But the great oil-burning liners have a different picture to present. The fire room is almost as neat and clean as is the engine room. The firemen do not seem to be overworked, as they step from one to another of their burners, looking through a series of peepholes to see that the oil is burning properly. Smudges of dirt are not uncommon on a man’s face and hands, perhaps, but the begrimed Vulcans of the coal-burning ships have no counterpart on the oil burners, and the coal dust and the dingy stokehold is a far cry.
The Majestic’s boilers do not depend upon natural draft, but a set of four powerful fans draws the warm air from the turbine rooms through two great air shafts about seven feet in diameter and forces it under pressure beneath the boilers. These great air shafts total 1,000 feet in length, and a part of the air they supply is led to each boiler. In addition to this supply for the furnaces there is a separate supply of fresh air for the crew of the boiler rooms. The boilers are all separate and any one or any series can be completely shut off from the others in case of necessity.
The steam that is generated in the forty-eight boilers of the Majestic is led by a complicated system of pipes to the turbines, which drive the propellers.
Formerly steamships universally used the reciprocating engine, but gradually the turbine is being adopted, until now the fastest ships are universally equipped with this later design.
A reciprocating engine is one that has one or more cylinders in which pistons are pushed back and forth by the steam which enters alternately one end of the cylinder and then the other, thus turning the shaft. This piston, running first up and then down, is joined to a “connecting rod” which in turn is connected to a “crank shaft” which is a continuation of the propeller shaft. As the piston moves up and down, one end of the connecting rod moves with it, for it is fastened by a hinge to the lower end of the piston rod which runs out of the bottom of the cylinder. The other end of the connecting rod is attached to the crank shaft which has a section of itself carried out to one side just as the shaft on which a grindstone is mounted is bent at right angles and attached to the handle. As the piston goes up, carrying the connecting rod with it, the off-centre section of the crank shaft is carried up also, as the handle of a grindstone is carried up when the operator begins to turn the wheel. When the piston has reached the top of its stroke the connecting rod has pulled the crank until it is pointing straight up. Then the steam pushes the piston down and the piston pushes the connecting rod, which in turn pushes the crank, so that the shaft is turned, just as you might turn a grindstone by hand, your arm representing the piston rod and connecting rod, and the handle representing the crank shaft.
The turbine, however, is a very different machine. There are no pistons and no other parts similar to those of the reciprocating engine. Instead there is a shaft on which is mounted a great steel wheel. Around the edge of this wheel are mounted thousands of little vanes, and the whole wheel works on the same principle as a windmill. A windmill carries a comparatively small number of vanes arranged somewhat as the blades of an electric fan or a propeller are arranged. When the wind blows against these “vanes” the wheel revolves. Now a turbine is a very highly developed example of this same principle, and the steam is led to it through pipes and directed against these vanes, which are small but are very numerous. The result is that this windmill type of engine revolves at a very rapid rate. One can get some idea of these turbines when he learns that the eight turbines of the Majestic contain a total of 900,000 vanes.
Turbines, however, have two major failings: First, they cannot be reversed—that is, a turbine can turn in only one direction—and second, they are most efficient when they operate at high speed. In order to use turbines on ships, then, it is necessary to have one turbine on each propeller shaft to drive the propeller ahead, and another turbine with which to drive it astern. Consequently, the Majestic has eight turbines—two to each propeller shaft.
The second difficulty is harder to overcome. Propellers are most efficient when they are run at comparatively slow speeds. Those on the Majestic are no exceptions to this rule, and at full speed are run at about one hundred eighty revolutions per minute. Turbines, however, are high-speed machines, capable generally of thousands of revolutions per minute. In order to utilize the power generated by the rapidly revolving turbine and transpose it into useful energy for use by the slowly turning propeller there must be some sort of reduction gear. The United States Navy has designed a number of its newest ships with an electric drive in which the high-speed turbines are used to generate electricity which is used to turn slow motors that drive the propellers. Another method is a reducing gear, similar in the work it does to the gears used in automobiles for “low” and “intermediate,” but necessarily very much greater in size. In the Majestic the arrangement is of the latter type—that is, the turbines are operated at high speeds, and through a series of gears the propeller shafts are turned at slower speeds.
THE GEORGE WASHINGTON