471 OF ENGINEERING FORMULÆ.
Marine Boilers.
In marine engines the “ nominal ” horse-power
varies so much from the actual or “ indicated ”
horse-power, that the former can no longer bs
taken as a basis for the calculation of boiler-power.
Many screw engines work up to more than six
times their nominal horse-power.
In marine compound engines the indicated
horse-power consumes from If to 2J lbs. of coal
per horse-power per hour, which requires the fol-
lowing proportions:—
Fire-grate area from'13 to ‘17 sq. ft. per I.H.P.
Heating surface „ 30 „5’0 „ „
Tube area „ ' 035 „ *05 „ „
Surface condensing
area, from.. .. 2 • 5 „3'0 „ „
Tubes for marine boilers are generally about
3j inches internal diameter, from 5 to 6 feet 6 inches
long, with an inclination of about | an inch to
the foot.
Stays, about 1| diameter.
Water spaces, 5 to 6 inches wide.
Height of firing plate above stoke-hole, 2 ft. 6 in.
Length of üi-e-grate, 5 feet to 7 feet 6 inches.
Length of fire-bars, from 2 feet 6 inches to 3 feet.,
Depth of fire-bars at centre = '12 of length.
Depth of fire-bars at ends = -06 of length.
Clearance of fire-bars, | to | inch.
Taper of tire-bars, | of depth.
Width of fire-bars at ends, f to 1 inch.
Minimum height of water, 6 inches over flues.
8 inches over tubes.