an estimate of the probable consumption of coals, per voyage, acorss the Atlantic of the "Great Britain," (late Mammoth,) steamer, founded on the basis of the nominal horse-power of the engines, accompanied with remarks on the speed that may be expected, and an assertion that, the area of the grate surface is "less than half the proper quantity." It will be my endeavour to point out, that nominal horse-power is a most inaccurate basis for calculations of this nature, and that no reliance can be placed on estimates deduced from such data. H.P. is a conventional expression which approximately represents the nett power exerted by a steam engine, only as long as engine manufacturers adherred to the limits introduced by Watt, of a load of 24 lb. per square inch, on the safety valves, and a light load on the piston moved by steam of full pressure throughout the stroke. It ceased to be of much value as soon as the safety-valve was loaded to from 5 to 8 lbs. per square inch, on board vessels, having what are still termed low pressure engines, inasmuch as they condense the steam; especially as ellgines are worked at the greatest possible load that the steam in the boiler can move. The rapidly increasing resistance of a vessel soon produces the full load on the piston. use. Nominal horse-power, also, becomes an expression daily of less value as expansion of steam on the cylinder is brought into more extended The elements of its calculations are, first-the area of the cylinder in square inches; second-the space passed over by the piston, in feet per minute; sometimes the actual speed is taken, which seldom varies much from Watt's rule, of 220 feet per minute; in other cases 200 feet is used, and also the constant of 7 lbs. nett steam pressure is employed to suit all cases. The nett pressure of 7 lb. of steam per square inch, obviously can move only a light load, less than was moved by Newcomen's engine in lifting water, and such steam can be supplied easily, and with much regularity, by means of the throttle-valve from 2 lbs. per square inch load on the safety-valve of the boiler. It is true that, 24 lb. load + 14 atmosphere, would give a pressure against a stoppage of the piston of 174 lbs. per square inch; but boilers of a moderate size could not, owing to irregularity of firing, guarantee a continued supply of steam of this pressure. An increased load is now often placed on the safety-valve of 5 or 6 lbs. per square inch, and this increase allows for some little boiler variation in the pressure of the steam, without affecting the working of the engine. Under these circumstances the surplus steam pressure acting on the connecting rod, is often from 10 to 12 lbs. per square inch, and the engine is worked at from 40 to 60 per cent. above the nominal horsepower. On the contrary, when expansion is used on a given cylinder the power exerted by the steam is reduced in a well-known ratio, subject to some slight interferences from clearance steam, and cooling; but the ratio of reduction of power is much less than the ratio of the reduction in the consumption of coal. Occasionally these sources of error-the increase of pressure from high steam, and the reduction of mean pressure from expansion, may correct each other. Such is their obvious tendency, but are we to trust to chance for a possible result, when a very simple basis for the connection of the total power exerted by the steam on the cylinder, and the power due to the water expended, as steam, at full pressure in the cylinder, (the measure of the engine's consumption of coal,) offers itself to our notice. This basis requires the pressure of the steam in the cylinder during the time the communication between it and the boiler is open, to be ascertained or assumed, and it also requires an addition to be made to the quantity of coal expended, in producing steam, for the engine, for blowing off, and other waste, attendant on the generation of such steam. We should never forget that, the engine is a machine to use steam, and that a boiler only produces it, and this distinction becomes of more value in expansion engines, in which the power exerted by the steam on the cylinder, after the steam-valve is closed, is obtained without any expenditure of coal. In full-pressure engines the power of steam due to its pressure and space, is nearly a direct measure of the coal expended in boilers of equal merits as evaporating machines; especially, if the excentric employed in shutting the valves saves a few cubic feet of steam equivalent to the clearance spaces. In expansion engines the cubic feet of a known or assumed steam pressure expended, must be measured from the point at which the communication with the boiler is closed. Now, as the cubic feet of steam of a given pressure expended per hour are easily known, and if not known, given conditions may be assumed approximately; the quantity of water expended as steam equally becomes known, and the quantity of coal necessary for its evaporation also known. Estimates founded on these principles, are