Part 18 (2/2)
Reciprocating (_i.e._ cylinder) engines, though brought to a high pitch of efficiency, have grave disadvantages, the greatest among which is the annoyance caused by their intense vibration to all persons in the vessel. A revolving body that is not exactly balanced runs unequally, and transmits a tremor to anything with which it may be in contact. Turn a cycle upside down and revolve the driving-wheel rapidly by means of the pedal. The whole machine soon begins to tremble violently, and dance up and down on the saddle springs, because one part of the wheel is heavier than the rest, the mere weight of the air-valve being sufficient to disturb the balance. Now consider what happens in the engine-room of high-powered vessels. On destroyers the screws make 400 revolutions a minute. That is to say, all the momentum of the pistons, cranks, rods, and valves (weighing tons), has to be arrested thirteen or fourteen times every second.
However well the moving parts may be balanced, the vibration is felt from stem to stern of the vessel. Even on luxuriously-appointed liners, with engines running at a far slower speed, the throbbing of the screw (_i.e._ engines) is only too noticeable and productive of discomfort.
We shall be told, perhaps, that vibration is a necessary consequence of speed. This is true enough of all vehicles, such as railway trains, motor-cars, cycles, which are shaken by the irregularities of the unyielding surface over which they run, but does not apply universally to s.h.i.+ps and boats. A sail or oar-propelled craft may be entirely free from vibration, whatever its speed, as the motions arising from water are usually slow and deliberate. In fact, water in its calmer moods is an ideal medium to travel on, and the trouble begins only with the introduction of steam as motive force.
But even steam may be robbed of its power to annoy us. The steam-turbine has arrived. It works a screw propeller as smoothly as a dynamo, and at a speed that no cylinder engine could maintain for a minute without shaking itself to pieces.
The steam-turbine is most closely connected with the name of the Hon.
Charles Parsons, son of Lord Rosse, the famous astronomer. He was the first to show, in his speedy little _Turbinia_, the possibilities of the turbine when applied to steam navigation. The results have been such as to attract the attention of the whole s.h.i.+pbuilding world.
The principle of the turbine is seen in the ordinary windmill. To an axle revolving in a stationary bearing are attached vanes which oppose a current of air, water, or steam, at an angle to its course, and by it are moved sideways through a circular path. Mr. Parsons' turbine has of course been specially adapted for the action of steam. It consists of a cylindrical, air-tight chest, inside which rotates a drum, fitted round its circ.u.mference with rows of curved vanes. The chest itself has fixed immovably to its inner side a corresponding number of vane rings, alternating with those on the drum, and so arranged as to deflect the steam on to the latter at the most efficient angle. The diameter of the chest and drum is not constant, but increases towards the exhaust end, in order to give the expanding and weakening steam a larger leverage as it proceeds.
The steam entering the chest from the boiler at a pressure of some hundreds of pounds to the square inch strikes the first set of vanes on the drum, pa.s.ses them and meets the first set of chest-vanes, is turned from its course on to the second set of drum-vanes, and so on to the other end of the chest. Its power arises entirely from its expansive velocity, which, rather than turn a number of sharp corners, will, if possible, compel the obstruction to move out of its way. If that obstruction be from any cause difficult to stir, the steam must pa.s.s round it until its pressure overcomes the inertia. Consequently the turbine differs from the cylinder engine in this respect, that steam _can_ pa.s.s through and be wasted without doing any work at all, whereas, unless the gear of a cylinder moves, and power is exerted, all steam ways are closed, and there is no waste. In practice, therefore, it is found that a turbine is most effective when running at high speed.
The first steam-turbines were used to drive dynamos. In 1884 Mr.
