(118.) Among the contrivances which have been proposed for remedying these defects of the common paddle wheel by introducing paddle boards capable of shifting their position as they revolve with the circumference of the wheel, the only one which has been adopted to any considerable extent in practice is that which is commonly known as Morgan's Paddle Wheel. The original patent for this contrivance was granted to Elijah Galloway, and sold by him to Mr. William Morgan. Subsequently to the purchase some improvements in its structure and arrangements were introduced, and it is now extensively adopted by Government in the Admiralty steamers. It was first tried on board His Majesty's steamer the Confiance; and after several successful experiments was ordered by the Lords of the Admiralty to be introduced on board the Flamer, the Firebrand, the Columbia, the Spitfire, the Lightning, a large war steamer called the Medea,[38] the Tartarus, the Blazer, &c. It has been tried by Government in several well-conducted experiments, where two vessels of precisely the same model, supplied with similar engines of equal power, and propelled, one by Morgan's paddle-wheels, and the other by the common paddle-wheels; when it was found that the advantage of the former, whether in smooth or in rough water, was quite apparent. One of the commanders in these experiments (Lieutenant Belson) states that the improvement in the speed of the Confiance, after being supplied with these wheels, was proportionately greater in a sea way than in smooth water; that their action was not impeded by the waves, since the variation of the velocity of the engine did not exceed one or two revolutions per minute: the vessel's way was never stopped, and there was no sensible increase of vibration on the paddle boxes during the gale. Another commander reported that on a comparison of the Confiance and a similar and equally powerful vessel, the Carron, the Confiance performed in fifty-four hours the voyage which occupied the Carron eighty-four hours in running. Independently of the great saving of fuel effected (namely, ten bushels per hour[39]), or the time saved in running the same distance, other advantages have been secured by the modification in question. On a comparison of the respective logs of the two vessels, it appeared that the Confiance had gained by the alteration in her wheels an increase of speed amounting to 2 knots on 7 in smooth water, and 2-1/2 knots on 4 to 4-1/2 knots in rough weather; that the action of the paddles did not bring up the engine or retard their velocity in a head sea; that in rolling their action assisted in righting the vessel; and that the wear and strain, as well on the vessel as on the engines, were materially reduced. With respect to the durability of these wheels, the commander of the Flamer reported in January, 1834, that in six weeks of the most tempestuous weather they found them to act remarkably well, without even a single float being shifted.[40]
This paddle-wheel is represented in figure 68. The contrivance may be shortly stated to consist in causing the wheel which bears the paddles to revolve on one centre, and the radial arms which move the paddles to revolve on another centre. Let A B C D E F G H I be the polygonal circumference of the paddle-wheel, formed of straight bars, securely connected together at the extremities of the spokes or radii of the wheel which turns on the shaft which is worked by the engine; the centre of this wheel being at O. So far this wheel is similar to the common paddle-wheel; but the paddle boards are not, as in the common wheel, fixed at A B C, &c., so as to be always directed to the centre O, but are so placed that they are capable of turning on axles which are always horizontal, so that they can take any angle with respect to the water which may be given to them. From the centres, or the line joining the pivots on which these paddle boards turn, there proceed short arms K, firmly fixed to the paddle boards at an angle of about 120°. On a motion given to this arm K, it will therefore give a corresponding angular motion to the paddle board, so as to make it turn on its pivots. At the extremities of the several arms marked K is a pin or pivot, to which the extremities of the radial arms L are severally attached, so that the angle between each radial arm L and the short paddle arm K is capable of being changed by any motion imparted to L; the radial arms L are connected at the other end with a centre P, round which they are capable of revolving. Now since the points A B C, &c., which are the pivots on which the paddle boards turn, are moved in the circumference of a circle, of which the centre is O, they are always at the same distance from that point; consequently they will continually vary their distance from the other centre P. Thus, when a paddle board arrives at that point of its revolution at which the centre P lies precisely between it and the centre O, its distance from P is less than in any other position. As it departs from that point, its distance from the centre P gradually increases until it arrives at the opposite point of its revolution, where the centre O is exactly between it and the centre P; then the distance of the paddle board from the centre P is greatest. This constant change of distance between each paddle board and the centre P is accommodated by the variation of the angle between the radial arm L and the short paddle board arm K; as the paddle board approaches the centre P this gradually diminishes; and as the distance of the