The Compound Engine and Twin-Screw Steamers
The Compound Engine
In common with metal hulls, the screw and the compound engine mark vast advances in the development of the ocean steamship. As early as 1850, the inefficiency of the simple engine, in which steam at low pressure was admitted. to the cylinder and thence to the condenser, was discussed by all marine engineers.
The first compound engine was introduced into an ocean steamship in 1856, when a sidewheeler of the Pacific Company was equipped with the improvement. Its success did not immediately warrant its general use. For nearly ten years more the simple engine prevailed, and it was not until 1865 that the advantages of the new type became patent.
In the China boats of the Holt Line, of Liverpool, the compound engine demonstrated its advantages, and with the sudden activity in freights in- duced by the opening of the Suez Canal, in 1869, there was a renewed demand for' the highest efficiency.
A recent technical work on Atlantic- steam navigation effectively displays the superiority of the multiple expansion engine. It points out that the first Cunard steamships could carry only two hundred tons of cargo and ninety passengers. could average only 8.7 knots an hour at the best, and that they consumed 4.7 pounds of coal per I. H. P. an hour.
In the first of the compound Cunarders the Bothnia—three thousand tons of cargo could be carried, three hundred and forty saloon passengers accommodated, and at an expenditure of only 2.2 pounds of coal per I. H. P., she could maintain an hourly average of thirteen knots.
The advantage of compound and triple expansion engines over the single expansion type is one that the lay reader may readily understand. In the simple type the steam is used to push a piston in one cylinder only.
After limited expansion, and still retaining a large percentage of its power, it is passed into the condensers, where it becomes water, and is then pumped back into the boilers. In this type steam is given to the engines at an extremely low pressure—often as low as ten pounds to the square inch. In the compound engine the pressure is much higher.
The steam is first introduced into a small cylinder, does its work there, and then expands into a larger cylinder, does more work, and then passes to the condensers. In the triple expansion engine the system is the same, with the addition of a third cylinder.
It is not to be understood that by doubling the expansion of the steam the engine's power is doubled, but that it is considerably increased; that higher speed is made possible, and that coal is saved.
The next step after this radical improvement in engines was the change in the fabric of the hulls -- the supplanting of iron by steel. About 1878, the perfection of the steel industry produced steel much stronger than iron at about an equal cost, and the success -- in 1879 -- of the Allan liner Buenos Ayrean, one of the first steel steamships, stimulated owners until today nearly every Clyde built steamer is constructed of that metal.
The Twin Screw Steamers
The American liner New York -- formerly the City of New York of the Inman Company -- was the first Atlantic passenger boat equipped with twin screws -- another tremendous stride in the trade. Their advantages are obvious, in the matter of speed, economy, safety, and many other details.
The perils that assail the single screw steamer are minimized in craft of the twin-screw type. The breaking of a shaft, the loss of a rudder, or the breaching of a midship section neither destroys nor cripples the high type liner of today. When one shaft breaks, the ship may be worked with the other engine, at only a small loss of speed.
Where the rudder is carried away or crippled, the ship may be worked by alternating the speed of the screws. In the event of collision amidships, the longitudinal bulkhead between the port and starboard engine rooms prevails against the flooding of more than half the section. The memorable accident to the Paris is an example.
In the first twin-screw liners, there was no provision to restrain the engines from racing -- that is, to prevent the machinery from rotating at destructive speed -- should the pressure upon the screw shafts be relieved. This contingency might result from the vessel pitching her screws clear of the water in stormy weather, or from the loss of a screw, or front the breaking of the shaft.
In the case of the Paris, the shaft broke outboard, the screw dropped off, and the engines, relieved of pressure, raced ahead at destroying velocity. Their vast energy, running wild, not only wrecked themselves, but also drove a hole in the ship's side. Only the longitudinal bulkhead saved her from filling and foundering. Had she been a single screw ship with no such protection, she would have gone under inevitably; but as it was, with one screw still in condition for work, she was able to make the Irish coast under her own steam, though almost on her beam-ends.
The New York and the Paris were the pioneers in a new era of Atlantic navigation. They far surpassed all other craft in size, strength, safety, and speed —excepting, of course, in point of size, that colossal freak, the Great Eastern.
For purposes of comparison it is necessary to give here some of the Great Eastern's dimensions. It is doubtful, however, whether she in any way .assisted in the development of the ocean liner, save in showing just what was not wanted.
In 1853, the British government advertised for tenders to carry the Indian and Australian mails, and the plans of the Great Eastern were drawn to obtain this contract. The tender was not accepted, yet the company kept on with the idea. She was designed by Brunel, was six hundred and ninety two feet over all, with eighty-three feet beam and fifty eight feet depth, and nineteen thousand gross tonnage.
A pair of paddlewheel engines of five thousand horsepower, and a pair of screw engines of six thousand horsepower propelled her. She cost a million pounds sterling, and proved a melancholy failure -- slow, unwieldy, costly, and uncomfortable. She was tried as liner, troopship, cable steamer, and floating museum, and was sent finally to the junk heap.