Section III.—Marine Engines

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Section III.—Marine Engines.

The changes which have now become completed in the marine steam-engine have been effected at a later date than those which produced the modern locomotive. On the American rivers the modification of the beam-engine since the time of Robert L. Stevens has been very slight. The same general arrangement is retained, and the details are little, if at all, altered. The pressure of steam is sometimes as high as 60 pounds per square inch.

Fig. 130.—Beam-Engine.

The valves are of the disk or poppet variety, rising and falling vertically. They are four in number, two steam and two exhaust valves being placed at each end of the steam-cylinder. The beam-engine is a peculiarly American type, seldom if ever seen abroad. Fig. 130 is an outline sketch of this engine as built for a steamer plying on the Hudson River. This class of engine is usually adopted in vessels of great length, light draught, and high speed. But one steam-cylinder is commonly used. The cross-head is coupled to one end of the beam by means of a pair of links, and the motion of the opposite end of the beam is transmitted to the crank by a connecting-rod of moderate length. The beam has a cast-iron centre surrounded by a wrought-iron strap of lozenge shape, in which are forged[380] the bosses for the end-centres, or for the pins to which the connecting-rod and the links are attached. The main centre of the beam is supported by a “gallows-frame” of timbers so arranged as to receive all stresses longitudinally. The crank and shaft are of wrought-iron. The valve-gear is usually of the form already mentioned as the Stevens valve-gear, the invention of Robert L. and Francis B. Stevens. The condenser is placed immediately beneath the[381] steam-cylinder. The air-pump is placed close beside it, and worked by a rod attached to the beam. Steam-vessels on the Hudson River have been driven by such engines at the rate of 20 miles an hour. This form of engine is remarkable for its smoothness of operation, its economy and durability, its compactness, and the latitude which it permits in the change of shape of the long, flexible vessels in which it is generally used, without injury by “getting out of line.”

Fig. 131.—Oscillating Engine and Feathering Paddle-Wheel.

For paddle-engines of large vessels, the favorite type, which has been the side-lever engine, is now rarely built. For smaller vessels, the oscillating engine with feathering paddle-wheels is still largely employed in Europe. This style of engine is shown in Fig. 131. It is very compact, light, and moderately economical, and excels in simplicity. The usual arrangement is such that the feathering-wheel has the same action upon the water as a radial wheel of double diameter. This reduction of the diameter of the wheel, while retaining maximum effectiveness, permits a high speed of engine, and therefore less weight, volume, and cost. The smaller wheel-boxes, by offering less resistance to the wind, retard the progress of the vessel less than those[382] of radial wheels. Inclined engines are sometimes used for driving paddle-wheels. In these the steam-cylinder lies in an inclined position, and its connecting-rod directly connects the crank with the cross-head. The condenser and air-pump usually lie beneath the cross-head guides, and are worked by a bell-crank driven by links on each side the connecting-rod, attached to the cross-head. Such engines are used to some extent in Europe, and they have been adopted in the United States navy for side-wheel gunboats. They are also used on the ferry-boats plying between New York and Brooklyn.

Fig. 132.—The Two Rhode Islands, 1836-1876.

Among the finest illustrations of recent practice in the construction of side-wheel steamers are those built for the several routes between New York and the cities of New England which traverse Long Island Sound. Our illustration exhibits the form of these vessels, and also shows well the modifications in structure and size which have been made during this generation. The later vessel is 325 feet long, 45 feet beam, 80 feet wide over the “guards,” and 16 feet deep, drawing 10 feet of water. The “frames” upon which the planking of the hull is fastened are of white-oak, and the lighter and “top” timbers of cedar and locust. The engine has a steam-cylinder 90 inches in diameter and 12 feet stroke of piston.[95] On each side the great saloons which extend from end to end of the upper deck are state-rooms, containing each two berths and elegantly furnished. The engine of this vessel is capable of developing about 2,500 horse-power. The great wheels, of which the paddle-boxes are seen rising nearly to the height of the hurricane-deck, are 371∕2 feet in diameter and 12 in breadth. The hull of this vessel, including all wood-work, weighs over 1,200 tons. The weight of the machinery is about 625 tons. The steamer makes 16 knots an hour when the engine is at its best speed—about 17 revolutions per minute—and its[383] average speed is about 14 knots on its route of 160 miles. The coal required to supply the furnaces of such a vessel and with such machinery would be about 3 tons per hour.[384] or a little over 21∕2 pounds per horse-power. The construction of such a vessel occupies, usually, about a year, and costs a quarter of a million dollars.

Fig. 133.—A Mississippi Steamboat.

The non-condensing direct-acting engine is used principally on the Western rivers, driven by steam of from 100 to 150 pounds pressure, and exhausts its steam into the atmosphere. It is the simplest possible form of direct-acting engine. The valves are usually of the “poppet” variety, and are operated by cams which act at the ends of long levers having their fulcra on the opposite side of the valve, the stem of which latter is attached at an intermediate point. The engine is horizontal, and the connecting-rod directly attached to cross-head and crank-pin without intermediate mechanism. The paddle-wheel is used, sometimes as a stern-wheel, as in the plan of Jonathan Hulls of one and[385] a half century ago, sometimes as a side-wheel, as is most usual elsewhere. One of the most noted of these steamers, plying on the Mississippi, is shown in the preceding sketch.

One of the largest of these steamers was the Grand Republic,[96] a vessel 340 feet long, 56 feet beam, and 101∕4 feet depth. The draught of water of this great craft was 31∕2 feet forward and 41∕2 aft. The two sets of compound engines, 28 and 56 inches diameter and of 10 feet stroke, drive wheels 381∕2 feet in diameter and 18 feet wide. The boilers were steel. A steamer built still later on the Ohio has the following dimensions: Length, 225 feet; breadth, 351∕2 feet; depth, 5 feet; cylinders, 173∕8 inches in diameter, 6 feet stroke; three boilers. The hull and cabin were built at Jeffersonville, Ind. She has 40 large state-rooms. The cost of the steamer was $40,000.

