more economical than the simple.
But when the compound engine itself was first introduced, the high pressure and with it, necessarily, high temperature steam, together with surface condensation, caused serious drawbacks to its efficiency, so that great changes had to be made in the internal arrangements of both engines and boilers. For instance, the high temperature produced great wear and tear in the cylinders, valves, valve faces, and so forth, while the boilers rapidly corroded under the influence of the feed water taken from the condenser. These evils, however, when more thoroughly understood, were provided against; and the enormous saving in fuel induced shipowners to adopt the compound engines: under careful engineers, they last as long as, and cost very little more for repairs than the ordinary common condensing engines which consumed twice the amount of fuel.
It will, thus, be seen that the great stride in economy in the marine engine is due to high pressure steam expansion, and surface condensation: and, with a view to further economy, pressures are still advancing, the difficulty now being to construct a boiler that will withstand these pressures, and, at the same time, fulfil the other requirements of a marine boiler at sea. With these objects in view a number of patent water tubular boilers have been made. In 1870 and 1871 three ocean-going steamers were fitted with Howard’s and one with Roots’ patent boilers to work at a pressure of 120 lbs. per square inch, but they were not very long at sea before they failed, and were condemned. Again, in 1870, two very large steamers, each of 800 nominal horse-power, built for one of the Atlantic lines, were fitted with improved water tubular boilers to work at 120 lbs. pressure, but the trial of the first set of boilers, which completely failed, led the owners to condemn them and supply both vessels with those of the ordinary type to work at a pressure of 80 lbs. per square inch.
The failure of these boilers entailed an immense loss to the owners, and detained the vessels over twelve months, besides rendering the large engines, which were designed to work at 120 lbs., much less efficient at the lower pressure than they were intended to be.
It will, thus, be seen that the primary obstacle to advancement in economy appears to be the boilers, and although their construction, for very high pressure, is an expensive experiment, there are no less than four different descriptions (all of them patented) now being built in this country for marine purposes, any one of which, if thoroughly successful, will be another great step in advance.
However great the saving, hitherto, effected in fuel, there is still a wide margin between the means used and the effect produced, and great room, in other respects, for improvement. Indeed, Mr. Froude’s late experiments, at the instance of the Admiralty, on the actual resistance of ships, show that, in the case of the Greyhound, the ship he experimented on, the efficiency, at a speed of 10½ knots, was only 51 per cent., showing a loss of 49 per cent. of the motive power, which was even greater when the speed was less.[465]
There remains, therefore, a very large and deeply interesting field of research; for, of all the heat produced, we utilise in the steam engine only a small proportion for the purposes of propulsion.[466] Nor have we yet reached perfection in our ships, so far as regards the best form for obtaining the greatest speed. I have already shown[467] that, in river navigation, the American steamers surpass in speed anything we have as yet accomplished; and that they have made various attempts towards the adoption of the flat floor or “skimming” process—in other words, to sail over the surface of the water rather than to force the ship through it, as in the case of the cigar ship and others.
Great skill required for building perfect ships,
To construct a perfect ship is itself a problem of the highest order, to which the attention of mathematicians and the knowledge, skill, and tact of naval architects have of late years been constantly directed, with as yet no examples of complete success, however much the ships of our own time surpass those of our forefathers. Nor can the construction of safe, effective, powerful, profitable, and durable engines and boilers for marine purposes be a matter of easy determination, as shown from the fact, that there are still continual failures, revealing many difficulties yet to be overcome. Again, the means of propelling the vessel through the water suggests questions as to the resistance of fluids, which hydro-dynamic science has hitherto failed fully to resolve. Finally, the combination of all these, so as to bring about to the greatest advantage the effect desired, is a still more arduous task which the skill of the naval architect, the mechanician, and the sailor, even when combined, has not yet overcome. To the perfecting of our steam-ships we must still continue to apply ourselves, if we would maintain the high maritime position we now hold; for it is, only, by the unwearied exertions of all who are employed in our varied branches of industry, and with the aid of wise and just laws, that England can hope to keep ahead of all other nations.
and especial importance to England of having the best ships.