Parsons made a turbine in which fifteen wheels of increasing size moved at the astonis.h.i.+ng rate of 300 revolutions per second, and developed 10 horse-power. In 1888 followed a 120 horse-power turbine, and in 1892 one of 2000 horse-power, provided with a condenser to produce suction. So successful were these steam fans for electrical work, pumping water and ventilating mines, that Mr. Parsons determined to test them as a means of propelling s.h.i.+ps. A small vessel 100 feet long and 9 feet in beam was fitted with three turbines--high, medium, and low pressure, of a total 2000 horse-power--a proportion of motive force to tonnage hitherto not approached. Yet when tried over the test course the _Turbinia_, as the boat was fitly named, ran in a most disappointing fas.h.i.+on. The screws revolved _too fast_, producing what is known as _cavitation_, or the scooping out of the water by the screws, so that they moved in a partial vacuum and utilised only a fraction of their force, from lack of anything to ”bite” on. This defect was remedied by employing screws of coa.r.s.er pitch and larger blade area, three of which were attached to each of the three propeller shafts. On a second trial the _Turbinia_ attained 32-3/4 knots over the ”measured mile,” and later the astonis.h.i.+ng speed of forty miles an hour, or double that of the fast Channel packets. At the Spithead Review in 1897 one of the most interesting sights was the little nimble _Turbinia_ rus.h.i.+ng up and down the rows of majestic wars.h.i.+ps at the rate of an express train.
[Ill.u.s.tration: _H.M.S. Torpedo Destroyer ”Viper.” This vessel was the fastest afloat, attaining the enormous speed of 41 miles an hour. The screws were worked by turbines, giving 11,000 horse-power. She was wrecked on Alderney during the Naval Manoeuvres of 1901._]
After this success Mr. Parsons erected works at Wallsend-on-Tyne for the special manufacture of turbines. The Admiralty soon placed with him an order for a torpedo-destroyer--the _Viper_--of 350 tons; which on its trial trip exceeded forty-one miles an hour at an estimated horse-power (11,000) equalling that of our largest battles.h.i.+ps. A sister vessel, the _Cobra_, of like size, proved as speedy.
Misfortune, however, overtook both destroyers. The _Viper_ was wrecked August 3, 1901, on the coast of Alderney during the autumn naval manoeuvres, and the _Cobra_ foundered in a severe storm on September 12 of the same year in the North Sea. This double disaster casts no reflections on the turbine engines; being attributed to fog in the one case and to structural weakness in the other. The Admiralty has since ordered another turbine destroyer, and before many years are past we shall probably see all the great naval powers providing themselves with like craft to act as the ”eyes of the fleet,” and travel at even higher speeds than those of the _Viper_ and _Cobra_.
The turbine has been applied to mercantile as well as warlike purposes. There is at the present time a turbine-propelled steamer, the _King Edward_, running in the Clyde on the Fairlie-Campbelltown route. This vessel, 250 feet long, 30 broad, 18 deep, contains three turbines. In each the steam is expanded fivefold, so that by the time it pa.s.ses into the condensers it occupies 125 times its boiler volume.
(On the _Viper_ the steam entered the turbine through an inlet eight inches in diameter, and left them by an outlet four feet square.) In cylinder engines thirty-fold expansion is considered a high ratio; hence the turbine extracts a great deal more power in proportion from its steam. As a turbine cannot be reversed, special turbines are attached to the two outside of the three propeller shafts to drive the vessel astern. The steamer attained 20-1/2 knots over the ”Skelmorlie mile” in fair and calm weather, with 3500 horse-power produced at the turbines. The _King Edward_ is thus the fastest by two or three knots of all the Clyde steamers, as she is the most comfortable. We are a.s.sured that as far as the turbines are concerned it is impossible by placing the hand upon the steam-chest to tell whether the drum inside is revolving or not!
Every marine engine is judged by its economy in the consumption of coal. Except in times of national peril extra speed produced by an extravagant use of fuel would be severely avoided by all owners and captains of s.h.i.+ps. At low speeds the turbine develops less power than cylinders from the same amount of steam, but when working at high velocity it gives at least equal results. A careful record kept by the managers of the Caledonian Steams.h.i.+p Company compares the _King Edward_ with the _d.u.c.h.ess of Hamilton_, a paddle steamer of equal tonnage used on the same route and built by the same firm. The record shows that though the paddle-boat ran a fraction of a mile further for every ton of coal burnt in the furnaces, the _King Edward_ averaged two knots an hour faster, a superiority of speed quite out of proportion to the slight excess of fuel. Were the _d.u.c.h.ess_ driven at 18-1/2 knots instead of 16-1/2 her coal bill would far exceed that of the turbine.