These vessels have now opened to commerce the whole extent of the great Mississippi basin, transporting a large share of the products of a section of country measuring a million and a half square miles—an area equal to many times that of New York State, and twelve times that of the island of Great Britain—an area exceeding that of the whole of Europe, exclusive of Russia and Turkey, and capable, if as thoroughly cultivated as the Netherlands, of supporting a population of between three and four hundred millions of people.

The steam-engine and propelling apparatus of the modern ocean-steamer have now become almost exclusively the compound or double-cylinder engine, driving the screw. The form and the location of the machinery in the vessel vary with the size and character of the ship which it drives. Very small boats are fitted with machinery of quite a different kind from that built for large steamers, and war-vessels have usually been supplied with engines of a design radically different from that adopted for merchant-steamers.

Fig. 134.—Steam-Launch, New York Steam-Power Company.

The introduction of Steam-Launches and small pleasure-boats[386] driven by steam-power is of comparatively recent date, but their use is rapidly increasing. Those first built were heavy, slow, and complicated; but, profiting by experience, light and graceful boats are now built, of remarkable swiftness, and having such improved and simplified machinery that they require little fuel and can be easily[387] managed. Such boats have strong, carefully-modeled hulls, light and strong boilers, capable of making a large amount of dry steam with little fuel, and a light, quick-running engine, working without shake or jar, and using steam economically.

Fig. 135.—Launch-Engine.

The above sketch represents the engine built by a New York firm for such little craft. This is the smallest size made for the market. It has a steam-cylinder 3 inches in diameter and a stroke of piston of 5 inches, driving a screw 26 inches in diameter and of 3 feet pitch. The maximum[388] power of the engine is four or five times the nominal power. The boiler is of the form shown in the illustrations of semi-portable engines, and has a heating-surface, in this case, of 75 square feet. The boat itself is like that seen on page 386, and is 25 feet long, of 5 feet 8 inches beam, and draws 21∕4 feet of water. These little machines weigh about 150 pounds per nominal horse-power, and the boilers about 300.

Some of these little vessels have attained wonderful speed. A British steam-yacht, the Miranda, 451∕2 feet in length, 53∕4 feet wide, and drawing 21∕2 feet of water, with a total weight of 33∕4 tons, has steamed nearly 181∕2 miles an hour for short runs. The boat was driven by an engine of 6 inches diameter of cylinder and 8 inches stroke of piston, making 600 revolutions per minute, driving a two-bladed screw 21∕2 feet in diameter and of 31∕3 feet pitch. Its machinery had a total weight of two tons. Another English yacht, the Firefly, is said to have made 18.94 miles an hour. A little French yacht, the Hirondelle, has attained a speed of 16 knots, equal to about 181∕2 miles, an hour. This was, however, a much larger vessel than the preceding. One of the most remarkable of these little steamers is a torpedo-boat built for the United States navy. This vessel is 60 feet long, 6 feet wide, and 5 feet deep; its screw is 38 inches in diameter and of 5 feet pitch, two-bladed, and is driven, by a very light engine and boiler, 400 revolutions per minute, the boat attaining a speed of 19 to 20 miles an hour. Another little vessel, the Vision, made nearly as great speed, developing 20 horse-power with engine and boiler weighing but about 400 pounds.

Yachts of high speed require such weight and bulk of engine that but little space is left for cabins, and they are usually exceedingly uncomfortable vessels. In the Miranda the weight of machinery is more than one-half the total weight of the whole. An illustration of the more comfortable and more generally liked pleasure-yacht is the Day Dream. The length is 105 feet, and the boat draws 51∕2[389] feet of water. There are two engines, having steam-cylinders 14 inches in diameter and of the same length of stroke, direct-acting, condensing, and driving a screw, of 7 feet diameter and of 101∕2 feet pitch, 135 revolutions a minute, giving the yacht a speed of 131∕2 knots an hour.

Fig. 136.—Horizontal, Direct-acting Naval Screw-Engine.

In larger vessels, as in yachts, in nearly all cases, the ordinary screw-engine is direct-acting. Two engines are placed side by side, with cranks on the shaft at an angle of 90° with each other. In merchant-steamers the steam-cylinders are usually vertical and directly over the crank-pins, to which the cross-heads are coupled. The condenser is placed behind the engine-frame, or, where a jet-condenser is used, the frame itself is sometimes made hollow, and serves as a condenser. The air-pump is worked by a beam connected by links with the cross-head. The general arrangement is like that shown in Figs. 137 and 138. For naval purposes such a form is objectionable, since its height is so great that it would be exposed to injury by shot. In naval engineering the cylinder is placed horizontally, as in Fig. 136, which is a sectional view, representing an horizontal, direct-acting naval screw-engine, with jet-condenser and double-acting air and circulating pumps. A is the steam-cylinder, B the piston, which is connected to the crank-pin by the piston-rod, D, and connecting-rod, E.[390] F is the cross-head guide. The eccentrics, G, operate the valve, which is of the “three-ported variety,” by a Stephenson link. Reversing is effected by the hand-wheel, C, which, by means of a gear, m, and a rack, k, elevates and depresses the link, and thus reverses the valve.

The trunk-engine, in which the connecting-rod is attached directly to the piston and vibrates within a trunk or cylinder secured to the piston, moving with it, and extending outside the cylinder, like an immense hollow piston-rod, is frequently used in the British navy. It has rarely been adopted in the United States.

Fig. 137.—Compound Marine Engine. Side Elevation.

Fig. 138.—Compound Marine Engine. Front Elevation and Section.