far more likely to give an approximation to truth, than deductions from the inaccurate data from which nominal horse-power is obtained. Not having any great regard for science in decimals, I conceive I may safely increase the cylinder capacity by 62 of a foot, making it 254 cubic feet in the Great Britain's engines, and a foot for half throw; this error, and those due to clearance spaces, which I also propose to omit, as unknown to me, are both less than the probable errors of observations or assumption in the present case; and consequently they may be neglected without much injury to the resulting estimates. The long average, and even the daily consumption under the same conditions are often at variance to a greater extent, I expect, than will prove to be the case with these calculations, whenever they are founded on data correctly taken. Taking the capacity of the cylinder at 254 cubic feet, and 9120 cylinders of steam per hour, (from 19 strokes per minute, and 8 cylinders of steam per revolution,) we have 2,316,480 cubic feet of steam per hour; and supposing such steam to be expended at an absolute or total pressure equal to one atmosphere on the cylinder, as the volume of steam for 1 of water at that pressure is about 1700, we have 2.316.480 1700 1362 cubic feet of water per hour expended in the cylinder exclusive of boiler waste. If good boilers can, exclusive of waste evaporate 7 lbs. per 1 lb. of coal, (other suppositions are easily made,) then of salt water Cubic Feet lbs. 1362 x 62.5 7 = 12160 lbs. of coal to 5.428 tons of coal per hour, and hence 130 tons per day, and this quantity becomes 65 do. 431 do. for steam cut off at half stroke, do. do. at one-third stroke. The horse-power due to the evaporation of 1362 cubic feet of water per hour must now be estimated. Taking the atmosphere at 14.72 lbs. per square inch, we have 2129 lbs. per square foot x 1700 cubic feet from each cubic foot of water expended in the cylinder 3,600,000 lbs. one foot high per hour, and 60,000 lbs. per minute. A gross power capable of producing a nett power of 33,000 lbs. one foot high on the connecting rod per minute. = Hence exclusive of waste, a cubic foot of water expended as steam is equivalent to one horse-power per hour. We are now enabled to approximate to the power when the steam is used two or three times expansively. In consequence of the assertion that the Great Britain's engines are only 1,000 horse-power, while their nominal horse-power, by the usual rule is obviously nearly 1,200, I have always concluded that these engines were intended to be worked expansively, which may be readily done to the greatest extent here calculated by four double acting cylinders driving one crank shaft, giving eight cylinders' action in one revolution; and, producing a great regularity of power, especially as the vessel itself acts as a regulator of power. Under these circumstances it appears to me that, less than one-half of the proper quantity of grate surface may be amply sufficient for the Great Britain; and, I think, the word usual in lieu of proper would have been a less objectionable term, in the Civil Engineers and Architects' Journal. Enough has been adduced to raise a doubt of the value of the data employed by the writer in that journal, in the calculation of the probable coal consumption. I now propose to advert to the probable speed. The large Atlantic steamers at present in use, are large steam ships. The Great Britain is an enormous steam boat, instead of her draught of water exceeding half her breadth, it will not quite amount to onethird of it, in fact she seems to be of the form that has been recommended by more than one of the correspondents in the Nautical Magazine. Consequently if the factor 1400 is correct for the common class of Atlantic steamers, as shewn by experience, it will not be likely to prove correct under such different conditions. The high velocities attained iu the experimental trials of Cunard's Packets were either from vessels with a medium, or, even light load, which could give but little indication of the sea rate of speed of the loaded vessel; this seems to have been a mean 8 miles per hour; and these very circumstances tend to show the propriety of the system adopted by the parties by whom the Great Britain was designed. If the screw is successful she will start as a moderately loaded vessel, and improve in speed as her coal is consumed, and her load is lessened. The performance of her engines will be unaffected by waves, except so far as they cause a greater resistance, and her impelling power will never be almost unable to reach the water, a result that has occasionally happened to some Atlantic steamers at the end of their voyages,-advantages that may balance the assumed greater speed of paddle wheels in fine weather. I would further remark that the quantity I have assumed as the consumption of coal in a full pressure engine, amounts to 10 lbs. per horsepower per nominal horse-power. We hear of 8 lbs. and even 64 lbs. per horse-power occasionally, but