As an outcome of these first trials the Caledonian Company are launching a second turbine vessel. Three high-speed turbine yachts are also on the stocks; one of 700 tons, another of 1500 tons, and a third of 170 tons. The last, the property of Colonel M'Calmont, is designed for a speed of twenty-four knots.
Mr. Parsons claims for his system the following advantages: Greatly increased speed; increased carrying power of coal; economy in coal consumption; increased facilities for navigating shallow waters; greater stability of vessels; reduced weight of machinery (the turbines of the _King Edward_ weigh but one-half of cylinders required to give the same power); cheapness of attending the machinery; absence of vibration, lessening wear and tear of the s.h.i.+p's hull and a.s.sisting the accurate training of guns; lowered centre of gravity in the vessel, and consequent greater safety during times of war.
The inventor has suggested a cruiser of 2800 tons, engined up to 80,000 horse-power, to yield a speed of forty-four knots (about fifty miles) an hour. Figures such as these suggest that we may be on the eve of a revolution of ocean travel comparable to that made by the subst.i.tution of steam for wind power. Whether the steam-turbine will make for increased speed all round, or for greater economy, remains to be seen; but we may be a.s.sured of a higher degree of comfort. We can easily believe that improvements will follow in this as in other mechanical contrivances, and that the turbine's efficiency has not yet reached a maximum; and even if our ocean expresses, naval and mercantile, do not attain the one-mile-a-minute standard, which is still regarded as creditable to the fastest methods of land locomotion, we look forward to a time in the near future when much higher speeds will prevail, and the tedium of long voyages be greatly shortened. Already there is talk of a service which shall reduce the trans-Atlantic journey to three-and-a-half days. The means are at hand to make it a fact.
_Note._--In the recently-launched turbine destroyer _Velox_ a novel feature is the introduction of ordinary reciprocating engines fitted in conjunction with the steam turbines. These engines are of triple-compound type, and are coupled direct to the main turbines. They take steam from the boilers direct and exhaust into the high-pressure turbine. These reciprocating engines are for use at cruising speeds. When higher power is needed the steam will be admitted to the turbines direct from the boilers, and the cylinders be thrown out of gear.
MECHANICAL FLIGHT.
Few, if any, problems have so strongly influenced the imagination and exercised the ingenuity of mankind as that of aerial navigation. There is something in our nature that rebels against being condemned to the condition of ”featherless bipeds” when birds, bats, and even minute insects have the whole realm of air and the wide heavens open to them.
Who has not, like Solomon, pondered upon ”the way of a bird in the air” with feelings of envy and regret that he is chained to earth by his gross body; contrasting our laboured movements from point to point of the earth's surface with the easy gliding of the feathered traveller? The unrealised wish has found expression in legends of Daedalus, Pegasus, in the ”flying carpet” of the fairy tale, and in the pages of Jules Verne, in which last the adventurous Robur on his ”Clipper of the Clouds” antic.i.p.ates the future in a most startling fas.h.i.+on.
Aeromobilism--to use its most modern t.i.tle--is regarded by the crowd as the mechanical counterpart of the Philosopher's Stone or the Elixir of Life; a highly desirable but unattainable thing. At times this incredulity is transformed by highly-coloured press reports into an equally unreasonable readiness to believe that the conquest of the air is completed, followed by a feeling of irritation that facts are not as they were represented in print.
The proper att.i.tude is of course half-way between these extremes.
Reflection will show us that money, time, and life itself would not have been freely and ungrudgingly given or risked by many men--hard-headed, practical men among them--in pursuit of a Will-o'-the-Wisp, especially in a century when scientific calculation tends always to calm down any too imaginative scheme. The existing state of the aerial problem may be compared to that of a railway truck which an insufficient number of men are trying to move. Ten men may make no impression on it, though they are putting out all their strength. Yet the arrival of an eleventh may enable them to overcome the truck's inertia and move it at an increasing pace.
Every new discovery of the scientific application of power brings us nearer to the day when the truck will move. We have metals of wonderful strength in proportion to their weight; pigmy motors containing the force of giants; a huge fund of mechanical experience to draw upon; in fact, to paraphrase the Jingo song, ”We've got the things, we've got the men, we've got the money too”--but we haven't as yet got the machine that can mock the bird like the flying express mocks the strength and speed of horses.
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