[391]In nearly all steam-vessels which have been built for the merchant service recently, and in some naval vessels, the compound engine has been adopted. Figs. 137 and 138 represent the usual form of this engine. Here A A, B B are the small and the large, or the high-pressure and the low-pressure cylinders respectively. C C are the valve-chests. G G is the condenser, which is invariably a surface-condenser. The condensing water is sometimes directed around the tubes contained within the casing, G G, while the steam is exhausted around them and among them,[392] and sometimes the steam is condensed within the tubes, while the injection-water which is sent into the condenser to produce condensation passes around the exterior of the tubes. In either case, the tubes are usually of small diameter, varying from five-eighths to half an inch, and in length from four to seven feet. The extent of heating-surface is usually from one-half to three-fourths that of the heating-surface of the boilers.

The air and circulating pumps are placed on the lower part of the condenser-casting, and are operated by a crank on the main shaft at N; or they are sometimes placed as in the style of engine last described, and driven by a beam worked by the cross-head. The piston-rods, T S, are guided by the cross-heads, V V, working in slipper-guides, and to these cross-heads are attached the connecting-rods, X X, driving the cranks, M M. The cranks are now usually set at right angles; in some engines this angle is increased to 120°, or even 180°. Where it is arranged as here shown, an intermediate reservoir, P O, is placed between the two cylinders to prevent the excessive variations of pressure that would otherwise accompany the varying relative motions of the pistons, as the steam passes from the high-pressure to the low-pressure cylinder. Steam from the boilers enters the high-pressure steam-chest, x, and is admitted by the steam-valve alternately above and below the piston as usual. The exhaust steam is conducted through the exhaust passage around into the reservoir, P, whence it it is taken by the low-pressure cylinder, precisely as the smaller cylinder drew its steam from the boiler. From the large or low-pressure cylinder the steam is exhausted into the condenser. The valve-gear is usually a Stephenson link, g e, the position of which is determined, and the reversal of which is accomplished, by a hand-wheel, o, and screw, m n p, which, by the bell-crank, k i, are attached to the link, g e. The “box-framing” forms also the hot-well. The surface-condenser is cleared by a single-acting air-pump,[393] inside the frame, at T. The feed-pump and the bilge-pumps are driven from the cross-head of the air-pump.

John Elder.

The successful introduction of the double-cylinder engine was finally accomplished by the exertions of a few engineers, who were at once intelligent enough to understand its advantages, and energetic and enterprising enough to push it forward in spite of active opposition, and powerful enough, pecuniarily and in influence, to succeed. The most active and earnest of these eminent men was John Elder, of the firm of Randolph, Elder & Co., subsequently John Elder & Co., of Glasgow.[97]

Elder was of Scotch descent. His ancestors had, for[394] generations, shown great skill and talent in construction, and had always been known as successful millwrights. John Elder was born at Glasgow, March 8, 1824, and died in London, September 17, 1869. He was educated at the Glasgow High-School and in the College of Engineering at the University of Glasgow, where, however, his attendance was but for a short time. He learned the trade under his father in the workshops of the Messrs. Napier, and became an unusually expert draughtsman. After spending three years in charge of the drawing-office at the engine-building works of Robert Napier, where his father had been manager, Elder became a partner in the firm which had previously been known as Randolph, Elliott & Co., in the year 1852. The firm commenced building iron vessels in 1860.

In the mean time, the experiments of Hornblower and Wolff, of Allaire and Smith, and of McNaught, Craddock, and Nicholson, together with the theoretical investigations of Thompson, Rankine, Clausius, and others, had shown plainly in what direction to look for improvement upon then standard engines, and what direction practice was taking with all types. The practical deductions which were becoming evident were recognized very early by Elder, and he promptly began to put in practice the principles which his knowledge of thermo-dynamics and of mechanics enabled him to appreciate. He adopted the compound engine, and coupled his cranks at angles of 180°, in order to avoid losses due to the friction of the crank-shaft in its bearings, by effecting a partial counterbalancing of pressures on the journals. Elder was one of the first to point out the fact that the compound engine had proved itself more efficient than the single-cylinder engine, only when the pressure of steam carried and the extent to which expansion was adopted exceeded the customary practice of his time. His own practice was, from the first, successful, and from 1853 to 1867 he and his partners were continually engaged in the construction of steamers and fitting them with compound engines.

[395]The engines of their first vessel, the Brandon, required but 31∕4 pounds of coal per hour and per horse-power, in 1854, when the usual consumption was a third more. Five years later, they had built engines which consumed a third less than those of the Brandon; and thenceforward, for many years, their engines, when of large size, exhibited what was then thought remarkable economy, running on a consumption of from 21∕4 to 21∕2 pounds.

In the year 1865 the British Government ordered a competitive trial of three naval vessels, which only differed in the form of their engines. The Arethusa was fitted with trunk-engines of the ordinary kind; the Octavia had three steam-cylinders, coupled to three cranks placed at angles of 120° with each other; and the Constance was fitted with compound engines, two sets of three cylinders each, and each taking steam from the boiler into one cylinder, passing it through the other two with continuous expansion, and finally exhausting from the third into the condenser. These vessels, during one week’s steaming at sea, averaged, respectively, 3.64, 3.17, and 2.51 pounds of coal per hour and per horse-power, and the Constance showed a marked superiority in the efficiency of the mechanism of her engines, when the losses by friction were compared.

The change from the side-lever single-cylinder engine, with jet-condenser and paddle-wheels, to the direct-acting compound engine, with surface-condenser and screw-propellers, has occurred within the memory and under the observation of even young engineers, and it may be considered that the revolution has not been completely effected. This change in the design of engine is not as great as it at first seemed likely to become. Builders have but slowly learned the principles stated above in reference to expansion in one or more cylinders, and the earlier engines were made with a high and low pressure cylinder working on the same connecting-rod, and each machine consisted of four steam-cylinders. It was at last discovered that a high-pressure single-cylinder[396] engine exhausting into a separate larger low-pressure engine might give good results, and the compound engine became as simple as the type of engine which it displaced. This independence of high and low pressure engines is not in itself novel, for the plan of using the exhaust of a high-pressure engine to drive a low-pressure condensing engine was one of the earliest of known combinations.

