THE THRUST OF THE MODERN LINER’S MIGHTY ENGINES is transmitted to the water by screw propellers that operate in the water in much the same way as the ordinary mechanical screw turns in wood. The propellers thus push the vessel forward. The illustration shows the four-
A QUARTER of a century ago the merchant fleets of the world were composed of steamers and sailing ships. To-
Whichever of these main types and variations is employed, the majority of modern power-
The screw propeller, however, is used for most sea-
Theoretically, each turn of the screw advances the ship a distance equal to the pitch, just as one turn of an ordinary carpenter’s screw advances it into a piece of wood a distance equivalent to the pitch. But, as wood is a solid, and water a fluid, it is not possible to imitate the motion exactly. When the marine screw revolves, the speed of its revolution has an effect upon the distance travelled. Because of the fluid nature of the medium in which the propeller is turning there is a “slip” for every revolution.
Almost all the fastest transatlantic liners, such as the “Queen Mary”, use four screws, and are referred to, therefore, as quadruple-
The screw itself is attached to a tail-
Here the shipowner’s difficulties begin. With all the bewildering array of fuels, engine types and arrangements available to-
A compound engine has a high-
This type of propelling machinery is essentially simple and reliable. It takes its energy from the boilers, which are mainly fired with Britain’s native fuel -
The air pump is necessary because of the condenser which converts the steam to water again. Unlike a locomotive, a ship, because of its long voyages, cannot afford continually to be taking fresh water on board and cannot “puff” the exhaust up a chimney. A few old tugs in New York Harbour are almost the only remaining vessels with non-
In the engine-
It was only a few years before the war of 1914-
The turbine is a rotary machine and, as such, is quite different from the engine that we have been discussing above, which is reciprocating. The turbine is rotary in that it comprises two portions, one of which turns in the other. The stator, or stationary part, has fixed blades, and the rotor, or moving part, has corresponding blades arranged on “wheels” on the main shaft, which revolve in grooves in the stator.
When the steam enters under pressure it hits the fixed blades and “bounces off” on to the blades of the rotor wheels, which, collectively, it turns, thus causing the propeller shaft to turn also. After leaving the turbine the steam goes through the same process as in the reciprocating engine. It returns to the boiler by way of the condenser and a hot well. The hot well is a device for bringing the water to a point conveniently near boiling-
The turbine differs from the reciprocating engine, in that it is not itself a reversible prime mover. A portion must, therefore, be incorporated in the turbine casing, in which the blades are set in the opposite direction, so that when steam is admitted to this part of the machine the shaft turns in the reverse direction. This is known as the astern turbine. For reasons of space, its size
can be only a fixed proportion of that of the ahead turbine. It is not practicable, therefore, with a turbine to have full power astern.
The turbine, because of its rotary nature, runs at high speed. It is not necessarily of benefit from the propulsive point of view for the turbine to be coupled directly to a propeller, for if the rate of revolutions is too high, the propeller “slips” idly round in the water, doing no useful work in thrusting the ship forward.
If the ship is fast and the hull is fine lined, high propeller speed, generally speaking, may be a matter of small importance; but cargo ships are fairly full bodied, and a slower running screw is necessary. When the turbine was adapted for cargo craft, it was necessary to introduce between the turbine and the shafting some form of reducing gear. Thus the propeller itself could turn at a number of revolutions suitable to the hull requirements, the reduction in some instances being from 6,000 to eighty revolutions a minute.
TURBINE BLADING is made of phosphor-
Once it was appreciated that speed reduction could take place by means of a toothed pinion on the end of the turbine, meshing with a large gear wheel on the propeller shaft, it was possible to arrange two, or even three turbines round a common gear-
A turbine of large diameter can always use low-
Another important type of machinery which is popular for certain types of ships is electric drive. The “Normandie”, one of the largest ships in the world, is electrically propelled. In this ship water-
Advantages of Electric Drive
One of the main advantages claimed for electric drive is that full power is available astern, since there are no astern turbines, and the electric motors can operate at full torque, as it is called, in either direction. The length of shafting is shorter, since the only connexion between the main prime-
A variation of the turbo-
GEARED TURBINES are the most compact type of prime mover for ship propulsion. The stator with its fixed blades and the rotor with its moving blades are contained, as explained in the text, in casings. This view shows the tops of the casings removed and the high-
The diesel system has been successfully employed, however, in a variety of ships of 3,000 hp downwards, including one particularly interesting group of oil tankers. Here the control to the main propelling motor is embodied on the bridge in a pedestal which contains also the controls for automatic gyro steering, or “Metal Mike”, as it is known. Thus, theoretically, the navigator can set the speed of his propelling motor by adjusting the handle on the pedestal. Her course having been set, the ship will run herself for as long as the controls are left untouched Diesel-
Scotch boilers and water-
The steam leaves what is called the stop-
A simple example is afforded by a three-
The motor ship now forms an important part of the world’s mercantile marine. In her cylinders fuel is rapidly burned, as the explosion may be chemically termed. The explosive action caused by the burning forces the pistons down and, as in a steam reciprocating engine, turns a crank.
The internal combustion engine does the two operations in one action and in one place. Therefore, until fairly recently the oil engine (or diesel engine as it is sometimes generically, though incorrectly, called) has been smaller, power for power, than the steam plant. But advances in steam propulsion have been so swift that with modem turbine plant, and even with some reciprocating engine plant, there is little to choose between the rival methods in space taken up, first cost, or in complexity. The diesel engine is confined entirely to oil, although some of the original engines of this type were designed for burning powdered coal as a fuel.
By comparing the first ocean-
THE RECIPROCATING ENGINE has been the friend of the marine engineers for many years and is still used for propelling a large number of vessels. This picture shows the cylinders which, of increasing size, provide various stages of expansion for the steam. An engine of this type is capable of developing about 3,000 hp at 98 rpm and is the kind of machine employed for driving large cargo steamers.
[From part 8, published 31 March 1936]
“The Queen Mary’s Engines” on this website.