(208.)
Among the many ways in which the steam-engine has ministered to the advancement of civilisation and the social progress of the human race, there is none more [Pg442] important or more interesting than its application to navigation. Before it lent its giant powers to the propulsion of ships, locomotion over the waters of the deep was attended with so much danger and uncertainty that, as a common proverb, it became the type and the representative of every thing which was precarious and perilous. The application, however, of steam to navigation has rescued the mariner and the voyager from many of the dangers of wind and water; and even in its present state, putting out of view its probable improvement, it has rendered all voyages of moderate length as safe, and very nearly as regular, as journeys over-land. As a means of transport by sea, the application of this power may be considered as established; and it is now receiving improvements by which its extension to the longest class of ocean voyages is a question not of practicability, but merely of profit.
The manner in which the steam-engine is rendered an instrument for the propulsion of vessels must in its general features be so familiar to every one as to require but short explanation. A shaft is carried across the vessel, being continued on either side beyond the timbers: to the extremities of this shaft, on the outside of the vessel, are fixed a pair of wheels constructed like undershot water-wheels, having attached to their rims a number of flat boards called paddle-boards. As the wheels revolve, these paddle-boards strike the water, driving it in a direction contrary to that in which it is intended the vessel should be propelled. The moving force imparted to the water thus driven backwards is necessarily accompanied by a re-action upon the vessel through the medium of the paddle-shaft, by which the vessel is propelled forwards. On the paddle-shaft two cranks are constructed, similar to the cranks already described on the axle of the driving wheels of a locomotive engine. These cranks are placed at right angles to each other, so that when either is in its highest or lowest position the other shall be horizontal. They are driven by two steam-engines, which are placed in the hull of the vessel below the paddle-shaft. In the earlier steam-boats a single steam-engine was used, and in that case the unequal action of the engine on the crank was equalised by a fly-wheel. This, however, has been long [Pg443] since abandoned in European vessels, and the use of two engines is now almost universal. By the relative position of the cranks it will be seen, that when either crank is at its dead points, the other will be in the positions most favourable to its action, and in all intermediate positions the relative efficiency of the cranks will be such as to render their combined action very nearly uniform.
The steam-engines used to impel vessels may be either condensing engines, similar to those of Watt, and such as are used in manufactures generally, or they may be non-condensing and high-pressure engines, similar in principle to those used on railways. Low-pressure condensing engines are, however, universally used for marine purposes in Europe and to some extent in the United States. In the latter country, however, high-pressure engines are also in pretty general use, on rivers where lightness is a matter of importance.
The arrangement of the parts of a marine engine differs in some respects from that of a land engine. The limitation of space, which is unavoidable in a vessel, renders greater compactness necessary. The paddle-shaft on which the cranks to be driven by the engine are constructed being very little below the deck of the vessel, the beam and connecting rod could not be placed in the position in which they usually are in land engines, without carrying the machinery to a considerable elevation above the deck. This is done in the steam-boat engines used on the American rivers; but it would be inadmissible in steam-boats in general, and more especially in sea-going steamers. The connecting rods, therefore, instead of being presented downwards towards the cranks which they drive, must, in steam-vessels, be presented upwards, and the impelling force received from below. If, under these circumstances, the beam were in the usual position above the cylinder and piston-rod, it must necessarily be placed between the engine and the paddle-shaft. This would require a depth for the machinery which would be incompatible with the magnitude of the vessel. The beam, therefore, of marine engines, instead of being above the cylinder and piston, is placed below them. To the top of the [Pg445] piston-rods cross pieces are attached of greater length than the diameter of the cylinders, so that their extremities shall project beyond the cylinders. To the ends of these cross pieces are attached by joints the rods of a parallel motion: these rods are carried downwards, and are connected with the ends of two beams below the cylinder, and placed on either side of it. The opposite ends of these beams are connected by another cross piece, to which is attached a connecting rod, which is continued upwards to the crank-pin, to which it is attached, and which it drives. Thus the beam, parallel motion, and connecting rod of a marine engine, is similar to that of a land engine, only that it is turned upside down; and in consequence of the impossibility of placing the beam directly over the piston-rod, two beams and two systems of parallel motion are provided, one on each side of the engine, acted upon by, and acting on the piston-rod and crank by cross pieces.
The proportion of the cylinders differs from that usually observed in land engines, for like reasons. The length of the cylinder of land engines is generally greater than its diameter, in the proportion of about two to one. The cylinders of marine engines are, however, commonly constructed with a diameter very little less than their length. In proportion, therefore, to their power their stroke is shorter, which infers a corresponding shortness of crank and a greater limitation of play of all the moving parts in the vertical direction. The valves and the gearing by which they are worked, the air-pump, the condenser, and other parts of the marine engines, do not materially differ from those already described in land engines.
Fig. 119.