The advantage of introducing double engines at sea is considerably greater than on land. The coal carried by a steam-vessel is not only an item of great importance in consequence of its first cost, but, displacing its weight or bulk of freight which might otherwise be carried, it represents so much non-paying cargo, and is to be charged with the full cost of transportation in addition to first cost. The best of steam-coal is therefore usually chosen for steamers making long voyages, and the necessity of obtaining the most economical engines is at once seen, and is fully appreciated by steamship proprietors. Again, an economy of one-fourth of a pound per horse-power per hour gives, on a large transatlantic steamer, a saving of about 100 tons of coal for a single voyage. To this saving of cost is to be added the gain in wages and sustenance of the labor required to handle that coal, and the gain by 100 tons of freight carried in place of the coal.

For many years the change which has here been outlined, in the forms of engine and the working of steam expansively, was retarded by the inefficiency of methods and tools used in construction. With gradual improvement in tools and in methods of doing work, it became possible to control higher steam and to work it successfully; and the change in this direction has been steadily going on up to the present time with all types of steam-engine. At sea this rise of pressure was for a considerable time retarded by the serious difficulty encountered in the tendency of the sulphate of lime to deposit in the boiler. When steam-pressure had risen to 25 pounds per square inch, it was[397] found that no amount of “blowing out” would prevent the deposition of seriously large quantities of this salt, while at the lower pressures at first carried at sea no troublesome precipitation occurred, and the only precaution necessary was to blow out sufficient brine to prevent the precipitation of common salt from a supersaturated solution. The introduction of surface-condensation was promptly attempted as the remedy for this evil, but for many years it was extremely doubtful whether its disadvantages were not greater than its advantages. It was found very difficult to keep the condensers tight, and boilers were injured by some singular process of corrosion, evidently due to the presence of the surface-condenser. The simple expedient of permitting a very thin scale to form in the boiler was, after a time, hit upon as a means of overcoming this difficulty, and thenceforward the greatest obstacle to the general introduction was the conservative disposition found among those who had charge of marine machinery, which conservatism regarded with suspicion every innovation. Another trouble arose from the difficulty of finding men neither too indolent nor too ignorant to take charge of the new condenser, which, more complicated and more readily disarranged than the old, demanded a higher class of attendants. Once introduced, however, the surface-condenser removed the obstacle to further elevation of steam-pressure, and the rise from 20 to 60 pounds pressure soon occurred. Elder and his competitors on the Clyde were the first to take advantage of the fact when these higher pressures became practicable.

The lightness of engine and the smaller weight of boiler secured when the simpler type of “compound” engine is used are great advantages, and, when coupled with the fact that by no other satisfactory device can great expansion and consequent economy of fuel be obtained at sea, the advantages are such as to make the adoption of this style of engine imperative for ship-propulsion.

[398]This extreme lightness in machinery has been largely, also, the result of very careful and skillful designing, of intelligent construction, and of care in the selection and use of material. British builders had, until after the introduction of these later types of vessels-of-war, been distinguished rather by the weight of their machinery than for nice calculation and proportioning of parts. Now the engines of the heavy iron-clads are models of good proportions, excellence in materials, and of workmanship, which are well worthy of study. The weight per indicated horse-power has been reduced from 400 or 500 pounds to less than half that amount within the last ten years. This has been accomplished by forcing the boilers—although thus, to some extent, losing economy—by higher steam-pressure, a very much higher piston-speed, reduction of friction of parts, reduction of capacity for coal-stowage, and exceedingly careful proportioning. The reduction of coal-bunker capacity is largely compensated by the increase of economy secured by superheating, by increased expansion, elevation of piston-speed, and the introduction of surface-condensation.

A good marine steam-engine of the form which was considered standard 15 or 20 years ago, having low-pressure boilers carrying steam at 20 or 25 pounds pressure as a maximum, expanding twice or three times, and having a jet-condenser, would require about 30 or 35 pounds of feed-water per horse-power per hour; substituting surface-condensation for that produced by the jet brought down the weight of steam used to from 25 to 30 pounds; increasing steam-pressure to 60 pounds, expanding from five to eight times, and combining the special advantages of the superheater and the compound engine with surface-condensation, has reduced the consumption of steam to 20, or even, in some cases, 15 pounds of steam per horse-power per hour. Messrs. Perkins, of London, guarantee, as has already been stated, to furnish engines capable of giving a horse-power with a consumption of but 11∕4 pound of coal. Mr.[399] C. E. Emery reports the United States revenue-steamer Hassler, designed by him, to have given an ordinary sea-going performance which is probably fully equal to anything yet accomplished. The Hassler is a small steamer, of but 151 feet in length, 241∕2 feet beam, and 10 feet draught. The engines have steam-cylinders 18.1 and 28 inches diameter, respectively, and of 28 inches stroke of piston, indicating 125 horse-power; with steam at 75 pounds pressure, and at a speed of but 7 knots, the coal consumed was but 1.87 pound per horse-power per hour.

The committee of the British Admiralty on designs of ships-of-war have reported recently: “The carrying-power of ships may certainly be to some extent increased by the adoption of compound engines in her Majesty’s service. Its use has recently become very general in the mercantile marine, and the weight of evidence in favor of the large economy of fuel thereby gained is, to our minds, overwhelming and conclusive. We therefore beg earnestly to recommend that the use of compound engines may be generally adopted in ships-of-war hereafter to be constructed, and applied, whenever it can be done with due regard to economy and to the convenience of the service, to those already built.”

The forms of screws now employed are exceedingly diverse, but those in common use are not numerous. In naval vessels it is common to apply screws of two blades, that they may be hoisted above water into a “well” when the vessel is under sail, or set with the two blades directly behind the stern-post, when their resistance to the forward motion of the vessel will be comparatively small. In other vessels, and in the greater number of full-power naval vessels, screws of three or four blades are used.

