottom and top surfaces of the blade are struck with this board, which is set at right angles with the spindle, a tapered thickness or guide-piece being attached when the blades are made face up. If the blades are to be cast face up, the moulds are carved out of the loam; if face down, then some pattern device is schemed, in order to give the cope impression. The piers or nowels are built of bricks and loam; but the copes are carried off in iron frames, so constructed that a good loam impression of the blade along with that portion of the hub may be lifted away, to be again returned when the whole mould has been dried, and all is ready for binding and ramming together in the pit.