Excerpt from Papers and Discussions on Engines, Boilers and Torpedo Boats: Reprinted From the "Transactions of the Institutions of Naval Architects," Volumes 1882 and 1883
Economy of coal in our steamships is a point of so great importance, that any step in this direction is worth chronicling. Steamships are now making long voyages, and at high rates of speed, which, till within a few years ago, were made by sailing ships. To accomplish these profitably, economy of coal is all-important. Every ton of coal saved means a ton of freight earned, and in many trades every stoppage to coal is a loss of time, besides the exorbitant rates for coal charged at these ports. Coal-saving, too, is equally important for high-speed steamers, as, unless their voyage is only one of the shortest duration, coal becomes the greatest weight the ship is burdened with.
The obvious direction in which to look for saving is increase of pressure. Unfortunately, as we get higher in pressure, we do not by any means gain in efficiency in anything like a proportionate degree, and the time must come, may indeed not be far away, when further increase of pressure will not pay. This time has not come yet.
When James Watt utilized the lower pressures, from 15 pounds downwards, he unquestionably had at his disposal the most prolific portion. Pressure gradually rose - I am speaking now more particularly of the marine engine - to 30 pounds, and sometimes 40 pounds, but, used as the steam was, in one cylinder, exhausting direct to the condenser, little additional economy was gained by this increase of pressure. The variation in temperature, over 300 degrees in the cylinder, was too great; steam was condensed at the commencement of the stroke, when it had its full work before it, and re-evaporated towards the end, when it had little left to do but go straight to the condenser. Steam-jackets ameliorated this action, but besides the difficulty of getting them attended to at sea, they effected their purpose imperfectly and at a very considerable sacrifice of steam.
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