Fig. 139.—Screw-Propeller.

The usual form of screw (Fig. 139) has blades of nearly equal breadth from the hub to the periphery, or slightly widening toward their extremities, as is seen in an exaggerated degree in Fig. 140, representing the form adopted for[400] tug-boats, where large surface near the extremity is more generally used than in vessels of high speed running free. In the Griffith screw, which has been much used, the hub is globular and very large. The blades are secured to the hub by flanges, and are bolted on in such a manner that their position may be changed slightly if desired. The blades are shaped like the section of a pear, the wider part being nearest the hub, and the blades tapering rapidly toward their extremities. A usual form is intermediate between the last, and is like that shown in Fig. 141, the hub being sufficiently enlarged to permit the blades to be attached as in the Griffith screw, but more nearly cylindrical, and the blades having nearly uniform width from end to end.

Fig. 140.—Tug-boat Screw.

[401]The pitch of a screw is the distance which would be traversed by the screw in one revolution were it to move through the water without slip; i. e., it is double the distance C D, Fig. 140. C D′ represents the helical path of the extremity of the blade B, and O E F H K is that of the blade A. The proportion of diameter to the pitch of the screw is determined by the speed of the vessel. For low speed the pitch may be as small as 11∕4 the diameter. For vessels of high speed the pitch is frequently double the diameter. The diameter of the screw is made as great as possible, since the slip decreases with the increase of the area of screw-disk. Its length is usually about one-sixth of the diameter. A greater length produces loss by increase of surface causing too great friction, while a shorter screw does not fully utilize the resisting power of the cylinder of water within which it works, and increased slip causes waste of power. An empirical value for the probable slip in vessels of good shape, which is closely approximate usually, is S = 4M∕A, in which S is the slip per cent., and M and A are the areas of the midship section and of the screw-disk in square feet.

Fig. 141.—Hirsch Screw.

[402]The most effective screws have slightly greater pitch at the periphery than at the hub, and an increasing pitch from the forward to the rear part of the screw. The latter method of increasing pitch is more generally adopted alone. The thrust of the screw is the pressure which it exerts in driving the vessel forward. In well-formed vessels, with good screws, about two-thirds of the power applied to the screw is utilized in propulsion, the remainder being wasted in slip and other useless work. Its efficiency is in such a case, therefore, 66 per cent. Twin screws, one on each side of the stern-post, are sometimes used in vessels of light draught and considerable breadth, whereby decreased slip is secured.

As has already been stated, the introduction of the compound engine has been attempted, but with less success than in Europe, by several American engineers.

The most radical change in the methods of ship-propulsion which has been successfully introduced in some localities has been the adoption of a system of “wire-rope towage.” It is only well adapted for cases in which the steamer traverses the same line constantly, moving backward and forward between certain points, and is never compelled to deviate to any considerable extent from the path selected. A similar system is in use in Canada, but it has not yet come into use in the United States, notwithstanding the fact that, wherever its adoption is practicable, it has a marked superiority in economy over the usual methods of propulsion. With chain or rope traction there is no loss by slip or oblique action, as in both screw and paddle-wheel propulsion. In the latter methods these losses amount to an important fraction of the total power; they rarely, if ever, fall below a total of 25 per cent., and probably in towage exceed 50 per cent. The objection to the adoption of chain-propulsion, as it is also often called, is the necessity of following closely the line along which the chain or the rope is laid. There is, however, much less difficulty than[403] would be anticipated in following a sinuous route or in avoiding obstacles in the channel or passing other vessels. The system is particularly well adapted for use on canals.

The steam-boilers in use in the later and best marine engineering practice are of various forms, but the standard types are few in number. That used on river-steamers in the United States has already been described.

Fig. 142.—Marine Fire-tubular Boiler. Section.

Fig. 142 is a type of marine tubular boiler which is in most extensive use in sea-going steamers for moderate pressure, and particularly for naval vessels. Here the gases pass directly into the back connection from the fire, and thence forward again, through horizontal tubes, to the front connection and up the chimney. In naval vessels the steam-chimney is omitted, as it is there necessary to keep all parts of the boiler as far below the water-line as possible. Steam is taken from the boiler by pipes which are carried from end to end of the steam-space, near the top of the boiler, the steam entering these pipes through small holes drilled on the other side. Steam is thus taken from the boiler “wet,” but no large quantity of water can usually be “entrained” by the steam.

A marine boiler has been quite extensively introduced[404] into the United States navy, in which the gases are led from the back connection through a tube-box around and among a set of upright water-tubes, which are filled with water, circulation taking place freely from the water-space immediately above the crown-sheet of the furnace up through these tubes into the water-space above them. These “water-tubular” boilers have a slight advantage over the “fire-tubular” boilers already described in compactness, in steaming capacity, and in economical efficiency. They have a very marked advantage in the facility with which the tubes may be scraped or freed from the deposit when a scale of sulphate of lime or other salt has formed within them by precipitation from the water. The fire-tubular boiler excels in convenience of access for plugging up leaking tubes, and is much less costly than the water-tubular. The water-tube class of boilers still remain in extensive use in the United States naval steamers. They have never been much used in the merchant service, although introduced by James Montgomery in the United States and by Lord Dundonald in Great Britain twenty years earlier. Opinion still remains divided among engineers in regard to their relative value. They are gradually reassuming prominence by their introduction in the modified form of sectional boilers.

Fig. 143.—Marine High-Pressure Boiler. Section.

