SCREW ENGINES OF THE "GREAT EASTERN."

Lyttelton Times, Volume VIII, Issue 516, 14 October 1857, Page 3

SCREW ENGINES OF THE "GREAT EASTERN."

The screw engines manufactured by Messrs. James Watt and Co., for the steamer Great Eastern; are the largest and most powerful engines ever yet constructed; There are four cylinders, each of eighty-four inches diameter, for. driving the screw of this vessel. The length of the stroke is four feet, and the makers reckon that the engines will make

about forty-five revolutions per minute. The cylinders lie on their sides, as is a common arrangement in screw engines: the piston-rods protrude through the ends of the cylinders nearest that central shaft oil the end of which the screw is hung, and the connecting rods attached to i.lie piston-rods engage cranks in the screw shaft and turn it round just in the same manner as the arms turn round a grindstone. The pistons, which are solid plugs, pressed by the steam backwards and forwards in the cylinders, communicate their reciprocating movements through the medium of the piston-rods to the connecting rods; and in this way the screw shaft is turned round, and the vessel is screwed forward in the water just as a screw augur turned round advances in a piece of wood, or a corkscrew in a cork.

The pressure of the steam in the boilers is 251b. on each square inch. The total heating surface in the boilers of the screw engines is about 30,000 square feet. The nominal power of the screw engines 1700-horse power; and, if they work up to four times their nominal power, which is not an unusual performance, the actual indicated power will be 6800-horse power. The area of that part of the cross section of the ship which falls beneath the water line is about 2000 square feet, when the vessel is drawing 28 feet of water. The estimated amount of water evaporated by the boilers of the screw engines per hour is 3150 cubic feet, and the area of the grate-bars is 1218 square feet.

The screw is formed with four blades. Its diameter is 24 feet, and its pitch or the distance wich it would advance during each revolution if it worked in a solid body, like a corkscrew penetrating a cork, is 44 feet.

In addition to the screw, the vessel is supplied with paddle-wheels driven by four engines, each of 72 inches diameter of cylinder, and 14 feet stroke, and rated collectively at 1000 nominal horses' power. If we suppose that these engines also work up to four times their nominal power, it will be quite safe to reckon the actual power effective in propelling the vessel at 10,000 indicator horses' power.

It is a question of much interest to determine what amount of speed this power will impart to the vessel. Messrs. James Watt and Co.'s anticipation is that the speed of the vessel will be about seventeen miles an hour; and from that to eighteen fmiles seems to be about the limit engineers have hitherto predicted. But we believe that these anticipations fall very far short of what the real speed will be, and which we do not hesitate to predict will turn out to be between twenty-four and twenty-five miles per hoar. No allowance has been made in the existing computations of the speed for the great size of the vessel; yet it is well known that large vessels are more easily propelled, relatively with their proportion of power, than small vessels, as is popularly manifested at every yacht race, when an allowance of time is made for the smallness of the vessels; and, in France, where the variation in the resistance consequent upon size has been carefully investigated, it is found that the velocities attained by similar vessels, but of different sizes, vary as the square root ot any linear dimension. A vessel, therefore, of twice the length of the Himalaya, and with four times the sectional area and four times the power, will be faster than the Himalaya in the proportion of the square root of 2 to the square root of 1, or 1.4 times; so that the Great Eastern, had she been built of the same size as the Himalaya, has proportion of power enough to attain a speed of seventeen miles an hour, she will with her existing dimensions, be 1.4 times faster, or go 23.8, nearly 24 miles an hour. By reckoning the resistance as proportionate to the immersed perimeter, the speed comes out as 23 miles. At the increased speed, however, the engines, if duly supplied with steam, will develop more power than at the computed speed, so that, in all probability, a speed of close upon twenty-five statute miles per hour will be at-

tamed,

The existing mode of estimating the resistance by the area of the immersed midship section is erroneous, except in the comparison of vessels of similar dimensions. It is in putting into motion a column of water by friction that the power of the engine in well-formed vessels is chiefly expended, and the magnitude of this column depends, not upon the area of the cross section, but the amount of rubbing surface it offers to the waters. The resistance of rivers is measured by the length, of the outline in th* cross section of the bed: and large rivers with the same declivity, run much more swiftly than small. Inriike manner it should be by the immersed perimeter of the cross section that the resistance of ships should be measured, and when this is done it will be seen how very much less is the proportionate resistance of large vessels. A speed of thirty miles an hour in steam vessels is not, we are persuaded, very distant of attainment. Nor does it appear probable that at high velocities the resistance will be found fto increase at the same rapid rate^ as at low. It is the adhesion of the water which at low speeds consumes power, and this adhesion moves the 'contiguous water because it is_ easier to do so than for the vessel to pass over it as if rubbing on a solid. But as with every increase of speed a thicker film of water adheres, the resistance occasioned by moving this mass of water will gradually become so great that it will be easier for the vessel to rub over the film than drag it with her; and when this takes place the friction will thereafter follow the law which obtains in the case of solid bodies, and the resistance will no longer increase as the square of the velocity. At what point the equilibrium between the adhesion and the friction will be attained is a question which experiment must determine j but the apprehension of the

fact that it will be attained at some determinate 'velocity gives warrant for the expectation of higher rates of speed in steam navigation than has heretofore been thought possible°of attainment.

The success of the Great Eastern as a commercial entripiisH depends mainly upon Ihm capabilities of realising some such speed as 25 miles an .hour. With such a speed she can command employment on any station, and can also compel the Government to give her a subvention for carrying the mails. With such an ordinary rate of speed as sixteen or seventeen miles an hour her commercial success is far more problematical. Moreover, with so large a cost a vital element is time. She must neither be a day longer on the voyage nor a day longer in harbour than is absolutely indispensable, 'but must be driven at such a rate as to make the capital productive. Taking the cost at £500,000, and the interest and depreciation at only ten per cent upon this amount, we have about £1000 a week of expense from this source alone. The actual atnount chargeable to this item will be very much more than is here reckoned, but the approximation is sufficient to show the importance of attaining and maintaining high speeds on this sole inducement. Of the twenty-five miles an hour we have no doubt whatever, if there is the steam; and the boilers will produce the steam if the draught is sufficient. In most steam vessels the draught is too sluggish, and the heat in the furnaces is not suflciently intense. It has been proposed to use anthracite coal in the Great Eastern, but this is an experiment, and Welsh coals can be more safely depended on. The bridges at the ends of the furnaces should be high, so as to retain a high temperature in the furnace, which both consumes the coal more effectively and compels more of the heat to enter the water in the region of the furnaces—thus leaving less work for the tubes or flues to do. There should be good steam jets in the chimneys, so as to ensure a strong draught. With these simple precautiona we have no fear of the speed; and, as the vessel is the strongest vesssel ever sent to sea, there can be no doubt of her complete seaworthiness. The auxiliary engines for turning the screw round when the screw engines are not at work are, in- our judgment, unnecessary; and every engine or piece of machinery not absolutely necessary is a complication and disparagement. Simplicity and fewness of parts to look after and keep in repair is a most important desideratum in steam navigation. Transcendentalism will not work in such a sphere, and in our judgment some of the refinements introduced into the Great Eastern might have been advantageously omitted. But in all its main features there can be no doubt whatever of the soundness of the design or of the excellence of the execution, while we believe that the performance of the vessel will exceed everything which the most sanguine supporters of the enterprise have ventured to anticipate, and will far outrun the prognostications which engineers, overlooking the element of size, have hitherto made.