We enter the stokehold through air-locks, closing one door before we can open the other, and find ourselves among sweating, grimy men, stripped to the waist. As though life itself depended upon it they shovel coal into the rapacious maws of furnaces glowing with a dazzling glare under the “forced-draught” sent down into the hold by the fans whirling overhead. The ignited furnace gases on their way to the outer air surrender a portion of their heat to the water from which they are separated by a skin of steel. Two kinds of marine boiler are used—the fire-tube and the water-tube. In fire-tube boilers the fire passes inside the tubes and the water outside; in water-tube boilers the reverse is the case, the crown and sides of the furnace being composed of sheaves of small parallel pipes through which water circulates. The latter type, as generating steam very quickly, and being able to bear very high pressures, is most often found in war vessels of all kinds. The quality sought in boiler construction is that the heating surface should be very large in proportion to the quantity of water to be heated. Special coal, anthracite or Welsh, is used in the navy on account of its great heating power and freedom from smoke; experiments have also been made with crude petroleum, or liquid fuel, which can be more quickly put on board than coal, requires the services of fewer stokers, and may be stored in odd corners unavailable as coal bunkers.
From the boiler the steam passes to the engine-room, whither we will follow it. We are now in a bewildering maze of clanking, whirling machinery; our noses offended by the reek of oil, our ears deafened by the uproar of the moving metal, our eyes wearied by the efforts to follow the motions of the cranks and rods.
On either side of us is ranged a series of three or perhaps even four cylinders, of increasing size. The smallest, known as the high-pressure cylinder, receives steam direct from the boiler. It takes in through a slide-valve a supply for a stroke; its piston is driven from end to end; the piston-rod flies through the cylinder-end and transmits a rotary motion to a crank by means of a connecting-rod. The half-expanded steam is then ejected, not into the air as would happen on a locomotive, but into the next cylinder, which has a larger piston to compensate the reduction of pressure. Number two served, the steam does duty a third time in number three, and perhaps yet a fourth time before it reaches the condensers, where its sudden conversion into water by cold produces a vacuum suction in the last cylinder of the series. The secret of a marine engine’s strength and economy lies then in its treatment of the steam, which, like clothes in a numerous family, is not thought to have served its purpose till it has been used over and over again.
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 ships 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 shipbuilding 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 circumference 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, passes 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 pass round it until its pressure overcomes the inertia. Consequently the turbine differs from the cylinder engine in this respect, that steam can pass 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 astonishing 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 ships. 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 fashion. 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 coarser 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 astonishing 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 rushing up and down the rows of majestic warships at the rate of an express train.