Marine boilers are now usually given the form shown in section in Fig. 143. This form of boiler is adopted where[405] steam-pressures of 60 pounds and upward are carried, as in steam-vessels supplied with compound engines, cylindrical forms being considered the best with high pressures. The large cylindrical flues, therefore, form the furnaces as shown in the transverse sectional view. The gases rise, as shown in the longitudinal section, through the connection, and pass back to the end of the boiler through the tubes, and thence, instead of entering a steam-chimney, they are conducted by a smoke-connection, not shown in the sketch, to the smoke funnel or stack. In merchant-steamers, a steam-drum is often mounted horizontally above the boiler. In other cases a separator is attached to the steam-pipe between boilers and engines. This usually consists of an iron tank, divided by a vertical partition extending from the top nearly to the bottom. The steam, entering the top at one side of this partition, passes underneath it, and up to the top on the opposite side, where it issues into a steam-pipe leading directly to the engine. The sudden reversal of its course at the bottom causes it to leave the suspended water in the bottom of the separator, whence it is drained off by pipes.

The most interesting illustrations of recent practice in marine engineering and naval architecture are found in the steamers which are now seen on transoceanic routes for the merchant service, and, in the naval service, in the enormous iron-clads which have been built in Great Britain.

The City of Peking is one of the finest examples of American practice. This vessel was constructed for the Pacific Mail Company. The hull is 423 feet long, of 48 feet beam, and 381∕2 feet deep. Accommodations are furnished for 150 cabin and 1,800 steerage passengers, and the coal-bunkers “stow” 1,500 tons of coal. The iron plates of which the sides and bottom are made are from 11∕16 to one inch in thickness. The weight of iron used in construction was about 5,500,000 pounds. The machinery weighed nearly 2,000,000 pounds, with spare gear and accessory apparatus.[406] The engines are compound, with two steam-cylinders of 51 inches and two of 88 inches diameter, and a stroke of piston of 41∕2 feet. The condensing water is sent through the surface-condensers by circulating-pumps driven by their own engines. Ten boilers furnish steam to these engines, each having a diameter of 13 feet, a length of 131∕2 feet, and a thickness of “shell” of 13∕16 inch. Each has three furnaces, and contains 204 tubes of an outside diameter of 31∕4 inches. All together, they have 520 square feet of grate-surface and 17,000 square feet of heating-surface. The area of cooling-surface in the condensers is 10,000 square feet. The City of Rome, a ship of later design, is 590 feet long, “over all,” 52 feet beam, 52 feet deep, and measures 8,300 tons. The engines, of 8,500 horse-power, will drive the vessel 18 knots (21 miles) an hour; they have six steam-cylinders (three high and three low pressure), and are supplied with steam by 8 boilers heated by 48 furnaces. The hull is of steel, the bottom double, and the whole divided into ten compartments by transverse bulkheads. Two longitudinal bulkheads in the engine and boiler compartments add greatly to the safety of the vessel.

The most successful steam-vessels in general use are these screw-steamers of transoceanic lines. Those of the transatlantic lines are now built from 350 to 550 feet long, generally propelled from 12 to 18 knots (14 to 21 miles) an hour, by engines of from 3,000 to 8,000 horse-power, consuming from 70 to 250 tons of coal a day, and crossing the Atlantic in from eight to ten days. These vessels are now invariably fitted with the compound engine and surface-condensers. One of these vessels, the Germanic, has been reported at Sandy Hook, the entrance to New York Harbor, in 7 days 11 hours 37 minutes from Queenstown—a distance, as measured by the log and by observation, of 2,830 miles. Another steamer, the Britannic, has crossed the Atlantic in 7 days 10 hours and 53 minutes. These vessels are of 5,000 tons burden, of 750 “nominal” horse-power (probably 5,000 actual).

Fig. 144.—The Modern Steamship.

[407-408]The modern steamship is as wonderful an illustration of ingenuity and skill in all interior arrangements as in size, power, and speed. The size of sea-going steamers has become so great that it is unsafe to intrust the raising of the anchor or the steering of the vessel to manual power and skill; and these operations, as well as the loading and unloading of the vessel, are now the work of the same great motor—steam.

The now common form of auxiliary engine for controlling the helm is one of the inventions of the American engineer F. E. Sickels, who devised the “Sickels cut-off,” and was first invented about 1850. It was exhibited at London at the International Exhibition of 1851. It consists[98] principally of two cylinders working at right angles upon a shaft geared into a large wheel fastened by a friction-plate lined with wood, and set by a screw to any desired pressure on the steering-apparatus. The wheel turned by the steersman is connected with the valve-gear of the cylinders, so that the steam, or other motor, will move the rudder precisely as the helmsman moves the wheel adjusting the steam-valves. This wheel thus becomes the steering-wheel. The apparatus is usually so arranged that it may be connected or disconnected in an instant, and hand-steering adopted if the smoothness of the sea and the low speed of the vessel make it desirable or convenient. This method was first adopted in the United States on the steamship Augusta.

The same inventor and others have contrived “steam-windlasses,” some of which are in general use on large vessels. The machinery of these vessels is also often fitted with a steam “reversing-gear,” by means of which the engines are as easily manœuvred as are those of the smallest vessels, to which hand-gear is always fitted. In one of these little auxiliary engines, as devised by the author, a small handle being adjusted to a marked position, as to the point marked “stop” on an index-plate, the auxiliary engine at once starts, throws the valve-gear into the proper position—as,[409] if a link-motion, into “middle-gear”—thus stopping the large engines, and then it itself stops. Setting the handle so that its pointer shall point to “ahead,” the little engine starts again, sets the link in position to go ahead, thus starting the large engines, and again stops itself. If set at “back,” the same series of operations occurs, leaving the main engines backing and the little “reversing engine” stopped. A number of forms of reversing engine are in use, each adapted to some one type of engine.

The hull of the transatlantic steamer is now always of iron, and is divided into a number of “compartments,” each of which is water-tight and separated from the adjacent compartments by iron “bulkheads,” in which are fitted doors which, when closed, are also water-tight. In some cases these doors close automatically when the water rises in the vessel, thus confining it to the leaking portion.

