SCREW ENGINES.

121. Q.—­What are the principal varieties of screw engines?

[Illustration:  Fig. 27.]

[Illustration:  Fig. 28.]

A.—­The engines employed for the propulsion of screw vessels are divided into two great classes,—­geared engines and direct acting engines; and each of these classes again has many varieties.  In screw vessels, the shaft on which the screw is set requires to revolve at a much greater velocity than is required in the case of the paddle shaft of a paddle vessel; and in geared engines this necessary velocity of rotation is obtained by the intervention of toothed wheels,—­the engines themselves moving with the usual velocity of paddle engines; whereas in direct acting engines the required velocity of rotation is obtained by accelerating the speed of the engines, and which are connected immediately to the screw shaft.

122. Q.—­Will you describe some of the principal varieties of geared engines?

A.—­A good many of the geared engines for screw vessels are made in the same manner as land engines, with a beam overhead, which by means of a connecting rod extending downwards, gives motion to the crank shaft, on which are set the cog wheels which give motion to pinions on the screw shaft,—­the teeth of the wheels being generally of wood and the teeth of the pinions of iron.  There are usually several wheels on the crank shaft and several pinions on the screw shaft; but the teeth of each do not run in the same line, but are set a little in advance of one another, so as to divide the thickness of the tooth into as many parts as there are independent wheels or pinions.  By this arrangement the wheels work more smoothly than they would otherwise do.

123. Q.—­What other forms are there of geared screw engines?

A.—­In some cases the cylinders lie on their sides in the manner of the cylinders of a locomotive engine.  In other cases vertical trunk engines are employed; and in other cases vertical oscillating engines.

124. Q.—­Will you give an example of a geared vertical oscillating engine?

A.—­The engines of a geared oscillating engine are similar to the paddle wheel engines (figs. 27 and 28), but the engines are placed lengthways of the ship, and instead of a paddle wheel on the main shaft, there is a geared wheel which connects with a pinion on the screw shaft.  The engines of the Great Britain are made off the same patterns as the paddle engines constructed by Messrs. John Penn & Son, for H.M.S.  Sphinx.  The diameter of each cylinder is 82-1/2 inches, the length of travel or stroke of the piston is 6 feet, and the nominal power is 500 horses.  The Great Britain is of 3,500 tons burden, and her displacement at 16 feet draught of water is 2,970 tons.  The diameter of the screw is 15-1/2 feet, length of screw in the line of the shaft, 3 feet 2 inches, and the pitch of the screw, 19 feet.