Part 2 (2/2)

Mr. Charles Atherton, the able and distinguished Chief Engineer of Her Majesty's Royal Dock Yard, at Woolwich, has published a volume, called ”Steams.h.i.+p Capability,” a smaller volume on ”Marine Engine Cla.s.sification,” and several elaborate papers for the British a.s.sociation, the Society of Arts, London, the a.s.sociation of Civil Engineers, and the Artisans' Journal, for the purpose of properly exposing the high cost of steam freight transport as based on the law above noticed, and the ruinous expense of running certain cla.s.ses of vessels of an inferior dynamic efficiency. When but a few weeks since in London, I asked the Editor of the ”Artisan,” if any engineer in England disputed the laws relative to power, on which Mr. Atherton based his arguments. He replied that he had never heard of one who did. I asked Mr. Atherton myself, if in the case of the newest and most improved steamers, with the best possible models for speed, he had ever found any defect in the law of, the resistance as the squares, and the power as the cubes of the velocity. He replied that he had not; and that he regarded the law as founded in nature, and had everywhere seen it verified in practice in the many experiments which it was his duty to conduct with steam vessels in and out of the Royal Navy. I think, therefore, that with all of these high authorities, the doctrine will be admitted as a law of power and speed, and consequently of the consumption of coal and the high cost of running steamers at mail speeds.

It is not my purpose here to discuss this law, or treat generally or specially of the theory of steam navigation. It will suffice that I point out clearly its existence and the prominent methods of its application only, as these are necessary to the general deduction which I propose making, that rapid steams.h.i.+ps can not support themselves on their own receipts. The general reader can pa.s.s over these formulae to p. 69, and look at their results.

I. TO FIND THE CONSUMPTION OF FUEL NECESSARY TO INCREASE THE SPEED OF A STEAMER.

Suppose that a steamer running eight miles per hour consumes forty tons of coal per day: how much coal will she consume per day at nine miles per hour? The calculation is as follows:

8^3 : 9^3 :: 40 : required consumption, which is, 56.95 tons. Here the speed has increased 12-1/2 per cent., while the quant.i.ty of fuel consumed increased 42-1/2 per cent.

Suppose, again, that we wish to increase the speed from 8 to 10, and from 8 to 16 miles per hour. The formula stands the same, thus:

Miles. Miles. Tons Coal. Tons Coal.

8^3 : 10^3 :: 40 : _x_, = 78.1 8^3 : 16^3 :: 40 : _x_, = 320.

II. TO FIND THE SPEED CORRESPONDING TO A DIMINISHED CONSUMPTION OF FUEL.

Murray has given some convenient formulae, which I will here adopt.

Suppose a vessel of 500 horse power run 12 knots per hour on 40 tons coal per day: what will be the speed if she burn only 30 tons per day?

Thus:

40 : 30 :: 12^3 : V^3 (or cube of the required velocity,) Or, reduced, 4 : 3 :: 1728 : V^3, Equation, 3 1728 = 5184 = 4V^3, Or, 5184/4 = Cube root of 1296 = 10.902 knots = V, required velocity.

Thus, we reduce the quant.i.ty of coal one fourth, but the speed is reduced but little above one twelfth.

III. RELATION BETWEEN THE CONSUMPTION OF FUEL, AND THE LENGTH AND VELOCITY OF VOYAGE.

The consumption of fuel on two or more given voyages will vary as the square of the velocity multiplied into the distance travelled. Thus, during a voyage of 1200 miles, average speed 10 knots, the consumption of coal is 150 tons: we wish to know the consumption for 1800 miles at 8 knots. Thus:

150 tons : C required Consumption :: 10^2 knots 1200 miles : 8^2, Knots 1800 miles.

Then, C 100 1200 = 150 64 1800,*

Or, C 120,000 = 17,280,000 Reduced to C = 1728/12 = 144 tons consumption.

Suppose, again, that we wish to know the rate of speed for 1800 miles, if the coals used be the same as on another voyage of 1200 miles, with 150 tons coal, and ten knots speed:

We subst.i.tute former consumption, 150 tons for C, as in the equation above, marked *, and V^2 (square of the required velocity) for 64, and have,

150 100 1200 = 150 V^2 1800, Or, 120,000 = 1800V^2, Reduced, 1200/18 = V^2, And V = square root of 66.66 = 8.15 knots.

From the foregoing easily intelligible formulae we can ascertain with approximate certainty the large quant.i.ty of coal necessary to increase speed, the large saving of coal in reducing speed, as well as the means of accommodating the fuel to the voyage, or the voyage to the fuel. It is not necessary here to study very closely the economy of fuel, as this is a question affecting the transport of freight alone.

When the mails are to be transported, economy of fuel is not the object desired, but speed; and, consequently, we must submit to extravagance of fuel. This large expenditure of coal is not necessary in the case of freights, as they may be transported slowly, and, consequently, cheaply. But one of the princ.i.p.al reasons for rapid transport of the mails is that they may largely antic.i.p.ate freights in their time of arrival, and consequently control their movements.

I recently had an excellent opportunity of testing the large quant.i.ty of fuel saved on a slight reduction of the speed, and give it as ill.u.s.trative of the law advanced. We were on the United States Mail steamer ”Fulton,” Captain Wotton, and running at 13 miles per hour.

Some of the tubes became unfit for use in one of the boilers, and the fires were extinguished and the steam and water drawn off from this boiler, leaving the other one, of the same size, to propel the s.h.i.+p.

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