Thus we have already seen a change in transoceanic lines from steamers like the Great Western (1837), 212 feet in length, of 351∕2 feet beam, and 23 feet depth, driven by engines of 450 horse-power, and requiring 15 days to cross the Atlantic, to steamships over 550 feet long, 55 feet beam, and 55 feet deep, with engines of 10,000 horse-power, crossing the Atlantic in 7 days; iron substituted for wood in construction, the cost of fuel reduced one-half, and the speed raised from 8 to 18 knots and over. In the earlier days of steamships they were given a proportion of length to breadth of from 5 to 6 to 1; in forty years the proportion increased until 11 to 1 was reached.

The whole naval establishment of every country has been greatly modified by the recent changes in methods of attack and defense; but the several classes of ships which still form the naval marine are all as dependent upon their steam-machinery as ever.

Fig. 145.—Modern Iron-Clads.

It is only recently that the attempt seems to have been made to determine a classification of war-vessels and to plan a naval establishment which shall be likely to meet[410] fully the requirements of the immediate future. It has hitherto been customary simply to make each ship a little stronger, faster, or more powerful to resist or to make[411] attack than was the last. The fact that the direction of progress in naval science and architecture is plainly perceivable, and that upon its study may be based a fair estimate of the character and relative distribution of several classes of vessels, seems to have been appreciated by very few.

In the year 1870 the writer proposed[99] a classification of vessels other than torpedo-vessels, which has since been also proposed in a somewhat modified form by Mr. J. Scott Russell.[100] The author then remarked that the increase so rapidly occurring in weight of ordnance and of armor, and in speed of war-vessels, would probably soon compel a division of the vessels of every navy into three classes of ships, exclusive of torpedo-vessels, one for general service in time of peace, the others for use only in time of war.

“The first class may consist of unarmored vessels of moderate size, fair speed under steam, armed with a few tolerably heavy guns, and carrying full sail-power.

“The second class may be vessels of great speed under steam, unarmored, carrying light batteries and as great spread of canvas as can readily be given them; very much such vessels as the Wampanoag class of our own navy were intended to be—calculated expressly to destroy the commerce of an enemy.

“The third class may consist of ships carrying the heaviest possible armor and armament, with strongly-built bows, the most powerful machinery that can be given them, of large coal-carrying capacity, and unencumbered by sails, everything being made secondary to the one object of obtaining victory in contending with the most powerful of possible opponents. Such vessels could never go to sea singly, but would cruise in couples or in squadrons. It seems hardly doubtful that attempts to combine the qualities of all classes in a single vessel, as has hitherto been[412] done, will be necessarily given up, although the classification indicated will certainly tend largely to restrict naval operations.”

The introduction of the stationary, the floating, and the automatic classes of torpedoes, and of torpedo-vessels, has now become accomplished, and this element, which it was predicted by Bushnell and by Fulton three-quarters of a century ago would at some future time become important in warfare, is now well recognized by all nations. How far it may modify future naval establishments cannot be yet confidently stated, but it seems sufficiently evident that the attack, by any navy, of stationary defenses protected by torpedoes is now quite a thing of the past. It may be perhaps looked upon as exceedingly probable that torpedo-ships of very high speed will yet drive all heavily-armored vessels from the ocean, thus completing the historic parallel between the man-in-armor of the middle ages and the armored man-of-war of our own time.[101]

Of these classes, the third is of most interest, as exhibiting most perfectly the importance and variety of the work which the steam-engine is made to perform. On the later of these vessels, the anchor is raised by a steam anchor-hoisting apparatus; the heavier spars and sails are handled by the aid of a steam-windlass; the helm is controlled by a steering-engine, and the helmsman, with his little finger, sets in motion a steam-engine, which adjusts the rudder with a power which is unimpeded by wind or sea, and with an exactness that could not be exceeded by the hand-steering gear of a yacht; the guns are loaded by steam, are elevated or depressed, and are given lateral training, by the same power; the turrets in which the guns are incased are turned, and the guns are whirled toward every point of the compass, in less time than is required to sponge and reload[413] them; and the ship itself is driven through the water by the power of ten thousand horses, at a speed which is only excelled on land by that of the railroad-train.

The British Minotaur was one of the earlier iron-clads. The great length and consequent difficulty of manœuvring, the defect of speed, and the weakness of armor of these vessels have led to the substitution of far more effective designs in later constructions. The Minotaur is a four-masted screw iron-clad, 400 feet long, of 59 feet beam and 261∕2 feet draught of water. Her speed at sea is about 121∕2 knots, and her engines develop, as a maximum, nearly 6,000 indicated horse-power. Her heaviest armor-plates are but 6 inches in thickness. Her extreme length and her unbalanced rudder make it difficult to turn rapidly. With eighteen men at the steering-wheel and sixty others on the tackle, the ship, on one occasion, was 71∕2 minutes in turning completely around. These long iron-clads were succeeded by the shorter vessels designed by Mr. E. J. Reed, of which the first, the Bellerophon, was of 4,246 tons burden, 300 feet long by 56 feet beam, and 241∕2 feet draught, of the 14-knot speed, with 4,600 horse-power; and having the “balanced rudder” used many years earlier in the United States by Robert L. Stevens,[102] it can turn in four minutes with eight men at the wheel. The cost of construction was some $600,000 less than that of the Minotaur. A still later vessel, the Monarch, was constructed on a system quite similar to that known in the United States as the Monitor type, or as a turreted iron-clad. This vessel is 330 feet long, 571∕2 feet wide, and 36 feet deep, drawing 241∕2 feet of water. The total weight of ship and contents is over 8,000 tons, and the engines are of over 8,500 horse-power. The armor is 6 and 7 inches thick on the hull, and 8 inches on the two turrets, over a heavy teak backing. The turrets contain each two 12-inch rifled guns, weighing 25 tons each, and,[414] with a charge of 70 pounds of powder, throwing a shot of 600 pounds weight with a velocity of 1,200 feet per second, and giving it a vis viva equivalent to the raising of over 6,100 tons one foot high, and equal to the work of penetrating an iron plate 131∕2 inches thick. This immense vessel is driven by a pair of “single-cylinder” engines having steam-cylinders ten feet in diameter and of 41∕2 feet stroke of piston, driving a two-bladed Griffith screw of 231∕2 feet diameter and 261∕2 feet pitch, 65 revolutions, at the maximum speed of 14.9 knots, or about 171∕2 miles, an hour. To drive these powerful engines, boilers having an aggregate of about 25,000 square feet (or more than a half-acre) of heating-surface are required, with 900 square feet of grate-surface. The refrigerating surface in the condensers has an area of 16,500 square feet—over one-third of an acre. The cost of these engines and boilers was £66,500.