These arrangements of a marine engine will be more clearly understood by reference to fig. 119. [35], in which is represented a longitudinal section of a marine engine with its boiler as placed in a steam-vessel. The sleepers of oak, supporting the engine, are represented at X, the base of the engine being secured to these by bolts passing through them [Pg446] and the bottom timbers of the vessel; S is the steam-pipe leading from the steam-chest in the boiler to the slides c, by which it is admitted to the top and bottom of the cylinder. The condenser is represented at B, and the air-pump at E. The hot well is seen at F, from which the feed is taken for the boiler; L is the piston-rod connected by the parallel motion a with the beam H, working on a centre K, near the base of the engine. The other end of the beam I drives the connecting rod M, which extends upwards to the crank which it works upon the paddle-shaft O. Q R is the framing by which the engine is supported. The beam here exhibited is shown on dotted lines as being on the further side of the engine. A similar beam similarly placed, and moving on the same axis, must be understood to be at this side connected with the cross head of the piston in like manner by a parallel motion, and with a cross piece attached to the lower end of the connecting rod and to the opposite beam. The eccentric which works the slides is placed upon the paddle shaft O, and the connecting arm which drives the slides may be easily detached when the engine requires to be stopped. The section of the boiler, grate, and flues, is represented at W U. The safety-valve y is enclosed beneath a pipe carried up beside the chimney, and is inaccessible to the engine-man; h are the cocks for blowing the salted water from the boiler; and I I the feed-pipe.
The general arrangement of the engine-room of a steam-vessel is represented in fig. 120.
The nature of the effect required to be produced by marine engines does not render either necessary or possible that great regularity of action which is indispensable in a steam-engine applied to the purposes of manufacture. The agitation of the surface of the sea will cause the immersion of the paddle-wheels to be subject to great variation, and the resistance produced by the water to the engine will undergo a corresponding change. The governor, therefore, and other parts of the apparatus, contrived for giving to the engine that great regularity required in manufactures, are omitted in nautical engines, and nothing is introduced save what is [Pg447] necessary to maintain the machine in its full working efficiency.
Fig. 120.
Fig. 121.
To save space, marine boilers are constructed so as to produce the necessary quantity of steam within the smallest possible dimensions. With this view a more extensive surface in proportion to the capacity of the boiler is exposed to the action of the fire. The flues, by which the flame and heated air are conducted to the chimney, are so constructed that the heat may act upon the water on every side in thin oblong shells or plates. This is accomplished by constructing the flues so as to traverse the boiler backwards and forwards several times before they terminate [Pg448] in the chimney. Such an arrangement renders the expense of the boilers greater, but their steam-producing power is proportionally augmented, and experiments made by Mr. Watt, at Birmingham, have proved that such boilers with the same consumption of fuel will produce, as compared with common land boilers, an increased evaporation in the proportion of about three to two.
Fig. 122.
Fig. 123.
The form and arrangement of the water-spaces and flues in marine boilers may be collected from the sections of the boilers used in some of the government steamers, exhibited in figs. 121, 122, 123. A section made by a horizontal plane passing through the flues is exhibited in fig. 121. The furnaces F communicate in pairs with the flues E, the air following the course through the flues represented by the arrows. The flue E passes to the back of the boiler, then returns to the front, then to the back again, and is finally carried back to the front, where it communicates at C with the curved flue B, represented in the transverse vertical section, fig. 122. This curved flue B finally terminates in the chimney A. There are in this case three independent boilers, each worked by two furnaces communicating with the same system of flues; and in the curved flues B, fig. 122., by which the air is finally conducted through the chimney, are placed three independent [Pg449] dampers, by means of which the furnace of each boiler can be regulated independently of the other, and by which each boiler may be separately detached from communication with the chimney. The letters of reference in the horizontal section, fig. 121., correspond with those in the transverse vertical section, fig. 122., E representing the commencement of the flues, and C their termination.
Fig. 124.
A longitudinal section of the boiler made by a vertical plane extending from the front to the back is given in fig. 123., where F, as before, is the furnace, G the grate-bars sloping downwards from the front to the back, H the fire-bridge, C the commencement of the flues, and A the chimney. An elevation of the front of the boiler is represented in fig. 124., showing two of the fire-doors closed, and the other two removed, displaying the position of the grate-bars in front. Small openings are also provided, closed by proper doors, by which access can be had to the under side of the flues between the foundation timbers of the engine for the purpose of cleaning them.
Each of these boilers can be worked independently of the others. By this means, when at sea, the engine may be worked by any two of the three boilers, while the third is being cleaned and put in order. In all sea-going steamers multiple boilers are at present provided for this purpose.
In the boilers here represented the flues are all upon the same level, winding backwards and forwards without passing one above the other. In other boilers, however, the flues, [Pg450] after passing backwards and forwards near the bottom of the boiler, turn upwards and pass backwards and forwards through a level of the water nearer its surface, finally terminating in the chimney. More heating surface is thus obtained with the same capacity of boiler.
The most formidable difficulty which has been encountered in the application of the steam-engine to sea-voyages has arisen from the necessity of supplying the boiler with sea-water instead of pure fresh water. The sea-water is injected into the condenser for the purpose of condensing the steam, and it is thence, mixed with the condensed steam, conducted as feeding water into the boiler.
