A tube that is split or that cannot be made steam tight by being re-mandrelled or expanded is plugged up at each end by means of either wooden or iron plugs. The best plan, however, is to use iron discs having a stepped diameter, so that one end will fit the bore of the tube, and the other will form a shoulder that will cover the end of the tube.
Each disc has a hole through its centre, so that a wrought iron rod or bolt may be passed through the hole and receive a nut at each end. Beneath the flange of each disc, a grummet of spun yarn and white lead is placed, so as to make a steam tight joint when the nuts are screwed home. This stays the tube plates as well as stopping the leaky tube.
If wooden plugs are used, they are made a driving fit in the tube bore, and driven through until they have passed the split, and a second wooden plug is driven tightly from the same end of the tube.
Black smoke is an evidence of incomplete or imperfect combustion, and may be, to a great extent, prevented by careful firing, as by feeding gradually and evenly, by the admission of the proper quantity of air, or by a jet of steam admitted above the dead plates.
The furnace bars are ordinarily of cast iron about 11⁄4 inches thick at the top, tapered towards the bottom, and with an air space of from 1⁄2 to 3⁄4 inch between them.
They require less air space for Welsh than for Newcastle coal, as the latter is the flaming or gaseous coal, and burns the fastest.
The quantity of coal burned in marine boiler furnaces is about 15 lbs. per square foot of fire grate area per hour; hence the quantity burnt per day with common average engines with 4 furnaces, 3 feet wide and 5 feet long, may be found by multiplying the area of the 4 furnaces (60 feet) by the number of lbs. (15) burned per foot of grate per hour, which will give the total lbs. weight burned per hour, which, divided by 112 lbs., will give the hundredweight burned per hour, and this, multiplied by the number of hours reckoned as constituting a day, gives the fuel consumption per day, based upon 15 lbs. coal per square foot of fire grate area.
The number of tons of steam coal burnt per day to drive an ordinary steamer of 40 feet beam 10 knots an hour by steam alone (or without sail), will depend upon the kind of engine used. Experience teaches us that with average vessels, the beam squared equals the consumption of coal for 40 days, in the case of an ordinary jet condenser engine; 50 days with a surface condensing engine; and 60 days with a compound engine; hence, in the present example, assuming the engine to be jet condensing, we may calculate the fuel consumption per day, for a vessel 40 feet beam giving 10 knots an hour, as follows:
The beam squared gives 1600 (40 × 40 = 1600), which divided by 40 (40 days) gives 40 tons per day. For surface condensing the 1600 would be divided by 50, giving 32 tons per day; and for a compound engine the 1600 would be divided by 60, giving 26 tons 131⁄3 cwt. per day.
It is obvious, however, that calculations of this kind, in which the ratio of expansion is not stated, are the merest approximations.