paddle board from P increases, the angle is likewise augmented. This change in the magnitude of the angle, which thus accommodates the varying position of the paddle board with respect to the centre P, will be observed in the figure. The paddle board D is nearest to P; and it will be observed that the angle contained between L and K is there very acute; at E the angle between L and K increases, but is still acute; at F it increases to a right angle; at G it becomes obtuse; and at K, where it is most distant from the centre P, it becomes most obtuse. It again diminishes at I, and becomes a right angle between A and B. Now this continual shifting of the direction of the short arm K is necessarily accompanied by an equivalent change of position in the paddle board to which it is attached; and the position of the second centre P is, or may be, so adjusted that this paddle board, as it enters the water and emerges from it, shall be such as shall be most advantageous for propelling the vessel, and therefore attended with less of that vibration which arises chiefly from the alternate depression and elevation of the water, owing to the oblique action of the paddle boards.[41]
(i) The relative value of the two wheels, namely, the common paddle-wheel, and that of Morgan has been investigated by Professor Barlow of the Military School, at Woolwich, and the results published in a paper of much ability in the Philosophical Transactions for 1834. By this paper it appears, that, when the paddles are not wholly immersed, the wheel of Morgan has no important advantage over the other, and only acquires one when the wheel wallows. But the most important of his inferences is that the common paddle is least efficient when in a vertical position, contrary to the usual opinion. From this we have a right to infer that the search for a form of wheel which shall always keep the paddle vertical is one whose success need not be attended with any important consequence. The superior qualities of Morgan's wheel when the paddles are deeply immersed is ascribed by Barlow to the lessening of the shock sustained by the common paddle-wheel when it strikes the water. This being the case, the triple wheel of Stevens is probably superior to that of Morgan in its efficiency, while it has the advantage of being far simpler and less liable to be put out of order.—A. E.
(119.) To form an approximate estimate of the limit of the present powers of steam navigation, it will be necessary to consider the mutual relation of the capacity or tonnage of the vessel; the magnitude, weight, and power of the machinery; the available stowage for fuel; and the average speed attainable in all weathers, as well as the general purposes to which the vessel is to be appropriated, whether for the transport of goods and merchandise, or merely of despatches and passengers. That portion of the capacity of the vessel which is appropriated to the moving power consists of the space occupied by the machinery and the space occupied by the fuel; the magnitude of the latter will necessarily depend upon the length of the voyage which the vessel must make without receiving a fresh supply of coals. If the voyage be short, this space may be proportionally limited, and a greater portion of room will be left for the machinery. If, on the contrary, the voyage be longer, a greater stock of coals will be necessary, and a less space will remain for the machinery. More powerful vessels, therefore, in proportion to their tonnage, may be used for short than for long voyages.
Pl. XII.
Taking an average of fifty-one voyages made by the Admiralty steamers, from Falmouth to Corfu and back during four years ending June, 1834, it was found that the average rate of steaming, exclusive of stoppages, was 7-1/4 miles per hour, taken in a direct line between the places, and without allowing for the necessary deviations in the course of the vessel. The vessels which performed this voyage varied from 350 to 700 tons burthen by measurement, and were provided with engines varying from 100 horse to 200 horse-power, with stowage for coals varying from 80 to 240 tons. The proportion of the power to the tonnage varied from 1 horse to 3 tons to 1 horse to 4 tons; thus, the Messenger had a power of 200 horses, and measured 730 tons; the Flamer had a power of 120 horses, and measured 500 tons; the Columbia had 120 horses, and measured 360 tons.
In general, it may be assumed that for the shortest class of trips, such as those of the Margate steamers, and the packets between Liverpool or Holyhead and Dublin, the proportion of the power to the tonnage should be that of 1 horse-power to every 2 tons by measure; while for the longest voyages the proportion would be reduced to 1 horse to 4 tons, voyages of intermediate lengths having every variety of intermediate proportion.
Steamers thus proportioned in their power and tonnage may then, on an average of weathers, be expected to make 7-1/4 miles an hour while steaming, which is equivalent to 174 miles per day of twenty-four hours. But, in very long voyages, it rarely happens that a steamer can work constantly without interruption. Besides stress of weather, in which she must sometimes lie-to, she is liable to occasional derangements of her machinery, and more especially of her paddles. In almost every long voyage hitherto attempted, some time has been lost in occasional repairs of this nature while at sea. We shall perhaps, therefore, for long voyages, arrive at a more correct estimate of the daily run of a steamer by taking it at 160 miles.[42]