Were all this vast steam-power developed, giving the vessel a speed of 15 knots, the ship, if used as a “ram,” would strike an enemy at rest with the tremendous “energy” of 48,000 foot-tons—equal to the shock of the projectiles of eight or nine such guns as are carried by the iron-clad itself, simultaneously discharged upon one spot.

But even this great vessel is less formidable than later vessels. One of the latter, the Inflexible, is a shorter but wider and deeper ship than the Monarch, measuring 320 feet long, 75 feet beam, and 25 draught, displacing over 10,000 tons. The great rifles carried by this vessel weigh 81 tons each, throwing shot weighing a half-ton from behind iron-plating two feet in thickness. The steam-engines are of about the same power as those of the Monarch, and give this enormous hull a speed of 14 knots an hour.

The navy of the United States does not to-day possess iron-clads of power even approximating that of either of several classes of British and other foreign naval vessels.

Fig. 146.—The Great Eastern.

The largest vessel of any class yet constructed is the Great Eastern (Fig. 146), begun in 1854 and completed in[415] 1859, by J. Scott Russell, on the Thames, England. This ship is 680 feet long, 83 feet wide, 58 feet deep, 28 feet draught, and of 24,000 tons measurement. There are four paddle and four screw engines, the former having steam-cylinders 74 inches in diameter, with 14 feet stroke, the latter 84 inches in diameter and 4 feet stroke. They are collectively of 10,000 actual horse-power. The paddle-wheels are 56 feet in diameter, the screw 24 feet. The steam-boilers supplying the paddle-engines have 44,000 square feet (more than an acre) of heating-surface. The boilers supplying the screw-engines are still larger. At 30 feet draught, this great vessel displaces 27,000 tons. The engines were designed to develop 10,000 horse-power, driving the ship at the rate of 161∕2 statute miles an hour.

The figures quoted in the descriptions of these great steamships do not enable the non-professional reader to form a conception of the wonderful power which is concentrated within so small a space as is occupied by their steam-machinery. The “horse-power” of the engines is that determined[416] by James Watt as the maximum obtainable for eight hours a day from the strongest London draught-horses. The ordinary average draught-horse would hardly be able to exert two-thirds as much during the eight hours’ steady work of a working-day. The working-day of the steam-engine, on the other hand, is twenty-four hours in length.

Fig. 147.—The Great Eastern at Sea.

The work of the 10,000 horse-power engines of the Great Eastern could be barely equaled by the efforts of 15,000 horses; but to continue their work uninterruptedly, day in and day out, for weeks together, as when done by steam, would require at least three relays, or 45,000 horses. Such a stud would weigh 25,000 tons, and if harnessed “tandem” would extend thirty miles. It is only by such a comparison that the mind can begin to comprehend the utter impossibility of accomplishing by means of animal[417] power the work now done for the world by steam. The cost of the greater power is but about one-tenth that of horse-power, and by its means tasks are accomplished with ease which are absolutely impossible of accomplishment by animal power.

It is estimated that the total steam-power of the world is about 15,000,000 horse-power, and that, were horses actually employed to do the work which these engines would be capable of doing were they kept constantly in operation, the number required would exceed 60,000,000.

Thus, from the small beginnings of the Comte d’Auxiron and the Marquis de Jouffroy in France, of Symmington in Great Britain, and of Henry, Rumsey, and Fitch, and of Fulton and Stevens, in the United States, steam-navigation has grown into a great and inestimable aid and blessing to mankind.

We to-day cross the ocean with less risk, and transport ourselves and our goods at as little cost in either time or money as, at the beginning of the century, our parents experienced in traveling one-tenth the distance.

It is largely in consequence of this ingenious application of a power that reminds one of the fabled genii of Eastern romance, that the mechanic and the laborer of to-day enjoy comforts and luxuries that were denied to wealth, and to royalty itself, a century ago.

The magnitude of our modern steamships excites the wonder and admiration of even the people of our own time; and there is certainly no creation of art that can be grander in appearance than a transatlantic steamer a hundred and fifty yards in length, and weighing, with her stores, five or six thousand tons, as she starts on her voyage, moved by engines equal in power to the united strength of thousands of horses; none can more fully awaken a feeling of awe than an immense structure like the great modern iron-clads (Fig. 145), vessels having a total weight of 8,000 to 10,000 tons, and propelled by steam-engines of as many horse-power,[418] carrying guns whose shot penetrate solid iron 20 inches thick, and having a power of impact, when steaming at moderate speed, sufficient to raise 35,000 tons a foot high.

Far more huge than the Monarch among the iron-clads even is that prematurely-built monster, the Great Eastern (Fig. 147), already described, an eighth of a mile long, and with steam doing the work of a stud of 45,000 horses.

Thus we are to-day witnessing the literal fulfillment of the predictions of Oliver Evans and of John Stevens, and almost that contained in the couplets written by the poet Darwin, who, more than a century ago, before even the earliest of Watt’s improvements had become generally known, sang:

“Soon shall thy arm, unconquered Steam, afar

Drag the slow barge, or drive the rapid car;

Or, on wide-waving wings expanded, bear

The flying chariot through the fields of air.”

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