ELECTRIC PROPULSION OF MERCHANT SHIPS. 281 



source of danger to turbine structures. Plate 121 shows a record for temperature 

 taken by a pyrometer situated between the nozzle and bucket of the last stage of a 

 marine turbine while the turbine was being operated at normal speed in the re- 

 verse direction. It will be observed that the high temperatures shown by that rec- 

 ord were produced in an extremely high vacuum by the introduction of small 

 amounts of steam. 



A turbine when operated in the reverse direction has a friction loss some- 

 thing like ten times as great as when it operates in a normal direction. In the 

 General Electric shops it has been discovered that reversing wheels of marine tur- 

 bines turn blue with heat when operated at normal speed in a vacuum of 20 inches. 

 While no definite information, can be given concerning the possible effects of 

 high superheat in reversals of a marine turbine, the facts here given indicate that 

 such effects may be serious and should, if possible, be avoided. A turbine which 

 is kept running in its normal direction is not subject to any large temperature vari- 

 ations. The economies incident to the use of superheat on shipboard are very great 

 and cannot long be neglected, although there have been few applications of super- 

 heat to American ships. The following extract from a letter from Van Nievelt, 

 Goudriaan & Co., Rotterdam, Holland, illustrates the superheat possibilities in en- 

 gine-driven ships: — 



"We are using during the last five years, in our multitubular boilers, 31^ -inch 

 tubes, very high funnels and Diamond blowers, and have no trouble at all in get- 

 ting sufficient steam. We have practically no leakage at the connections of the 

 pipes and boxes. The original pipes are still in use. The capacity of a 20-ton 

 evaporator is sufficient for supplying feed water." 



"Three of our steamers have been running half a year without superheaters 

 with a coal consumption of 24 to 25 tons. After fitting superheaters the consump- 

 tion was about 22 tons, making a saving of at least 10 per cent." 



In electrically driven ships the gain is quite as great as is here shown, and no 

 practical difficulties can result even from degrees of superheat which would be 

 impracticable with reciprocating engines. 



Efficiency of Transmission. — The selection for comparison of a ship of low 

 power is unfavorable to the electric drive in the matter of transmission efficiency, 

 the conditions being better for this method in ships of higher power. The genera- 

 tor designed for this case has an efficiency of 95.6 per cent and the motor 95.9 per 

 cent, making the transmission efficiency, including cable loss, etc., 91.6 per cent. In 

 machinery designed for certain high-power ships, the efficiency is as high as 94 

 per cent. 



To determine the efficiency of gear transmission as compared with the figure 

 given above, very careful tests have been made at Schenectady. A 2,400 horse- 

 power ship turbine was connected through two sets of double-reduction gearing 

 to a generator, and the steam consumption was tested at various degrees of load 