in these cases we may expect to find that the engines have been worked in some degree expansively; but this system cannot be carried out to its full extent with low pressure steam, except by the plan adopted in the Great Britain of a large amount of cylinder capacity in proportion to the boiler. High pressure steam to produce the usual mean pressure by expansion, in a given cylinder, is a different question. As regards the minor questions of the proper, or, rather usual size of the air pump and condensers, it is obvious the air pump might be reduced in proportion to the expansion, to extract the less amount of injection water and liberated air; yet in the only district in which expansion engines are in use, Watt's proportions nearly are preserved to condense steam of from 7 to 10 lbs. below atmospheric pressure. The condensers are stated to be nearly three times as large "as experience shows to be necessary," the tendency, however, of a large condenser is favorable to a rapid reduction of steam pressure on the opening of the eduction valve, and, under the assumed conditions of three times expansion, the steam at the end of the stroke will be of a pressure of 5 lbs., that is 9 or 10 lbs. under the atmosphere. By means of a condenser as large as the cylinder this pressure is reduced to about 3 lbs. at once, without any aid from injection water. Still, in engines with a slow stroke, the size of the condenser may be of less importance, but I should like to ascertain whether the practice of using small condensers has been occasioned by want of space in steamers, or, in consequence of experiments definitively proving their advantages. In conclusion, I would advert to the stowage of 1000 tons of coal on board the Great Britain, which would give a consumption of 50 tons per day for 20 days, and it would afford a considerable allowance, in case I have taken the water evaporation too high per lb. of coal; and for my omitting to notice, the proportional increased fraction of engine resistance to the mean steam pressure, when reduced by expansion, which tends to prevent the practical advantages of expansion from actually reaching CAPT. MIDGLEY ON A VOYAGE TO THE GOLD COAST. 25 the theoretical. I will further add, that the sketches of the Great Britain's boilers referred to, are at variance with my recollections of them. It should also be remembered that, when the Great Britain was commenced, we had little or no experience of the larger class of iron vessels. Since that period the Nemesis ran against one of the outer rocks of Scilly at full speed; and the dents in the stem, and in one of the iron plates of the bow were repaired for less than £30. The Nemesis also encountered a gale of wind for three days off the Cape of Good Hope, without injury,-circumstances in favour of the views of the parties who recommended the plan of building the Great Brtain of iron. I have ventured to call the attention of your readers to conditions and principles, well known perhaps, but as essential to the formation of a correct opinion respecting steam-vessels, as the knowledge of the diameter and length of the guns of any class of ships of war is, in ascertaining the actual force of the vessel. Nov. 14th, 1842. Your obedient servant, J. S. ENYS. NAUTICAL OBSERVATION ON A VOYAGE TO THE GOLD COAST. Liverpool Shipmasters' Association, Old Hall Street, Dec. 16th 1842. SIR.-The accompanying interesting notes were handed to me by Captain Midgley, for insertion in the Journal of this Association, into which they have been copied. And as he kindly placed them at my disposal, I forward them to you, as I know no better means of rendering them generally useful, than through your very valuable Magazine. To the Editor of the Nautical Magazine. I have, &c., SAMUEL MOSS, Master of the Rooms. THE extensive tract of sea between the meridian of 14° W. and 9° E., and the parallel of 9° N. and 3° S., is yet imperfectly known by the majority of navigators; being bordered by a range of coast, in a great measure peopled by an uncivilized and semi-barbarous race, who require peculiar address and management in the method of dealing with them for the produce of their country. I shall suppose that, a vessel bound to this coast* has got out of the channel, or, at least clear of Tuskar and the Smalls, in which case the master's mind will be comparatively at ease, to what it was when he had less sea-room in the narrows of the channel. But whilst the ship is in soundings she will be more or less exposed to a northerly, or N.N.E. indraught of tide or current, which sets nine hours out of every twelve, even at a consider • All vessels bound to the Gold Coast should be provided with the charts constructed from Capt. Vidal's Survey of it, and recently published by the Admiralty. We noticed these charts in p. 174, of our volume for 1841, and preceded that notice with Capt. Vidal's own account of the survey. With these charts no master of a vessel can plead ignorance of the coast.-ED. N.M. ENLARGED SERIES.-NO. 1.-VOL. FOR 1843. E |