The reason why the casing and rotor are made larger and larger as one proceeds from the inlet towards the exhaust or outlet is that the steam must, if all its
energy is to be extracted, expand as it goes and the enlargement provides room for this expansion.
One of the great advantages of the turbine is that the steam is always entering at the same end. In the cylinder of a reciprocating engine the steam enters alternately. It comes in hot but as it does its work and finally goes out it becomes very much cooler: the next lot of steam which enters, therefore, is chilled by the cool walls of the cylinder which have just been cooled by the departure of the previous lot of steam: so heat is wasted. Wasted heat means fuel lost, and as any given ship can only carry a limited quantity of fuel, wasted heat means less range and more frequent returns to the base to coal or to "oil."
Also let me remark again upon the simplicity of the turbine as opposed to the other sort. The latter consists of a mass of moving and swaying rods and cranks, to work among which, as the engineers have to do, is a terrifying and nerve-racking experience. The turbine, on the other hand, has its only working part enclosed. It is difficult to tell, by looking at it, whether a turbine is at work or not, so silent and still is it, so self-contained. The reciprocating engine-room is noisy and full of turmoil: the turbine room is weirdly still by comparison.
On the whole, too, it makes better use of the steam which it uses, but it has one decided drawback. It will not reverse, which the other type of engine does readily.
This means that two turbines have to be coupled together, one with the blades so set that the steam
drives it round correctly to produce motion ahead and the other set the opposite way so that it drives the vessel astern. The steam can be sent through either turbine at will and so motion can be obtained in either direction. Whichever turbine is in use the other revolves idly.
Unfortunately it is impossible to make a turbine to go slowly and yet be efficient. Consequently, slow steamers cannot use turbines, but for warships, which are nearly all fast boats, it has almost displaced the older type of engine.
The Curtiss turbine is different from the Parsons in that the steam encounters periodically, in its passage through, a partition perforated with funnel-shaped holes. Between the partitions it passes blades upon which it acts just as already described. The chief effect of this is to permit the machine being made of a rather more convenient shape and size. Other varieties of turbine are more or less combinations of the two ideas underlying these two.
When we look at a locomotive in motion we always see steam coming out of the funnel, but we never see that in the case of a steamer. That is because all the energy of the steam is taken and used in the latter case, while in the former much valuable energy goes off up the funnel, making a puffing noise instead of doing useful work.