MARINE ENGINES.

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the bracket fitted to the side of the ship; the sole of the block isalso bolted to the bracket, as shown in Fig. 266.

In some ocean Steamers the paddle-wheel shaft was arranged sothat it could be disconnected from the engine when the ship wasunder sail alone. The simplest plan for effecting this is by fittinga disc on the end of the shaft, instead of the usual crank, the discliaving a hoop with a projectinglug piece for taking the crankpin; the hoop is forged all in onepiece, and is held in position witha fast-and-loose collar on theround disc; the grip is attained byfriction blocks, or wedges, firmlyscrewed between the disc and thehoop on the circumferential line,the disc being keyed to the shaftwith one or more keys. There-fore when the friction blocksare released, the shaft and discrevolve independently of theengine, the motion of the vesselthrough the water, driven by thesails,causing the paddle wheels torevolve. In this way the progressof the vessel is not impeded somuch as it would be were the floats stationary, and offering a greatresistance for the wind to overcome.

The paddle wheel of a ship may be compared to an ordinarycarriage wheel, any point in the circumference describing a cycloidcurve. The circumferential distance a carriage wheel travels overis an exact measure of the distance the carriage has gone; but asthe paddle wheel acts in a yielding fluid, the distance travelledover by it is not an exact measure of the vessel’s progress throughthe water. The difference is termed the slip of the paddle, andranges from one-fourth to one-fifth of the circumferential distancethe paddle wheel has gone over, which of course must be measuredon the mean centre of propulsion of the floats, and not on theextreme diameter.

The reciprocating parts of marine engines are generally balancedwith suitable weights, and notwithstanding that the cylinders of

Fig. 266.—Pillow Block for Paddle Wheel.

, Pillow Block, b, Cap for do. c c, Holding-downbolts.