The speed of ships with the best constructed screw propellers is fully equal to that of paddle-wheel vessels; and when two vessels of the same size, and with engines of equal power, one fitted with paddles, and the other with the screw, are fastened stem and stern together, in a trial of strength, the screw propeller has been found to have the advantage, and to pull its antagonist along at the rate of one or two miles an hour.
The difficulty at first experienced in the application of the screw propeller was to communicate a sufficiently rapid motion to the shaft to which it is fixed; but, by the employment of direct-acting engines, this difficulty has been for the most part overcome. The power is generally first applied to drive a large cog-wheel, the teeth of which take into the teeth of a smaller cog-wheel fixed to the propeller shaft, and in this manner the velocity is sufficiently increased.
In 1852 the proportion of screw to paddle-wheel vessels building in the Clyde was as 43 to 30. The advantages of the propeller are becoming every year more appreciated, and it is rapidly superseding the paddle-wheel.
In the steam-boats of the United States the engines are constructed on the high-pressure principle; and by working with steam of the pressure of 100 pounds on the square inch, and with larger paddle-wheels, their boats attain a speed exceeding sixteen miles an hour. But numerous explosions of boilers on the North American rivers have operated as a caution against the introduction of high-pressure engines in steam-boats in this country. The dread of high-pressure steam was early impressed by the destructive explosion of the boiler of a steam-vessel at Norwich in 1817, which led to a long parliamentary inquiry into the subject; and the subsequent loss of life by the explosion of the "Cricket" on the Thames, has tended to strengthen the apprehension of high-pressure steam engines. For river use, however, when fresh water is always at command for generating the steam, there appears to be no more cause for fear of high-pressure engines in boats than on railways, provided the boilers are constructed with sufficient care. The experiments made by Mr. Fairbairn on the strength of boilers, the results of which were communicated at the meeting of the British Association in 1853, prove, that by increasing the number and strength of the "stays," or internal supports, of the boilers, they may be made, if sufficiently strong, to resist any possible pressure; and that the square shape, which was supposed to be the weakest, offers, on the contrary, peculiar facilities for giving increased strength. In one of these experiments made to determine the ultimate strength of the flat surfaces of boilers, when divided into squares of sixteen inches area, the boiler did not give way until it had sustained the enormous pressure of 1,625 pounds on the square inch.
It might be desirable, in the construction of steam boilers, to adopt the same principle that is introduced in the building of gunpowder mills, one-half of which is built in strong masonry, whilst the other is made of wood. By this means, when an explosion does occur, much less damage is done, for the lighter part only is blown away, which does little injury. In the same manner, steam engine boilers might be constructed with a small portion comparatively weaker, so that if it gave way there would not be much damage done. Safety-valves are intended to act in that manner; and if they were properly constructed, they would sufficiently answer the purpose, and guard against the possibility of danger; but the numerous accidents that occur with boilers provided with imperfect safety-valves, show that there is a necessity for some more effectual protection. Engineers are not sufficiently alive to the importance of improvements in this respect. They supply an engine with safety-valves, which would answer the purpose if kept in proper condition; but they do not make effectual provision against careless management and reckless misconduct. Some years since, a gentleman in America sent to the author a description, with drawings, of a safety-valve that combined the principles of the safety-plug without its inconvenience; it being so contrived that when the boiler became too hot, it melted some fusible metal which previously held down the valve, and then a weight pulled it open to allow an ample escape of steam; but when the heat was lowered, the valve again closed. This was shown to an eminent engineer for his opinion. He pronounced it to be very ingenious, and that it would, no doubt, answer the purpose; but he said, "An improved safety-valve is not wanted, those in use being quite sufficient for the purpose."
In steam-ships, where salt water is used for generating the steam, the incrustation on the sides of the boilers becomes a serious annoyance. It obstructs the communication of heat from the furnace to the water, and the metal is thus liable to become red-hot. Numerous plans have been adopted for the purpose of preventing the accumulation of salt on the sides of the boiler, the most common of which is to allow the water, when saturated with saline matter, to escape, and then to fill the boiler afresh. Among other contrivances for effecting the same purpose, without the waste of heating power which the change of water occasions, is Mr. Hall's plan of condensing the steam in dry condensers, cooled externally, so that the distilled water may be used again and again. This plan though theoretically good, is not much adopted; for the condensation of steam cannot be so well accomplished by that means as when a jet of cold water is thrown directly into the condenser. The principle of the dry condenser has, however, been lately made available in a new kind of engine, wherein the combined action of steam and of spirit vapour is applied as the propelling power.
Steam-boats had been for many years in extensive use on the rivers and seas of Europe and America before it was thought practicable to make voyages in them across the Atlantic. At the meeting of the British Association at Liverpool in 1837, that subject was brought forward for consideration, and it was then attempted to be shown, by calculations of the quantities of coal requisite for such a voyage, that steam communication with America would not be profitable, if it could be accomplished, as the coal would occupy so much of the tonnage as to leave scarcely any space for passengers and goods. Within a few months afterwards those calculations were set at nought by the "Sirius" and the "Great Western," which successfully crossed the Atlantic with passengers and cargo, the former in nineteen days from Cork, and the latter in sixteen. At the present time, steam-packets are constantly crossing from New York to Liverpool in eleven days.
Steam-ships now find their way to India and even to Australia, though the necessity of taking in coals at depôts supplied from England not only prolongs the time, but adds so materially to the cost, as to render steam communication with those distant places scarcely practicable with profit, since no freight can pay for the expense of coaling under such circumstances. To overcome that difficulty, it was proposed to build ships large enough to carry a supply of coals sufficient for the voyage there and back. One of those ships has been built for the Eastern Steam Navigation Company by Mr. J. Scott Russell, from the plans of Mr. Brunel, which is 675 feet long, 83 feet broad, and 60 feet deep. It is adapted to carry 6,000 tons burthen, in addition to the engines and requisite quantity of fuel, and to accommodate 2,000 passengers. This monster ship has been built on what is called the "wave principle" of ship-building, with long concave bows. It is to be propelled by the combined powers of the paddle-wheel and the screw. The engines for the former consist of 4 oscillating cylinders, 16 feet long and 74 inches in diameter, and the screw is to be worked by 4 separate engines, with cylinders of 84 inches in diameter. The speed which the "Great Eastern" is estimated to attain is 24 miles an hour, and it is calculated that the voyage to Australia will be accomplished in 30 days. There seems, at present, but small prospect of those calculations being realized, for the great cost incurred in launching the vessel and other expenses have exhausted the funds of the company by whom the ship was constructed.
Another company has, however, been formed for the purpose of completing, if possible, this great experiment in Steam Navigation; and the opinion so strongly expressed by Mr. Fairbairn at the recent meeting of the British Association at Leeds, of the strength of the monster ship, will give additional stimulus to their exertions. The ship is built on the same principle of construction as the Britannia Bridge over the Menai Straits, and it was stated by Mr. Fairbairn that it might be supported out of water, either in the centre or at each end, without injury.