CHAPTER XII.
STEAM NAVIGATION.
Propulsion by paddle-wheels. — Manner of driving them. — Marine Engine. — Its form and arrangement. — Proportion of its cylinder. — Injury to boilers by deposites and incrustation. — Not effectually removed by blowing out. — Mr. Samuel Hall's condenser. — Its advantages. — Originally suggested by Watt. — Hall's steam saver. — Howard's vapour engine. — Morgan's paddle-wheels. — Limits of steam navigation. — Proportion of tonnage to power. — Average speed. — Consumption of fuel. — Iron steamers. — American steam raft. — Steam navigation to India. — By Egypt and the Red Sea to Bombay. — By same route to Calcutta. — By Syria and the Euphrates to Bombay. — Steam communication with the United States from the west coast of Ireland to St. John's, Halifax, and New York.
(114.) Among the various ways in which the steam engine has ministered to the social progress of our race, none is more important and interesting than the aid it has afforded to navigation. Before it lent its giant powers to that art, locomotion over the waters of the deep was attended with a degree of danger and uncertainty, which seemed so necessary and so inevitable, that, as a common proverb, it became the type and representative of everything else which was precarious and perilous. The application, however, of steam to navigation has rescued the mariner from much of the perils of the winds and waves; and even in its actual state, apart from the improvements which it is still likely to receive, it has rendered all voyages of moderate length as safe and as regular as journeys over land. We are even now upon the brink of such improvements as will probably so extend the powers of the steam engine, as to render it available as the means of connecting the most distant points of the earth.
The manner in which the steam engine is commonly applied to propel vessels must be so familiar as to require but short explanation. A pair of wheels, like common under-shot water-wheels, bearing on their rims a number of flat boards called paddle boards, are placed one at each side of the vessel, in such a position that when the vessel is immersed to her ordinary depth the lowest paddle boards shall be submerged. These wheels are fixed upon a shaft, which is made to revolve by cranks placed upon it, in the same manner as the fly-wheel of a common steam engine is turned. It is now the invariable custom to place in steam vessels two engines, each of which works a crank: these two cranks are placed at right angles to each other, in the same manner as the cranks already described upon the working axles of locomotive engines. When either crank is at its dead point, the other is in full activity, so that the necessity for a fly wheel is superseded. The engines may be either condensing or high-pressure engines; but in Europe the low-pressure condensing engine has been invariably used for nautical purposes. In the United States, where steam navigation had its origin, and where it was, until a recent period, much more extensively practised than in Europe, less objection was felt to the use of high-pressure engines; and their limited bulk, their small original cost, and simplicity of structure, strongly recommended them, more especially for the purposes of river navigation.
(g.) The original type of nearly all the engines used in steam navigation was that constructed at Soho by Watt and Bolton for Mr. Fulton, and first used by him upon the Hudson river. This had the beam below the piston-rod as in the English boat-engines, but the cylinder above deck, as in the American. From this primitive form, the two nations have diverged in opposite directions. The Americans, navigating rivers, and having speed for their principal object, have not hesitated to keep the cylinder above deck, and have lengthened the stroke of the piston in order to make the power cut on a more advantageous point of the wheel. Compactness has been gained by the suppression of the working beam.
On the other hand, the English, having the safe navigation of stormy seas as their more important object, have shortened the cylinder in order that the piston-rod may work wholly under the deck, and the arrangement of Fulton's working beam has been retained by them. In this way there can be no doubt, that they have lost the power of obtaining equal speed from a given expenditure of power, and those conversant in the practice and theory of stowing ships may well doubt whether security is not also sacrificed.—A. E.
The arrangement of the parts of the maritime engine differs in some respects, from that of the land engine. Want of room renders greater compactness necessary; and in order to diminish the height of the machine, the working beam is transferred from above the cylinder to below it. In fact, there are two beams, one at each side of the engine, which are connected by a parallel motion with the piston, the rods of the parallel motion extending from the lower part of the engine to the top of the piston-rod. The working end of the beam is connected with the crank by a connecting rod, presented upwards instead of downwards, as in the land engine. The proportion of the length and diameters of the cylinders differ from those of land engines for a like reason: to save height, short cylinders with large diameters are used. Thus, in an engine of 200 horse-power, the length of the cylinder is sometimes 60 inches, and its diameter 53 inches: the valves and the gearing which work them, the air-pump, condenser, and other parts of the machine, do not differ materially from those already described in the land engines.
(h.) The action of machinery may be rendered more equable by using two engines, each of half the power, instead of a single one. If one of these be working with its maximum force when the other is changing the direction of its motion, the result of their joint action will be a force nearly constant. Such a combination was invented by Mr. Francis Ogden, and has been used in several steam boats constructed under his directions. It would however be far more valuable in other cases, particularly where great uniformity in the velocity is indispensable.
This method has now become almost universal in the engines used in the English steam boats, each of which has usually two, both applied to the same shaft, and therefore capable of being used singly or together to turn the paddle wheels. In the American steam boats, although two engines have been often applied, each usually acts upon no more than one of the wheels. We can see no other good reason for this, than that our engineers do not wish to be thought to copy Mr. Ogden.—A. E.
The nature of the work which the marine engine has to perform, is such, that great regularity of action is neither necessary nor possible. The agitation of the surface of the sea will cause the immersion of the paddle-wheels to vary very much, and the resistance to the engine will undergo a corresponding change: the governor, and other parts of the apparatus already described, contrived for imparting to the engine that extreme regularity which is indispensable in its application to manufactures, are therefore here omitted; and nothing is introduced except what is necessary to maintain the engine in its full working power.