SUPERHEATED STEAM IN MARINE PRACTICE. 189 



"It is to be remembered that it is the existence of water round the piston and 

 valves that enables leakage to be fifty times as great as if there was no water." 



70. Callendar and Nicholson, in describing their well-known experiments, say 

 that "hot water leaks through a crack and past valves and piston with far greater 

 facility than does steam." 



71. Boiler capacity as an asset is real, but rather difficult to express in figures. 

 This may, in the minds of some, not be of particular moment. What is meant by 

 boiler capacity in this sense is that, assuming a given power with saturated steam 

 and we applied a superheater, the same indicated horse-power is obtainable with, 

 say, 8 per cent less fuel and 10 per cent less steam. If, for any reason, it was de- 

 sired to force the boilers and engines, and to burn the same amount of fuel as with 

 the saturated steam, the output measured by engine horse-power will be increased 

 more than 10 per cent. Of course all ships may not have engines capable of de- 

 veloping this increased output on demand, but a great many do. For ships which 

 have boilers relatively small in comparison with the engines, there appear to be 

 great possibilities in fitting them with superheaters. 



72. Greater revenue cargo is obtained by virtue of the fact that decreased 

 tonnage of bunker coal is required for a given voyage. The saving in dead cargo 

 can be converted to paying cargo. The added income from this source should be 

 considered in connection with the possible use of superheated steam. 



SUMMARY AND OUTLOOK FOR SUPERHEATED STEAM. 



73. It is the writer's opinion that the situation, as regards superheaters and 

 the use of highly superheated steam, may be summed up by stating that designs 

 suitable from the standpoint of efficiency, simplicity, accessibility and durability 

 are available for shipowners. Also that the difficulties which discouraged the 

 earlier designers have been overcome as regards detail designs and operating con- 

 ditions, and, therefore, the installation, operation and maintenance of high-degree 

 superheaters appears to be not only possible, but practical and beneficial. Gray, in 

 his paper several times previously mentioned, states the following: — 



"The number of steamers fitted with superheaters during recent years is an 

 indication of successful working under superheat conditions. The following fig- 

 ures, though far from being a complete record, are worthy of note and considera- 

 tion : — 



No. of 

 steamers Total I. H. P. 



Schmidt's superheaters 1,042 Cylindrical boilers 1,322,092 



Hide 20 Cylindrical boilers 



Yarrow 13 Water tube boilers 261,000 



Babcock & Wilcox superheaters. . 29 Water tube boilers 316,675 



Thornycroft superheaters Water tube boilers 60,000 



