BUCKINGHAM: BALANCING OF TURBINKs 199 



any important sacrifice of other qualities of the turbine, and fur- 

 thermore that if the steam and propeller thrusts are balanced at full 

 speed the balance will be maintained approximately at much 

 lower speeds,— a matter of more consequence in naval than in 

 commercial practice. 



It may be regarded as a certainty that the standard direct- 

 connected installation of the immediate future will have turbines 

 of the mixed wheel-and-drum type driving independent shafts, 

 each unit being complete in itself. 



The operative advantages of the independent shaft system are 

 obvious; and the superiority of the mixed drum-and-wheel con- 

 struction is such that it is fast superseding the pure drum turbine, 

 while the pure multicellular turbine has seldom been used and is 

 now obsolete in marine practice. The drum construction is more 

 compact than the wheel construction and permits of the use of 

 more blade rows in a prescribed length, beside reducing windage. 

 It should be used at the low pressure (L.P.) end and should extend 

 toward the high pressure (H.P.) end as far as the steam volume 

 and blade lengths are great enough that ample radial clearances 

 can be used without serious leakage losses. Above this point, 

 the use of impulse- wheel stages contracts the leakage ring to 

 the diameter of a shaft bushing and permits of unlimited radial 

 clearances. At the H.P. end the use of impulse stages with 

 variable partial admission offers several well recognized advan- 

 tages. The following remarks, therefore, refer only to the mixed 

 turbine, and they relate to the problem of deciding where the 

 drum should begin. 



With a single rotor and unidirectional flow aft, the steam 

 thrust, due in the main to the difference of pressure on the ends 

 of the drum, is available for balancing the propeller t hrust. From 

 the speed-power curve of the hull the thrust may be found for 

 the assigned speed at which a balance is desired; and from the 

 estimated propulsive coefficient the shaft horse-power of the 

 turbine computed. If the terminal conditions of pressure, 

 superheat, and vacuum are specified, together with the revo- 

 lutions per minute, we may then, from the data on efficiency 

 obtained by experiment on stages of the general style to be used, 



