MARINE ENGINES.

525

of condensing surface. The air pump for the surface System maybe less than for the injection sj^stem, ranging from one-twelfth toone-fifteenth of the cubical contents of the cylinder; it is, however,often made of the same capacity as the injection System, in theevent of that System being adopted, in case of accident to thesurface condenser.

Feed pump .—The number of cubic inches of water required forthe boilers varies of course according to the evaporation of waterin the boilers. To find the water required, multiply the cubiccontents in feet of steam for one cylinder for an entire revolutionby the cubic inches of water required for the steam used, that isthe quantity required to be boiled off, and the result will give thecubical contents of the feed pump of the single-acting type. Thisresult is only for one cylinder, and generally double the contentsis allowed, so that in the event of one of the pumps getting out ofOrder the other one will be able to keep up the supply. The areaof the water passages in the grating, if india rubber valves are used,is found by the same rule as for the air pumps; for brass valves,divide the pump’s diameter by 1 "3, which gives the diameter of thevalve. For the sake of uniformity, the bilge pump is the same sizeas the feed pump.

Screw propeller .-—The screw’s disc, or the circle represented bythe diameter of the screw, should have an area of about 1 foot forevery 275 square feet of midship area or immersed section; whilethe actual area of the blades may be put down at about 1 squarefoot for every 7 to 10 square feet in the midship section. Thethickness of the roots of the blades for cast iron is found by multi-plying the diameter of the screw by 75, this tapering to 1 % inchat the point on the average. When the screw is made of gun-metalmultiply the diameter by -44, which gives the thickness at the root,and all the other parts in proportion. To find the pitch of thescrew, multiply 6080, the lineal feet in a knot or nautical mile, bythe speed in knots per hour that is required, dividing the result bythe number of revolutions of the screw multiplied by 60 minutes,and the quotient will give the pitch of the screw as it were workingin a solid nut. A certain amount, however, must be allowed forslip. Deduct that amount from 100, which we will name the divisor,then multiply the pitch as already found by 100, and divide thatquantity by the divisor: hence the formula—

100 —slip : pitch as found : : 100 : actual pitch.